Selective Soldering PCB Technical Details: What & How

Selective soldering is a soldering process used to solder electronic components to printed circuit boards (PCBs) by applying solder only to the areas that require solder. This targeted approach differs from traditional wave soldering, which solders entire circuit boards by passing them over waves of molten solder.

Selective soldering provides more control and precision in the soldering process. It allows intricate solder joints to be created while minimizing the risk of bridging between closely spaced component leads. This makes it well-suited for soldering complex, densely populated PCB assemblies.

Benefits of Selective Soldering

Selective soldering offers several advantages over wave soldering:

• Reduced solder waste - Solder is only applied where needed instead of coating the entire board
• Minimal component stress - Sensitive components avoid excess heat exposure
• Suitability for advanced components - Compatible with bottom-terminated components like BGAs which cannot withstand wave soldering
• Flexibility - Different solder alloys can be applied as needed across a single board
• Easy rework - Defective joints can be reworked quickly by resoldering only affected areas

How Selective Soldering Works

Selective soldering is typically performed using specialized equipment that allows for automated, precision solder application:

1. Flux Application

• Flux is sprayed or foamed onto areas of the PCB requiring solder
• Flux removes surface oxides and facilitates solder flow

2. Preheating

• Board passes under infrared heaters or convection heaters
• Heats board to a temperature just below the solder liquidus point
• Prevents thermal shock to components when solder is applied

3. Solder Dispensing

• Miniature solder pump or drop-jet nozzle dispenses precise volumes of solder
• Solder alloy matches terminal composition
• Nitrogen atmosphere prevents oxidation

4. Cooling and Cleaning

• Board moves into cooling tunnel to solidify solder joints
• Any residual flux is removed in cleaning step

Key Process Parameters

Achieving high quality, reliable solder joints requires optimizing and controlling key process parameters:

Temperature

• Preheat temperature - Typically 100-150°C; brings board/components close to solder liquidus temperature
• Solder temperature - Depends on alloy; around 30-50°C above liquidus preferred for good flow/wetting
• Peak temperature - Holding at 20-40°C over liquidus for 1-2 secs gives sound solder joints

Atmosphere

• Nitrogen level - N2 blanketing prevents oxidation and improves solderability
• Humidity - Ensuring dry atmosphere (<40% RH) eliminates flux spattering

Time

• Preheat time - 2-4 minutes to gradually heat board and components without damage
• Dwell time - 0.5-3 secs of liquidus peak temp needed for good wetting and bond formation

Board Parameters

• Layout - Spacing, density, thermal reliefs designed not to impede selective soldering
• Cleanliness - Rigorous cleaning/descaling ensures pads are fluxable and solderable
• Flatness - Board warp and twist minimized to enable consistent heating

Suitable Components for Selective Soldering

Selective soldering provides a compliant, no-stress soldering process capable of soldering advanced component packages not suited to wave soldering:

BGA/CSP Components

• Bottom-terminated, cannot withstand turbulent waves
• Sensitive to stress from socketing required in reflow process
• Selective soldering solders balls without component removal

Connectors

• Tall profiles exceed typical solder wave height
• Selective “mini-wave” possible at each joint opening

Bottom Terminated Components

• QFNs, LEDs and other bottom-terminated parts incompatibile with reflow process
• Ideal for inverted selective soldering

Thru-Hole Components

• Selective “spot soldering” precisely solders leads/lugs without bridging
• Capable of soldering mixed SMT and thru-hole assemblies

Defect Prevention in Selective Soldering

While selective soldering produces superior solder joint quality compared to wave soldering, defects can still occur if the process is not properly setup and maintained:

DefectRoot CauseCorrective ActionInsufficient solderLow solder bath temperatureIncrease temperature to improve meltingCold solder jointInadequate heating of pad and componentIncrease preheat temp/time; adjust soldering speedSolder ballingHigh/turbulent wave causing splashingLower solder flow volume and pressureBridgingExcessive solder depositionOptimize solder nozzle setup; change fluxIcicles/webbingHigh temperatures causing flux pyrolysisLower preheat temperature; change to no-clean fluxSolder beadingContamination preventing wettingImprove cleaning process; change fluxDewettingOxidized surface; non-solderable finishUse more active flux; alter ENIG to immersion tin

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Monitoring key process metrics allows defects to be traced back to specific root causes for correction before large quantities of boards are impacted.

Selective Soldering Applications

The precision, flexibility and low stress of selective soldering makes it the preferred soldering technique for:

High Density Interconnects (HDI)

• Microvias, fine features prone to bridging defects in wave soldering
• Eliminates shorts between tightly spaced component pads

LED Lighting

• Bottom-terminated high brightness LEDs incompatible with standard reflow
• Provides thermal relief needed to protect sensitive LED components

Automotive Electronics

• Capable of soldering components to rigorous automotive quality standards
• Facilitates rework of safety-critical vehicle electronics

Power Electronics

• Fluxing and soldering large copper bus bars without excessive heat damage
• Creation of multi-alloy solder joints on mixed metallurgy boards

Medical Electronics

• Biocompatible precision soldering for active implantable devices
• X-ray transparency allows internal inspection of hidden solder joints

Conformal Coating Integration

For boards that require conformal coating material to be applied for added protection, cure ovens can be integrated into the selective soldering line following the soldering process. This enables immediate coating material application after soldering while components are still hot and outgassing is promoted. Multiple spray, dispense and spray methods exist for conformal coating integration.

Summary

The closed environment, precise nature of the selective soldering process enables defect-free soldering of challenging component configurations and PCB designs. When key parameters are properly managed, it surpasses wave soldering for producing robust, high reliability solder joints in automated production environments. Integration with conformal coating makes selective soldering a flexible, comprehensive advanced PCB assembly finishing solution.

Frequently Asked Questions

What are the limitations of selective soldering?

The main limitations involve the size of boards able to be processed. Very large boards are difficult to heat evenly without shadowing effects. High mass boards also require longer process times to reach thermal equilibrium. Selective soldering also has lower throughput compared to wave soldering since boards are run individually instead of continuously.

Is manual application possible?

While nearly all selective soldering is automated, manual systems do exist using handheld solder baths or mini-wave machines. This gives more flexibility but loses the precise process control of automated selective equipment. Manual selective soldering requires very skilled operators.

Can thru-hole and SMT components be mixed?

One of the strengths of selective soldering is its ability to reliably solder mixed technology boards with SMT and thru-hole components assembled. Both component types can even reside in a single solder joint. Soldering parameters can be optimized to suit all package styles present.

What training is required to operate selective equipment?

Selective soldering systems utilize automated programming of recipes which is quite user-friendly. However basic training on mechanics, changeover, optimization, troubleshooting and maintenance is still required. IPC certification courses are available covering all aspects of selective soldering.

Can selective soldering reduce manufacturing costs?

For complex, advanced PCB assemblies that cannot be reliably wave soldered, implementing selective soldering can decrease overall costs through improved first pass yields and reduced rework expenses. In simpler cases it may increase production costs over wave soldering but provides substantial quality benefits.