Solder is an essential material used to form both mechanical and electrical connections in printed circuit board (PCB) assembly. But with the variety of solder alloys available with different properties, how do you determine the “best” solder type for your particular PCB application? There is no single answer, as the optimum solder choice depends on factors like:

• Application requirements – e.g. operating temperatures, vibration levels
• PCB design considerations – e.g. pad sizes, pitch between joints
• Process characteristics – e.g. soldering methods used, production volumes
• Reliability expectations – e.g. product lifetime, expected thermal cycles
• Compliance needs – e.g. lead-free requirements, military specifications

By weighing specifications against these factors, PCB designers and production engineers can zero in on the right solder materials for their boards. This article provides guidance on selecting the best solder for PCBs based on different criteria.

Common Solder Alloy Compositions Used in PCBs

There are a variety of solder alloys to choose from, classified by their melting temperatures. Here are some of the most common compositions used for PCB work along with their characteristics:

There are some general tradeoffs between cost, melting point, wettability (solder spreading), mechanical strength and thermal cycle lifetime when evaluating solder alloys. Higher tin compositions (like SN100C) typically have better wetting behavior, but reduced temperature resistance compared to silver or gold-containing high melt solders.

Determining the Best Solder – Key Selection Criteria

Several critical factors should be evaluated when figuring out the most appropriate solder materials for your PCB application. These include:

Solder Joint Reliability Needed

How many thermal excursions can solder joints withstand before the product quality degrades? Cycle counts above 1000+ generally require SAC alloys or high melt solder.

Operating Temperature Range

Maximum service temperatures influence solder liquidus to avoid re-flowing. High melt solders like AuSn are used for very high temperature PCBs.

Vibration/Shock Resistance

Mechanical stability requirements dictate alloy strength. Silver/copper strengthens lead-free joints for vibration-prone assemblies.

Environmental Resistance

Solder corrosion potential should match reasonably expected environments. Gold containing alloys excel in chemical resistance.

PCB Design and Pitch

Joint sizes and spacing impact options. Small joints and fine pitch demand quality wetting behaviour with sufficient capillary action.

Production Process Details

The soldering process used (hand soldering, SMT reflow, wave/selective soldering) determines alloy melting suitability.

We’ll now dive deeper into each of these criteria to identify ideals solders types for differing PCB applications based on their unique requirements and specifications.

Best Solders for Thermal Reliability and Operating Temperatures

A primary factor determining best solder match is the temperature conditions the PCB assembly will encounter in operation and the expected thermal cycle lifetime at those temperatures without degradation of solder joints. Higher temperature products demand high melt solder alloys.

Low Temperature PCBs

For applications only reaching 60-100°C with short lifetime thermal requirements, lead-free SN100C alloy may provide the best combination of lower melting point, good wetting and moderate cost. Other options are 63/37 SnPb or SAC alloys.

Moderate Temperature PCBs

Where operational temps range from -40°C to +125°C and products expect 5-10 year lifetimes, SAC305 or SAC405 lead-free solders offer reliable performance with melt points of 217°C – 221°C to withstand those peak temperatures.

High Reliability and Harsh Environments

In aviation, military and automotive products seeing extreme temperatures (-55°C to +150°C) and long 20-30 year expected cycle lives, high melt solder alloys with silver, copper and/or gold are superior choices despite higher costs. This includes alloys like SnAgCu, SACX plus and AuSn.

Extreme Environments

For specialized electronics operating in excess of 260°C-300°C ambient environments, gold-based solders like Au88Ge12 and Au80Sn20 with melting points over 280°C+ are often the only viable option. Alumina substrates are typically used in place of traditional PCBs.

Vibration Tolerance and Mechanical Shock Resistance

Solder alloy composition choices also impact mechanical joint strength and resilience to vibration stresses and shocks. Silver and copper strengthen solder alloys.

Low Vibration

If electronics will not undergo significant vibration exposure, lower cost SN100C or SAC305 provide adequate mechanical strength while enhancing wetting and flow performance.

Moderate Vibration/Shock

Where electronics experience regular vibrations <10G RMS and some shock risks, silver-containing SAC405 or SACX Plus lead-free solders give better vibration resistance.

High Vibration and Shock

For mechanically demanding applications seeing continual vibration profiles exceeding 30G RMS and large shock threats, solder joint failure risks climb. Here high-percentage silver (Ag3.5-4%) SAC alloys, silver-copper composite solders and gold alloys like Au80Sn20 maximize joint integrity.

Environmental Resistance Requirements

Operating environments dictate material durability needs. Solder alloy corrosion vulnerability, humidity tolerance and contamination risks guide ideal choices.

Controlled Environments

For PCBs used in clean, dry indoor ambient environments, economical SAC305 or Sn/Cu offer decent corrosion allowance and minimize cost.

Mixed Environments

Where electronics experience both indoor and occasional outdoor usage with humidity and chemical splash risks, SAC405 solder resists corrosion better. Conformal coatings also improve protection.

Harsh Outdoor/Chemical Environments

In chemically caustic plants, marine vessels or contaminated outdoor settings, gold-content solders like Au80Sn20 provide maximal corrosion and contamination protections despite their steep costs.

Solder Suitability for Fine Pitch PCB Components

As PCB components shrink in size with expanding demands for miniaturization and circuit density, fine component pitch poses soldering and capillary flow challenges needing special solders.

Standard Pitch Components

Where component pins and pads utilize ≥ 0.65mm pitch spacing, economical SAC305 or SN100C alloys offer reliable soldering and wetting performance.

Mid-Range Fine Pitch

For moderatedensity designs with 0.45mm – 0.65mm pitch ICs, BGAs or connectors, SN100C and SAC305 may remain suitable, but issues can arise needing solder masks to limit bridging potential.

Extremely Fine Pitch

High density PCB assemblies populated with ≤ 0.4mm pitch CSPs, 0201 discrete packages or microBGAs demand pastes matching SN100C or AuSn20 solders to maximize reflow capillary effects for forming reliable, bridging-free joints.

Production Process Soldering Selection Factors

Beyond product specifications, manufacturing methods influence best solder choices balancing process-optimized alloys against assembly soldering equipment capabilities and limitations.

Hand Soldering

Operators hand soldering larger thru-hole PCB components have flexibility using general 63/37 SnPb or no-clean SN100C cores along with appropriate fluxes. Low melt alloys improve manual soldering.

SMT Reflow

For SMT boards with primarily SMD parts, lead-free SAC305 solder paste remains an industry mainstay for compatibility with standard reflow ovens. Adjustable profile ovens support other alloys.

Wave Soldering

High volume PTH soldering depends on solder pot and pump compatibility. Common wave solders include SN100C and SAC305 for hotter dual-wave equipment, or lower-melt SnPb and SnCu for single pot use.

Selective Soldering

Precision solder nozzle equipment offers flexibility for low-volume applications to match suitable alloys like SnPb or SN100C with fluxes to selective solder SMD, PTH or solder repair needs.

Summary Comparison of Key Factors for Selecting Best PCB Solder

This table summarizes how some of the critical considerations and tradeoffs impact selecting the ideal solder type for various PCB requirements:

Matching PCB requirements to candidate solders provides an optimal starting point for selecting solder materials, with final suitability confirmation via soldering evaluations. As needs evolve, advanced alloy options continue expanding to address technical demands.

Frequently Asked Questions on Best Solders for PCBs

What is the most commonly used solder?

The lead-free SAC305 alloy (96.5Sn/3Ag/0.5Cu) is currently the most popular PCB solder, used widely since the transition from tin-lead solders. It offers a good balance of properties for many consumer electronics applications at reasonable cost.

Is lead-free solder better than leaded?

Lead-free SAC alloys provide comparable or superior thermal cycle lifetime, mechanical shock resistance and electrical conductivity to traditional SnPb solders. Process adaptations were required when switching to lead-free, but performance has matched or exceeded lead.

Can you mix lead and lead-free solder?

Avoid mixing lead and lead-free alloys on the same PCB assembly whenever possible. The differing melting points and material properties can impact solder joint reliability. In some cases, mixing cannot be prevented, demanding extra process care.

What problems can occur from using the wrong solder?

Potential risks from using unsuitable solder include weakened mechanical integrity, fractured joints under thermal stress, increased electrical resistance, overheating damage, solder balls/beading or cracked joints leading to intermittent contacts or total failure.

How important is it to match solder alloy to paste?

Carefully matching solder paste metal loads to the barrel, wire or board finish alloy is extremely important to achieve optimal soldering quality. Minor alloy differences can impact melting, wetting and flow dynamics. Pastes are formulated specifically for their target solders.