Introduction

Flexible printed circuits (FPCs) are widely used to interconnect components in products like mobile phones, wearables, displays, medical devices etc. where flexibility is required. FPCs use thin and flexible polymer materials as the base substrate unlike rigid printed circuit boards (PCBs). However, this flexibility also makes FPCs prone to bending and twisting stresses during handling, assembly and use. Stiffeners can be attached to FPCs to provide localized reinforcement and protect certain areas from these stresses.

This article will provide a detailed overview of FPC stiffeners – their role, materials used, design considerations, attachment methods and applications.

What is a FPC Stiffener?

An FPC stiffener is a relatively thick, rigid structure that is fixed to the FPC using adhesives at strategic locations. It helps prevent excessive flexing, twisting or bending of the FPC in those areas during manufacturing, assembly and usage. Stiffeners protect delicate components as well as the FPC substrate itself from damage due to handling and mechanical stresses. They also provide stiffness for connectors and interface points to external devices.

FPC stiffeners are commonly made using materials like stainless steel, FR4, polyimide and acrylics. Adhesives such as epoxies and acrylics are used to bond the stiffener securely to the FPC. The stiffener often includes holes and cutouts to allow mounting of ICs, connectors, buttons and other components.

Need for Stiffeners in FPCs

While the thin, flexible construction of FPCs is advantageous for many applications, it also creates some unique challenges which FPC stiffeners help overcome:

• The thin polyimide substrate can get damaged by repeated flexing and twisting stresses during assembly and usage. Stiffeners reinforce areas prone to such stresses.
• Components on FPCs are also susceptible to failures and cracked joints due to excessive vibration or shock loads without support. Stiffeners protect fragile components.
• Flexible sections of FPCs by nature are prone to frequent handling and manipulation during fabrication, component placement and testing. Stiffeners prevent damage to copper traces and joints.
• Display driver ICs are sensitive to bending stresses due to the brittle silicon. Stiffeners support these components on display driver FPCs.
• Connectors and interface points to other PCBs or cables undergo mechanical stresses which can crack solder joints. Stiffeners prevent flexing damage.
• Stiffness is required during handling, clamping and assembly of FPCs using automatic equipment. Targeted stiffeners facilitate this.
• Stiffened areas can be grabbed or held firmly during manual assembly and repair procedures without causing damage due to bending.
• Components mounted on stiffened islands remain stable for inspection and rework. Flexible FPCs can distort from flatness.
• Stiffeners allow mounting of components on both sides of FPCs without short circuits due to touch. Open areas can be kept flexible.

FPC Stiffener Materials

Stiffeners designed for FPCs need to meet requirements like dimensional stability, mechanical stiffness, easy machinability or stamping, moisture resistance and adhesion compatibility with the FPC substrate and components. Here are some common materials used:

Plastics

Polyimide sheets offer a balance of stiffness, temperature resistance, dimensional stability and cost-effectiveness. Popular grades include Vespel, Duratron and Kapton. However, polyimide absorbs some moisture which may be unsuitable for high humidity environments. It has limited chemical resistance against solvents.

Acrylics like PMMA provide good moisture and chemical resistance along with stiffness and stability. PMMA stiffeners retain flexibility and do not become brittle over time. They adhere well to FPC substrates using acrylic adhesives. However, acrylics have relatively lower temperature ratings.

Polycarbonate offers high tensile strength for mechanical rigidity combined with temperature resistance exceeding 155°C. It also offers flame retardant properties required in some applications. Polycarbonate is prone to environmental stress cracking however.

LCP (liquid crystal polymer) grades like Zenite offer the best temperature resistance (>240°C) and stiffness among plastic options. LCP has excellent chemical resistance and ultra-low moisture absorption. But it is an expensive material choice.

Metals

Stainless steel offers extremely high stiffness and environmental resistance. It can withstand repeated flexing without cracking. Steel can be easily stamped or machined to desired shapes. It needs nickel/gold plating for soldering compatibility. Stainless steel substantially increases the weight of the FPC assembly.

Aluminum provides good stiffness-to-weight ratio and corrosion resistance while being lighter than steel. It is also low cost and easy to fabricate, but not as rigid as steel for the same thickness. Plating may be needed for soldering.

Copper has excellent thermal conductivity to dissipate heat from components. This lightweight metal stiffener can be readily soldered to the FPC copper traces without plating. But copper is prone to corrosion and needs protective coatings.

Kovar alloy has matching coefficient of thermal expansion (CTE) as silicon chips, minimizing solder joint stresses. However, kovar is not easily soldered and is mainly used as leadframes for ICs rather than stiffeners.

Brass offers good stiffness and manufacturability though not as strong as steel. It is prone to corrosion and needs protective nickel-tin plating typically. Soldering compatibility is also excellent.

Composites

FR-4 combines good stiffness, dimensional stability and cost-effectiveness. It can withstand processing temperatures over 150°C. Being fiberglass reinforced epoxy, it bonds well with the FPC substrate. However, FR-4 absorbs moisture and is not suitable for all environments.

CEM-1 laminates consisting of woven glass fabric in a cyanate ester resin provide the highest thermal capabilities as stiffeners, withstanding over 280oC. They have very low outgassing and moisture absorption. But CEM-1 material cost is also high.

Carbon fiber composites offer extremely high rigidity for their weight, along with corrosion resistance and ambient temperature tolerance exceeding 200°C. The high cost of carbon fiber is the main deterrent.

FPC Stiffener Design Considerations

Some key factors to consider during design of FPC stiffeners are:

Stiffness requirements – The amount of flex that needs to be prevented guides required stiffness. This helps select the material and thickness.

Area coverage – Partial patches or strips may suffice for localized reinforcement versus covering larger areas. Stiffener footprint impacts weight, cost and assembly considerations.

Component space – Cutouts, holes or voids need accommodation in the stiffener for mounting components, connectors, displays etc.

Routing provisions – Stiffener location should avoid blocking interconnection paths between components located on both sides of the FPC.

FPC material and layers – More layers and thicker dielectric require higher rigidity for equivalent stiffening. Stiffener adhesive selections depend on the FPC substrate.

Assembly and inspection – Stiffener should not hamper component placement, clamping, machine handling or manual assembly. Visual inspections and rework access need consideration.

Weight and thickness – Overall thickness after stiffener installation should meet product specifications. Weight limits steer material selection for wearable and portable devices.

Environment – Temperature range, chemical exposure, moisture levels and outgassing drive stiffener material selection.

Thermal dissipation – Heat dissipation may require thermally conductive stiffeners under high power components. Electrical and thermal insulation is needed for some applications.

Reworkability – Strippable stiffener adhesives may be preferred to allow rework of underlying components after stiffener attachment.

FPC Stiffener Attachment Methods

The stiffener must be securely fixed to the FPC substrate to prevent separation or movement during use. The common joining methods include:

Adhesive bonding – Acrylic, epoxy or polyimide adhesives are commonly used to bond the stiffener. Adhesive selection depends on materials and environmental needs. Die-cutting can create custom bondline shapes.

Soldering – For metal stiffeners, solder paste can attach and electrically ground it. Reflow attaches stiffener but risks heat damage to FPC substrate. Selective hand soldering is preferred.

Press-in – Interlocking press-fit features on metal stiffeners physically lock into the FPC substrate when compressed together. No adhesives are needed. Allows stiffener removal for repairs.

Clips – Metal spring clips or fasteners screwed into the stiffener mechanically clamp it to the FPC substrate firmly. Allows easy installation and removal.

Screws – Screws passing through clearance holes in the FPC substrate can thread into tapped holes in the stiffener to clamp it together. Supports reworkability.

Riveting – Rivets installed in the stiffener are peened over onto the FPC substrate to capture it. Allows stiffener removal by drilling out rivets.

Typical Applications of FPC Stiffeners

Some common applications where FPC stiffeners provide important benefits include:

Display driver FPCs – Stiffeners reinforce areas under driver ICs and connectors to prevent damage due to handling and vibration in devices like mobile phones, tablets, televisions etc.

Battery FPC interconnects – Mechanical robustness to withstand vibration and shocks over long device lifetimes make stiffeners essential for battery connector FPCs.

Medical products – For patient monitoring patches, implants and wearable devices, stiffeners allow robust interface connectors while keeping sensors flexible.

Automotive FPCs – Stiffeners used in cabling and sensors prevent damage due to continuous vibration and thermal cycles over vehicle lifetime.

Robotics – Frequent robotic arm and gripper movements require local stiffening on flex cables interconnecting movable sections for extended mechanical endurance.

Consumer Electronics – Stiffeners reinforce FPC connections to PCBs in smartphones, laptops and portable electronics that undergo drops and shocks.

Industrial – FPCs used in motion control systems, pneumatic valves, instrumentation etc. use stiffeners to create stable component mounting interfaces and robust cabling.

Benefits of FPC Stiffeners

Some key benefits provided by FPC stiffeners include:

• Reinforce connector areas for increased mechanical robustness and reliability
• Support fragile components like driver ICs and prevent failures
• Allow clamping and assembly handling of FPC without damage
• Improve stability for component placement, inspection and rework
• Prevent copper trace damage due to repeated bending stresses
• Absorb vibration and shock loads instead of transmitting them to FPC
• Facilitate manual and automated assembly and fabrication processes
• Allow double-sided component mounting for increased density without shorts
• Enable reliable performance over expected product lifetime
• Protect flexible cable segments from complex motion stresses

FAQs

Do FPC stiffeners help in heat dissipation?

Yes, choosing a thermally conductive stiffener under high power components allows heat spreading into the stiffener effectively. This reduces local hot spots.

Can multiple small stiffeners be used instead of one large one?

Yes, multiple discrete stiffener patches can be used for localized reinforcement and component protection instead of a single large stiffener.

Is the stiffener always in direct contact with FPC substrate?

Not always – standoffs can create an airgap between stiffener and FPC to facilitate inspection while still providing mechanical protection.

What tolerance is essential in FPC stiffener fabrication?

As stiffeners align FPC substrates for assembly, typical machining tolerance held is around ±0.10 mm. For placement near connectors or components, ±0.05mm tolerance or better is recommended.

How thick is a typical FPC stiffener?

Stiffener thickness depends on rigidity required but is typically in 0.3mm to 1mm range. Stainless steel can be as thin as 0.1mm but plastics need to be thicker for equivalent strength.

The importance of FPC stiffener

The Flex PCB stiffener refers to a stiffener attached to the back of the FPC when a widget such as a connector is attached to the FPC.

From the point of view of reinforcing materials, various materials, including metals and resins, can be provided depending on the application.

FPC（Flexible Printed Circuit）it is a soft and flexible structure. This is the biggest feature of FPC, but due to its soft structure, when connecting a small part such as a connector to an FPC, it is necessary to place a reinforcing plate on the back side of the electronic component to which the connector is attached.

But why do you want to put a stiffener? There are three main reasons for this. 1 solder resist off measure 2 solder pad drop countermeasure As shown in the right side of Figure 1, when the connector mounted on the FPC is inserted and removed, the FPC is soft, so external force is applied to the terminal portions of some connectors. As a result, the welded portion may fall off directly. Similarly, since the copper foil portion (pad) of the FPC may also easily fall off from the underlying structure, the base portion. This is where the mat falls off.

In either case, as shown in the left part of Fig. 1, as long as a sufficiently stiff reinforcing plate is placed on the FPC, it is possible to prevent the force from being concentrated on the terminal portion, and the solder/pad can effectively prevent the falling off.

3 Anti-substrate warpage countermeasures during reflow soldering

If the reinforcing plate is not attached during reflow soldering, the FPC will bend and the correct soldering may not be possible.

At the same time, even if it is difficult to weld, the mounting portion sometimes bends, which causes the mounting failure. (parts fall off)

In order to keep the mounting surface flat and securely mounted, the reinforcing plate is essential.

Introduction

Printed circuit boards (PCBs) form the backbone of all electronic devices. They provide the mechanical support structure to mount various electronic components and interconnect them through conductive copper tracks to form functioning circuits. The copper tracks and pads on the PCB that connect the various components need to be protected from oxidation and corrosion to maintain the electrical conductivity and solderability. This is achieved by applying different types of surface finishes over the exposed copper on the PCB.

There are various types of surface finishes available for PCBs, each with their own sets of pros and cons. Selecting the right surface finish for a PCB design requires carefully weighing factors like cost, solderability, oxidation resistance, contact resistance, assembly process compatibility, reworkability and more. The most common finishes used in PCB fabrication today include:

• Hot Air Solder Leveling (HASL)
• Immersion Silver (IAg)
• Immersion Tin (ISn)
• Electroless Nickel Immersion Gold (ENIG)
• Organic Solderability Preservatives (OSP)
• Electroless Nickel Electroless Palladium Immersion Gold (ENEPIG)

This article provides a detailed overview of these popular PCB surface finishes, their characteristics, typical applications and process methods used to apply them.

PCB Surface Finishing Processes

Surface finishing of PCBs is typically done after the boards have gone through the complete fabrication process of lamination, drilling, metallization and imaging. The exposed copper tracks are coated with the selected surface finish to protect the copper from oxidation and maintain solderability. There are various techniques used for depositing the different finish layers:

Electroplating

This involves submerging the PCB in a electrolytic bath containing the coating metal ions (tin, silver, gold etc.) and applying electric current to facilitate deposition of the metal layer onto the copper through reduction reactions. Electroplating allows deposition of uniform, smooth and shiny metallic coatings.

Electroless Plating

In this process, the PCB is dipped in a aqueous solution containing the plating metal (nickel, gold, palladium etc.) and a reducing agent. The reducing agent reacts with the metal ions leading to autocatalytic deposition of the metal layer onto the copper surface without external current.

Hot Air Solder Leveling

In this technique, the PCB is passed over a molten solder wave. The solder adheres to the copper tracks forming a coating. Hot air knives immediately blow hot air to flatten out the solder layer and give a smooth finish.

Organic Surface Preservatives

The PCB is dipped in an organic solution containing additives like imidazole, benzimidazole etc. which form an invisible thin coating and prevent copper oxidation.

With this background on the commonly used PCB surface finishing processes, let us now look at the popular finishes in more detail.

Hot Air Solder Leveling (HASL)

Hot air solder leveling using tin-lead alloy was the most prevalent PCB finish for many decades due to its low cost, ease of application, repairability and acceptable performance. However, due to the toxicity of lead and subsequent ban on lead usage in electronic assembly, lead-free solders have become standard for HASL finish.

Composition

The HASL finish consists of a thin layer of solder alloy coated over the copper conductors on the PCB. Common solder alloys used are:

• Sn96.5/Ag3/Cu0.5 (SAC305) – Melting point 217??C
• Sn95.5/Ag3.8/Cu0.7 (SAC387) – Melting point 219??C
• Sn99.3/Cu0.7 (SAC0307) – Melting point 227??C

These lead-free solders provide comparable performance to tin-lead solders. The thickness of the finish typically ranges from 1 to 3 mils.

Process

The steps in the HASL process are:

1. Flux application – The PCB goes through a foam fluxer unit containing flux solution to clean the copper and prepare it for soldering.
2. Preheating – The board is preheated to about 150??C to evaporate solvents in the flux and activate it.
3. Solder coating – The hot PCB (typical solder bath temperature 240-260??C) passes over a pumped solder wave which deposits and evenly coats the molten solder over the entire board surface.
4. Hot air leveling – High pressure hot air knives (245-255??C) immediately blow down on the solder coated board. This flattens out the solder, removing any peaks and valleys, giving an even, smooth finish.
5. Cooling and cleaning – The board cools down as it exits the HASL system. Residual flux is washed off in a cleaning unit.

Advantages

• Low cost process due to high throughput.
• No special PCB fabrication processes required.
• Provides good solderability. The solder finish itself acts as a permanent coating.
• Easy rework of components by locally applying solder.
• Repairable finish unlike ENIG, ISn etc. Damaged boards can be re-finished.

Disadvantages

• Can cause unevenness on small component leads and test pins.
• Flux residue entrapment can lead to corrosion and dendrite growth.
• Thickness control harder versus plating processes.
• Exposed finish prone to oxidation requiring nitrogen storage.
• Thermal fatigue issues due to CTE mismatch with copper.
• Unsuitable for fine pitch components due to bridging risk.
• Lead contamination risk in solder bath affects long term reliability.

Applications

HASL continues to be used for cost-sensitive applications like:

• Consumer electronics
• Power supplies
• LED lighting

It is often avoided for high reliability or fine pitch boards. The most common lead-free solder used is SAC305 which provides good solderability.

Immersion Silver (IAg)

Immersion silver finish has emerged as a popular and cost-effective replacement to HASL for many applications. It avoids the unevenness and bridging issues faced with HASL for fine pitch ICs and offers better coplanarity. The immersion silver plating process is simple and does not need any special PCB fabrication adjustments.

Composition

The immersion silver deposit consists of pure silver. The plating thickness is typically 2 to 5 micro-inches (0.05 to 0.125 microns).

Process

Immersion silver plating involves the following steps:

1. Surface preparation – The copper pads and traces are microetched to remove oxides and activate the surface.
2. Silver deposition – The PCB is immersed in a silver plating solution containing a silver salt (typically silver nitrate) and a reducing agent. The reducing agent facilitates deposition of silver ions onto the copper surfaces.
3. Rinsing and drying – The plated board is rinsed thoroughly in deionized water to remove any chemicals. It is then hot air dried.

The immersion silver process typically takes just 2-3 minutes and can easily be integrated into standard PCB fabrication lines.

Advantages

• Provides excellent solderability comparable to HASL.
• Uniform thickness deposition across pads, leads and test pins.
• Solves issues of HASL like icicling, flagging and bridging.
• Lower cost than other common finishes – ENIG, ENEPIG etc.
• Suitable for fine pitch components.
• RoHS compliant and halogen free process.

Disadvantages

• Silver tarnishes and oxidizes over time which can degrade solderability. Shelf life is limited.
• Not easily repairable unlike HASL. Damaged boards need to be re-plated.
• Slightly higher cost compared to HASL process.
• Does not prevent whisker growth like immersion tin.

Applications

The low cost, excellent solderability and fine pitch compatibility of immersion silver has made it the most widely used surface finish today for a range of applications:

• Cellphones, tablets and portable electronics
• Computer motherboards and graphic cards
• Telecommunication hardware
• Automotive electronics

It provides drop-in replacement capability for HASL. The limited shelf life means Ag plated boards are best assembled within 6-12 months.

Immersion Tin (ISn)

Immersion tin finish provides an economical way to protect copper traces from oxidation while retaining excellent solderability. The matte tin coating prevents whisker growth which has made it popular for high reliability applications.

Composition

The immersion tin deposit on PCBs consists of pure tin. The thickness typically ranges from 1 to 5 micro-inches (0.025 – 0.125 microns).

Process

The immersion tin plating process involves the following steps:

1. Surface preparation – Microetching cleans the copper surfaces and activates them for plating.
2. Tin deposition – The PCB is immersed in a heated tin salt solution containing a reducing agent. This causes deposition of Sn ions as a thin uniform metallic tin layer.
3. Rinsing and drying – the plated board is thoroughly rinsed and then dried using hot air.

The immersion tin plating process adds minimal cost to PCB fabrication.

Advantages

• Provides excellent solderability. Comparable to HASL for leaded and lead-free solders.
• Lower cost compared to other platings – ENIG, ENEPIG.
• Uniform thickness across component leads, test pads etc.
• Matte grey finish prevents tin whisker growth risks.
• Suitable even for ultra-fine pitch components.
• Long shelf life with good oxidation resistance.

Disadvantages

• Can have adhesion issues during thermal cycling leading to cracking.
• Not easily repairable compared to HASL process.
• Higher cost versus HASL process.
• Dull cosmetic finish.

Applications

The whisker resistance, stable solderability and fine pitch compatibility of immersion tin make it well suited for high reliability applications:

• Aerospace and defense electronics
• High-end telecom, 5G and networking gear
• Automotive electronics
• Medical equipment
• Industrial electronics

Its ability to prevent tin whiskering has led to the wide adoption of this finish in these segments.

Electroless Nickel Immersion Gold (ENIG)

The electroless nickel immersion gold (ENIG) finish provides an excellent combination of durability, solderability, bondability, conductivity and corrosion resistance. This had made it the finish of choice for high-reliability and high-performance electronics over the past several decades, despite its higher cost.

Composition

ENIG finish consists of a bottom layer of nickel-phosphorus alloy topped by a thin layer of immersion gold. Typical thickness is:

• Electroless nickel: 80 to 250 micro-inches (2-6 microns)
• Immersion gold: 3 to 15 micro-inches (0.08 – 0.4 microns)

Process

The ENIG process involves sequential plating of nickel and gold layers:

1. Surface preparation – A microetch step removes oxides and activates the copper surface.
2. Nickel plating – The PCB is immersed in an autocatalytic nickel bath where the exposed copper catalyzes deposition of nickel-phosphorus without any electrical current.
3. Gold plating – The nickel layer is coated with a thin top layer of immersion gold by dipping in a gold plating bath containing a reducing agent.
4. Rinsing and drying – Finally, the board is thoroughly rinsed in DI water and hot air dried.

Advantages

• Provides excellent solderability combined with long shelf life.
• Gold layer prevents copper/nickel oxidation.
• Wear and abrasion resistant finish.
• Uniform thickness deposition across component leads, test pads etc.
• Good for both leaded & lead-free soldering.
• Strong wire bondability.
• Excellent corrosion and tarnish resistance.
• Good electrical conductivity compared to other finishes.

Disadvantages

• Relatively higher cost finish.
• Not easily repairable or reworkable finish.
• Nickel barrier prone to cracking under thermal fatigue.
• Black pad risk due to thick nickel layer.

Applications

The superior reliability and performance characteristics have established ENIG as the finish of choice for critical applications:

• High speed digital electronics – CPUs, GPUs, chipsets, memory
• Aerospace and defense electronics
• Telecommunication infrastructure
• Automotive electronics
• High-end consumer electronics

ENIG is also used where component wire bonding is needed. The thickness of the nickel layer is adjusted based on required corrosion resistance and current density.

Organic Solderability Preservatives (OSP)

Organic solderability preservatives provide an inexpensive way to protect copper traces from oxidation without the cost of metal plating. The organic coating prevents tarnishing while allowing soldering when needed.

Composition

OSP deposits consist of organic inhibitors like imidazoles, benzimidazoles, benzotriazoles etc. They form monolayers with a thickness of only a few molecules.

Process

The OSP application process is quick and involves:

1. Surface preparation – The copper is microetched and activated prior to OSP deposition.
2. OSP treatment – The PCB is immersed in a room temperature OSP solution containing organic inhibitors. These coat the copper surfaces.
3. Rinsing and drying – Residual treatment solution is rinsed off, followed by drying.

The OSP process adds minimal fabrication cost and can easily be integrated.

Advantages

• Extremely low cost compared to metal finishing.
• Simple room temperature process.
• Minimal effect on production process flow.
• Provides temporary protection against oxidation.
• Minimal risk of insertion issues versus thick metal layers.

Disadvantages

• Shelf life limited to about 6 months.
• Multiple heat cycles can degrade solderability.
• Not suitable for high temperature or high reliability applications.
• Repairing and reworking is difficult.
• No wire bonding possible. Only for SMT parts.

Applications

The ultra-low cost of OSP makes it attractive for cost sensitive consumer electronic products with limited shelf lives:

• Mobile phones
• Tablets and laptops
• IoT devices
• Portable electronics
• LED lighting

For long-life or high temperature products, other finishes like ENIG or Immersion Silver are used.

Electroless Nickel Electroless Palladium Immersion Gold (ENEPIG)

ENEPIG finish provides a cost-effective alternative to ENIG for many applications by replacing the immersion gold layer with electroless palladium + immersion gold. This reduces the overall gold consumption while providing excellent shelf life.

Composition

ENEPIG deposits consist of:

• Electroless nickel: 2 to 5 microns
• Electroless palladium: 0.05 – 0.3 microns
• Immersion gold: 0.025 – 0.05 microns

Process

ENEPIG plating involves the following steps:

1. Surface preparation using microetching
2. Electroless nickel plating
3. Electroless palladium plating
4. Immersion gold – Double dip process
5. Rinsing and drying

Advantages

• Low gold consumption provides cost benefit over ENIG.
• Shelf life, solderability and conductivity close to that of ENIG finish.
• Good corrosion and oxidation resistance.
• Palladium/Gold resists black pad risk.
• Uniform deposition across component leads and pads.

Disadvantages

• More expensive than tin, silver or OSP finishes.
• Slightly lower bondability than with thick immersion gold.
• Repair and rework is difficult.

Applications

The ENEPIG process allows reducing thickness (and cost) of gold layer without compromising shelf life or solderability. It is commonly used for:

• High reliability telecommunications hardware
• Aerospace and defense electronics
• Automotive electronics
• Medical electronics equipment
• High-end industrial electronics

Faster depletion of thinner gold layer limits reworkability.

Summary of Common PCB Finishes

Finish	Typical Thickness	Pros	Cons	Applications
HASL	1-3 mils	Low cost, repairable, good solderability	Thermal fatigue issues, uneven deposits, bridging risks	Cost-sensitive electronics
Immersion Silver	0.05-0.125 ??m	Low cost, excellent solderability, suitable for fine pitch	Oxidation and tarnishing issues, short shelf life	Cellphones, computers, telecom hardware
Immersion Tin	0.025-0.125 ??m	Whisker-free, excellent shelf life and solderability	Dull cos

How to know the surface finish from PCB color?

From the PCB surface, there are three main colors: gold, silver, and light red color. The PCB with gold color is the most expensive, the silver color is cheap, and the light red color is the cheapest.

You could know if the manufacturer have cut corners or not, from the surface color.

Besides, the circuit inside the board is mainly pure copper. The Copper is easily oxidized when exposed to air, so the outer layer must have the above protective cover.

1. Gold color

Some people say that gold color is copper, that is not right.

Please refer to the picture with gold plating on the board as below:

The golden circuit board is the most expensive and is the real gold. Although it is thin, it also accounts for nearly 10% of the cost of the board.

There are two advantages for using gold, One is for welding conveniently and the other is for anti-corrosion.

As shown below, this is the golden finger of the memory stick 8 years ago. It is still golden and shining.

The gold-plated layer is widely used in circuit board component pads, gold fingers, connector shrapnel etc.

If you find that some boards are all silver, it must be cut corners. We call it “costdown.”

Generally speaking, the motherboards of mobile phone are gold-plated boards, but computer motherboards and small digital boards are not gold-plated.

Please refer to the board of iPhone X as below, The exposed parts are all gold plated.

2. Silver color

Gold color is gold, silver color is silver? Of course not, it is tin.

The silver board is called a HASL board. Spraying tin on the outer layer of the copper, it also could help to welding, but it is not as stable as gold.

There is no effect on already welded components for HASL board. However, if pads are exposed to the air for a long time, such as ground pads and pin sockets etc, It is easy to oxidize and rust, resulting in poor contact.

All the small digital products are HASL boards. There is only one reason: cheap.

3. Light red color

OSP(Organic Solderability Preservatives), it is organic, not metal, So it is cheaper than the HASL process.

The only function of the organic membrane is to ensure the inner copper foil will not be oxidized before PCB soldering.

This membrane evaporates as soon as it is heated. Then you could solder the copper wire and components together.

But it is easy to be corroded. An OSP board can’t be soldered if it is exposed to the air for more then 10 days.

There are many OSP processes in computer motherboards. Because the board size is too large.

Printed circuit boards (PCB) need protective coatings on the copper traces to prevent oxidation and improve solderability. Gold is commonly used as PCB surface finish due to its high conductivity, corrosion resistance and ability to form reliable solder joints. The two main gold finishing processes used are immersion gold and electroplated gold.

This article explains immersion gold and gold plating for PCBs, compares their properties, process differences, reliability and costs to help understand their pros and cons. It also provides guidance on selecting the right gold finish for a PCB application based on considerations like cost, durability, complexity etc.

Introduction to PCB Gold Finishing

PCBs have exposed copper pads and traces that are prone to oxidation and corrosion when left unprotected. This negatively affects soldering performance. Gold finishing deposits a thin layer of gold on the copper conductors which provides excellent protection.

Gold is used due to its unique properties:

• Highly conductive
• Does not oxidize or corrode
• Bio-compatible and lead-free
• Good solderability and wire bondability
• Surface hardness

Gold finishing is typically applied over nickel underplating on PCBs. The nickel layer protects copper from leaching into the solder during high temperature soldering processes.

Read More about PCB surface finished:

• HASL Lead Free
• Immersion Gold (ENIG)
• ENEPIG
• OSP PCB
• Immersion Tin
• Immersion Silver
• Hard Gold PCB
• Gold Plating Process

What is PCB Immersion Gold?

Immersion gold or electroless gold involves depositing a layer of gold on the PCB conductors by immersing the boards in an aqueous gold solution containing ions of a reducing agent like sodium borohydride.

The electrons from the reducing agent provide energy to reduce gold ions in the solution to metallic gold which deposits on the PCB surface.

The thickness of the gold layer can be controlled by immersion time, solution parameters and temperature typically ranging from 3 to 8 micro-inches. The deposition occurs uniformly on the entire PCB surface.

What is Gold Plating on PCBs?

Gold plating refers to the electrolytic deposition of gold on PCBs using electrical current. The PCB acts as the cathode and gold ions in solution deposit on the PCB surface in the presence of an electrical potential.

Thickness can be precisely controlled ranging from 10 to 100 micro-inches. Only selected areas like fingers, pads, traces that are connected to the current get plated.

Comparing Immersion Gold vs Gold Plating

Process Differences

Immersion Gold	Gold Plating
Auto-catalytic chemical process. No external electrons supplied.	Electrolytic process requires external DC current to drive deposition.
Uniform deposition on all surfaces.	Selective deposition only on surfaces connected to current.
Lower process control and thickness uniformity.	More process control and thickness uniformity.
Suitable for simple board geometries.	Suitable for complex board geometries.
No power supplies or process control needed.	Power supplies and process control instrumentation required.

Performance Comparison

Parameter	Immersion Gold	Gold Plating
Corrosion Resistance	Good	Excellent
Hardness	Lower (~100 Knoop)	Higher (130-160 Knoop)
Adhesion	Moderate	Very good
Solderability	Good	Excellent
Wire Bondability	Moderate	Excellent
Wear Resistance	Lower	Higher
Purity	Moderate (~98%)	High (~99.9%)
Durability	Moderate (~1-2 years)	Very high (5-10 years)

Cost Comparison

Immersion Gold	Gold Plating
Lower equipment costs.	Higher equipment costs.
Lower process control costs.	Additional process control instrumentation needed.
Higher solution costs due to lack of solution recovery.	Lower solution costs due to >99% solution recovery.
Overall lower cost process.	Overall higher cost process.

PCB Applications of Immersion Gold vs Gold Plating

Immersion Gold is Typically Used for:

• Double sided and multilayer boards
• High density interconnect (HDI) boards
• For finer features below 100 microns
• For board features with spacing <150 microns
• Budget consumer electronics assemblies
• Shorter service life products

Gold Plating is Ideal for:

• High reliability electronics – aerospace, medical, automotive
• Mission critical electronics like servers
• High current power electronics
• Low voltage digital logic boards
• Complex and dense board geometries
• Long field life products

Pros and Cons of Immersion Gold vs Gold Plating

Advantages of Immersion Gold

• Simple process with no electrical instrumentation
• Lower equipment and processing costs
• Uniform coverage on all surfaces
• Faster processing time
• Suitable for fine features and PCBs with dense spacing

Disadvantages of Immersion Gold

• Lesser control on thickness uniformity and reproducibility
• No possibility of selective plating
• Lower purity, hardness and bondability
• Less wear resistance and durability

Advantages of Gold Plating

• Excellent thickness process control and uniformity
• High purity, bondability, hardness and wear resistance
• Selective plating ability
• Superior solderability and corrosion resistance
• High durability with thickness up to 100 micro-inches

Disadvantages of Gold Plating

• Requires current supply and monitoring instrumentation
• Slower than immersion with higher cost
• Complex for boards with dense, small features
• Higher initial capital investment

Choosing Between Immersion Gold vs Gold Plating

Factors Favoring Immersion Gold

• Cost sensitive products
• Consumer electronics with shorter life cycles
• Very fine feature PCBs below 100 microns
• Quick turnaround boards
• Simpler board geometries

Factors Favoring Gold Plating

• High reliability products like medical, defense, aerospace
• Mission critical enterprise server boards
• Complex, dense board geometries with various feature sizes
• Very flat gold finish needed for wire bonding
• Long field operational life expectancy

Conclusion

In summary, immersion gold provides a faster and cheaper process while electrolytic gold plating offers superior durability, solderability and selective plating capability. For low cost consumer PCBs with fine features, immersion gold usually suffices. But for complex, high performance boards with long service lives, gold plating may be the optimal choice.

The PCB application, cost targets, reliability needs and product life cycle should be evaluated to decide between immersion gold vs gold plating finish. With new technological advances, hybrid techniques can also potentially combine the advantages of both processes.

FAQs

Q1. Is it possible to first use immersion gold and then gold plate?

Yes, it is possible to deposit initial immersion gold layer followed by gold plating to thicken critical areas needing more gold like contact fingers while minimizing costs.

Q2. Does solder mask affect choice of gold finishing process?

Yes, solder mask clearance constraints like 75-100 microns favor using immersion gold which can coat beneath tight spaces. Gold plating requires more spacing.

Q3. Which gold finishing is suitable for ENEPIG surface finish?

ENEPIG refers to electroless Ni, electroless Pd and immersion Au finish. It already uses immersion gold and provides excellent solderability. Gold plating over ENEPIG is generally not required.

Q4. Can immersion gold withstand multiple reflow cycles?

Immersion gold can withstand around 1-3 reflow cycles as compared to 4-6 for hard gold. Recommended thickness is 50-100 microinches for multiple reflow capability.

Q5. Does porosity of copper affect gold finishing?

Yes, highly porous copper can trap process chemicals causing issues. A nickel barrier underlayer helps mitigate porosity issues for both immersion gold and gold plating processes.