Soldering is a critical process used to attach electronic components to printed circuit boards (PCB) by melting solder to form permanent electromechanical bonds. The two most common soldering methods used in PCB assembly are wave soldering and reflow soldering. Both techniques have their own advantages and disadvantages.
This article provides an in-depth comparison between wave soldering and reflow soldering processes, equipment, applications etc. to understand their key differences and help select the right approach.
Overview of Wave and Reflow Soldering
Wave Soldering involves passing the bottom side of a PCB over a pumped wave of molten solder to solder component leads. The wave solder machine maintains a continuous recirculating wave of liquid solder within a tank.
Wave Soldering Process
The key stages in wave soldering process are:
- Flux application – Flux is sprayed on the bottom side of the PCB to remove oxides.
- Preheat – PCB is preheated to about 100°C to dry flux and prepare surfaces.
- Wave contact – The PCB travels over the solder wave, contacting the wave for 1-5 seconds allowing soldering of leads.
- Cool down – PCB passed through cool air jets to solidify solder before next processing step.
Reflow Soldering Process
The stages in a typical reflow soldering process are:
- Paste printing – Solder paste is deposited on PCB pads using stencil.
- Component placement – Components are accurately placed on board.
- Reflow – The PCB passes through preheat, reflow and cool down zones to melt and solidify solder.
- Inspection – Post reflow inspection performed.
Wave Soldering Equipment
The major equipment units in a wave soldering line are:
- Fluxer – Applies flux on PCB bottom side by spraying or foaming.
- Preheater – Uses IR heaters or convection heaters to warm PCB and components.
- Solder wave – Molten solder pumped and maintained at optimum level by nitrogen or air knives.
- Cooling section – Fans or water cooled heat sinks to solidify solder quickly.
Support mechanisms include conveyor, board supports, easy load/unload systems. Parameters like temperatures, speed are controlled by a computer.
Reflow Soldering Equipment
The primary equipment used in reflow soldering are:
- Solder paste printer – High precision, high speed printer to deposit solder paste on PCB.
- Pick and place machine – Automates placement of components on boards.
- Reflow oven – Has different thermal zones to heat and cool PCB Assembly. Convection, IR, vapor phase ovens used.
- Post reflow inspection – AOI machines inspect quality after soldering.
The process is highly automated with conveyor belts moving PCBs between machines.
Joint Quality Comparison
Wave Soldering Advantages
- Even solder deposition on pad surfaces.
- No tombstoning since components not yet placed.
- Less voiding as solder wets pads directly.
- No head-in-pillow defects.
Reflow Soldering Advantages
- Generally lower defects as solder paste can hold components in place.
- Better for fine pitch components due to precise paste deposition.
- Even heating in oven avoids thermal gradients.
- Double-sided reflow possible.
Wave Soldering Application
- Through hole components
- Wires, connectors soldered to PCB edge
- Single-sided PCB population
- Surface mount parts cannot be wave soldered.
- Tall components may get dislodged.
- Shadowing can block solder flow under components.
Reflow Soldering Application
- Surface mount components like SMD ICs, resistors, capacitors.
- High density boards with small components.
- Double sided SMD boards.
- Through hole components may fall out before reflow.
- Large components and boards need customized oven profiles.
- Lead-free solders like SAC alloy compatible but can require higher temperatures.
- Drossing and alloy contamination can be issues.
- Nitrogen curtains and mini-waves aid lead-free soldering.
- Easily adapted for lead-free solders through modified profiles.
- Closed, controlled oven environment gives better process control.
- Separate lead-free paste printers may be needed.
Process Speed Comparison
- Very high throughput of upto 35,000 joints per hour.
- Multiple boards can be processed simultaneously.
- Continuous in-line processing minimizes non-value added time.
- Limited by serial nature of process steps.
- Typical throughput of 2,500 – 6,000 joints per hour.
- Higher automation and vision inspection add cycle time.
Floor Space Needs
- Entire line from fluxer to cooler can fit in under 10 m length.
- Compact footprint due to in-line arrangement.
- Operators only needed at load/unload ends.
- Printer, oven, AOI require significant space between them.
- Typically need over 25 m long line.
- Operators required at multiple points through process.
- High thermal efficiency as solder constantly maintained molten.
- Preheating is only major energy need.
- Heat recovery reduces energy usage.
- Repeated heating of oven and paste deposits consumes more energy.
- Forced convection ovens consume high power.
- Optimization of preheat and reflow profile can help reduce consumption.
Equipment Cost Comparison
- Initial equipment cost of $50,000 to $150,000
- Low maintenance except for periodic pump rebuilds, nozzle cleaning etc.
- Printer, placement machine, oven total investment of $200,000 to $500,000.
- Higher maintenance for paste printer stencils, nozzles etc.
- Solder paste costs extra.
Wave Soldering Defects
- Shadowing leading to solder skips.
- Flux residue trapping under components.
- Bridging between leads and pads.
- Disturbed joints due to wave turbulence.
Reflow Soldering Defects
- Insufficient paste deposit causing poor wetting.
- Tombstoning of small components.
- Reflow voids due to poor paste application or profiles.
- Head-in-pillow defects under BGAs.
Process Control Factors
Wave Soldering Controls
- Solder wave parameters – height, speed, temperature, dross levels.
- Flux amount, heating levels in preheaters.
- Conveyor speed.
- Board supports, carriers, jigs.
Reflow Soldering Controls
- Paste printer setup – stencil, pressure, print speed.
- Accuracy of pick and place machines.
- Reflow oven – zone temps, conveyor speed.
- Paste condition and volume.
Wave Soldering Inspection
- Visual inspection at several stages – post flux, post solder, end of line.
- For solder defects, placement issues, residue etc.
- ICT testing for connectivity.
Reflow Soldering Inspection
- Main inspection after reflow by optical AOI equipment.
- Checks for insufficient or excess solder, tombstoning etc.
- ICT testing also required to confirm electrical integrity.
|Fluxing, preheat, solder wave, cooling
|Paste print, component place, reflow, cooling
|SMD components primarily
|Possible with higher temperatures
|Easily adapted through oven profiles
|Very high, upto 35000 joints/hr
|Limited by serial steps, 2500-6000 joints/hr
|Compact inline arrangement
|More space needed between machines
|High thermal efficiency
|Repeated oven heating consumes more energy
|$50K to $150K
|$200K to $500K
|Shadowing, flux residue, solder bridging
|Insufficient paste, tombstoning, reflow voids
|Wave parameters, preheat, conveyor speed
|Paste print, oven profile, paste volume
|Post-solder visual and ICT
|Post-reflow AOI and ICT
Applications of Wave and Reflow Soldering
Wave Soldering Use Cases
- High mix, medium volume PCB production
- Automotive electronics manufacturing
- Telecom infrastructure equipment
- Industrial control boards
Reflow Soldering Applications
- Low to medium volume, high mix consumer electronics
- Cellphone and portable device manufacturing
- High density digital boards for computing
- Advanced driver assistance automotive electronics
Choosing the Right Process
When to use wave soldering
- For through hole components
- Double sided TH boards
- Large boards
- High production volumes
- Low precision component placement
When to use reflow soldering
- For surface mount components
- Fine pitch parts below 0.5 mm
- Dense component placement
- Sensitive components that cannot withstand wave turbulence
- Lower to medium production volumes
In summary, while both wave and reflow soldering achieve the same end goal of permanently soldering components to PCBs, they have significant differences in their process steps, equipment involved, defect modes, process control needs and applications. Selecting the right soldering technology is vital based on product requirements like component types, precision, volumes etc. Using a combination of both methods is also commonly done to leverage their respective strengths for optimal quality and throughput. With the electronics manufacturing industry moving towards greater automation, miniaturization and flexibility, innovations in soldering technologies will continue playing a key enabler role.
Q1. Is it possible to use both wave and reflow soldering on a single PCB assembly?
Yes, it is common to use both methods in a mixed technology assembly. First the surface mount devices are soldered using reflow soldering. Then the through hole components are soldered by passing the board through the wave soldering machine.
Q2. How to decide between wave or reflow soldering for a product?
If using primarily through hole components, wave soldering is preferable. For boards with dense SMD components, fine pitch ICs below 0.5mm, BGAs etc. reflow soldering is more suitable.
Q3. What are no-clean soldering processes?
No-clean processes use solder paste and flux chemistries that do not require cleaning after soldering. This saves time and cost. They are common for reflow soldering, while wave soldering typically needs cleaning.
Q4. Does solder alloy affect the choice of process?
Yes, high temperature alloys like SAC305 may be difficult to use for wave soldering requiring higher preheat temperatures. Reflow ovens can be programmed for these alloys more easily.
Q5. How to reduce voids in reflow soldering?
Adjusting the reflow profile, using pause steps, proper ramp down, optimized paste deposit volumes and outgassing are some ways to reduce voids with reflow soldering.
In PCBA processing, There are two common soldering methods: Reflow soldering and wave soldering.
What is the function of reflow soldering, what is the function of wave soldering, and where are their differences?
PCBA is Printed Circuit Board +Assembly, That is to say, the PCB board board passes through the SMT line and DIP line, the whole process we call it PCBA. This is a commonly used method in China, While the standard way of writing in Europe and America is PCB’A, adding ” ‘ “, which is an official idiom.
1. Reflow soldering
Reflow soldering is a process in which a solder paste (a sticky mixture of powdered solder and flux) is used to temporarily attach one or thousands of tiny electrical components to their contact pads, after which the entire assembly is subjected to controlled heat. The solder paste reflows in a molten state, creating permanent solder joints. Heating may be accomplished by passing the assembly through a reflow oven or under an infrared lamp or by soldering individual joints [unconventionally] with a desoldering hot air pencil.
2. Wave Soldering
Wave soldering is a bulk pcb soldering process used in the manufacture of printed circuit boards. The circuit board is passed over a pan of molten solder in which a pump produces an upwelling of solder that looks like a standing wave. As the circuit board makes contact with this wave, the components become soldered to the board. Wave soldering is used for both through-hole printed circuit assemblies, and surface mount. In the latter case, the components are glued onto the surface of a printed circuit board (PCB) by placement equipment, before being run through the molten solder wave. Wave soldering is mainly used in soldering of through hole electronic components.
3. The difference between wave soldering and reflow soldering
(1) Wave soldering is a solder solder that forms a solder peak to solder components;
Reflow soldering is a high temperature hot air to form a reflowed molten solder to solder components.
(2) During reflow soldering, there is solder on the pcb before the furnace, and only the coated solder paste is melted and welded after soldering;
During wave soldering, there is no solder on the pcb before the furnace, and the solder wave generated by the welder coats the solder on the pads to be soldered.
(3) Reflow soldering is suitable for SMD electronic components, and wave soldering is suitable for pin electronic components.