Lead-free: Using Vapor Phase Reflow in Lead-free Processing

Estimated reading time: 6 minutes

During lead-free processing, vapor phase reflow offers the option to eliminate the variable of higher temperature processing.

By Steve Fraser and Chris Munroe

uccessfully implementing lean manufacturing practices in an electronics manufacturing service (EMS) environment requires process optimization, as well as adjustments to material flow planning and stocking practices. When developing a Synchronous Flow Manufacturing process, one company* paid attention to developing strategies for reflowing and wave soldering that allowed process flexibility in terms of product mix. Another benefit of this was the ability to perform lead-free soldering at conventional reflow temperatures.

The focus on equipment optimization included working with equipment suppliers to develop unique systems for rapid changeover. Vapor phase equipment was selected because of its ability to support throughput and upcoming lead-free requirements. Some advantages over convection reflow systems include:

• The oven uses an inert Teflon-based liquid that when heated to the boiling point, creates a vapor blanket, through which PCBs are passed. Vapor condensation on the PCB creates heat transfer.
• The vapor condensation atmosphere has better thermal transfer properties than IR or convection systems. This allows for even heat transfer on PCBs with large thermal masses.
• The vapor blanket provides an oxygen-free environment during soldering, improving solderability.
• Improved thermal properties ease solder profiling that can be established based on PCB technology and size.

The ability to solder lead-free alloys at conventional temperatures is a significant benefit in the EMS environment because many OEMs are in varying phases of lead and hazardous substance elimination, depending on product types and customer bases. While the main driver of eliminating lead and other hazardous substances is the Restriction on the Use of Hazardous Substances (RoHS) Directive, Japanese OEMs also have implemented schedules for eliminating lead from manufacturing processes and components as part of internal initiatives adopting more environmentally-friendly manufacturing practices. The result is that most EMS providers are facing a range of customer requests for process capability vs. a uniform strategy.

Lead-free processing in IR or convection ovens involve processing PCBAs at higher temperatures. Disadvantages include the risk of thermal stress and issues associated with damage to heat-sensitive components at higher peak temperatures. Vapor phase offers the option to eliminate the variable of higher temperature processing.

This company* has been producing SMT assemblies with lead-free solder since 2004, and can support the use of tin/silver and tin/silver/copper alloys.

The vapor phase lead-free solder process operates at a maximum temperature of 230°C, whereas many convection or IR reflow processes range from 250°C to over 300°C in some zones of the reflow process, depending on the type of PCBAs being processed. Vapor phase is compatible with typical PCB finishes, including HASL, bare copper with OSP, copper/nickel/gold, immersion tin and immersion silver.

Laboratory Tests

In 2003, a process qualification study was initiated with a test laboratory*2 to qualify and validate the assembly process using Surface Insulation Resistance (SIR) electrical performance and Ion Chromatography testing on a test board.*3 The study used IPC Class 3 level performance per IPC ANSI J-STD 001C. The analysis was conducted using ionic cleanliness evaluation by Ion Chromatography IPC-TM-650, method 2.3.28 and SIR characterization IPC-TM-650, method 2.6.3.3A.

Five board processes were measured:

Process 1 - Current Process

• PCBs screen printed by hand using one*4 solder paste
• Hand-placed SMT components from kits*3
• Reflow solder using a convection reflow oven*5
• Hand-placed thru-hole components
• Wave solder using a wave solder machine*6
• A VOC-free no-clean flux*7
• Finished PCBAs were placed into an ESD bag and marked “Current Process”

Process 2 - New Process A

• PCBs screen printed by hand using one solder paste*4
• Hand-placed SMT components from kits*3
• Reflow solder using a reflow oven*8
• One fluid*9
• Hand-placed thru-hole components
• Wave solder using a wave solder machine*6
• A VOC-free no-clean flux*7
• Finished PCBAs were placed into an ESD bag and marked “New Process A”

Process 3 - New Process: B

• PCBs screen printed by hand using one*4 solder paste
• Hand-placed SMT components from kits*3
• Reflow solder using a vapor phase reflow oven*8
• One fluid*9
• Finished PCBAs were placed into an ESD bag and marked “New Process B”

Process 4 - Unprocessed Controls

• HASLed bare-board surface finish with a soldermask*10

Process 5 - Production PCBA

• PCBAs processed on a production SMT line
• PCBs screen printed*11 using one*4 solder paste
• Assembly equipment*12
• Reflow using a vapor phase reflow oven*8
• Finished PCBAs placed into an ESD bag and marked “Production PCBA”

Cleanliness Data Summary

All processed assemblies in the evaluation passed cleanliness testing criteria with low levels of chloride, bromide and weak organic acid (WOA) processing contaminants. The cleanliness criteria of J-STD-001C, Appendix B, on all patterns, showed low levels of contaminants.

SIR Data Summary

All process assemblies in the evaluation passed the SIR testing with high levels of surface resistivity. The SIR criteria of J-STD-001C, Appendix B, on all patterns, showed levels above the limits of 100 MΩ of resistance. All processed assemblies for current water-soluble fluxes and new production water-soluble fluxes passed SIR and visual criteria of the J-STD-001C, Appendix B test conditions. A diagram of the SIR test environment used in this evaluation can be seen in Figure 1.

Figure 1. SIR test environment evaluations

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Project Test Vehicle

A process evaluation substrate, which has a four-layer MLB with a total of 31 test sites for SIR testing; IPC B-24 test patterns (no mask, striped mask, full mask); a row of 0805 chips; a row of 1206 chips; BGA, QFP and LCC test patterns with and without solder masks; and a row of DIPS for thru-hole processes were developed.

IC Analysis - Residue Summary

Ion chromatography detected these primary anion residues: chloride (Cl-), bromide (Br-) and WOAs (typical for PWBs and assemblies). The following substances also were tested for (and found below detectable levels) nitrates (NO3-), phosphates (PO42-), methane sulfonic acid (MSA), sulfates (SO42-), WOAs and conductive organic-element residues.

Bromide

Bare boards and assemblies all showed low levels of bromide that could be fully attributed to fire-retardant materials in the laminate and mask.

Chloride

Bare-board and processed assemblies all showed low levels of chloride, and pose minimal risk of electromigration problems.

Weak Organic Acids

All samples showed similar levels of WOA, below recommended limits and posed minimal risk of electromigration.

Conclusion

1. Ionic cleanliness levels for current and new processes showed similarly low levels of chloride, bromide and WOA. Current production boards showed similarly low levels of chloride and WOA, indicating the process change created little difference.
2. The electrical performance for the SIR showed good high resistance readings for all samples - from current to new processes. Vapor phase reflow showed good electrical performance when compared to the entire group of SIR test samples.
3. All boards*3 using current production and new process conditions with the vapor phase reflow passed IPC SIR criteria, meeting ANSI J-STD 001, Appendix B process qualifications.

While vapor phase soldering eliminates the process variables associated with higher temperature soldering processes, it does not eliminate other challenges associated with removing lead and other hazardous substances, including the availability of a full range of components with lead-free terminations, continually changing requirements related to RoHS and ambiguous customer requirements. The solution continues to focus on customer needs on a case-by-case basis. Information on controlled reflow profiles, reduced temperature exposure, solder joint reliability and field application results are being monitored continuously to validate data in support of customer process changeover.

* EPIC Technologies *2 Foresite, Inc., Kokomo, Ind. *3 Umpire boards, Foresite, Inc. *4 Alpha UP78 solder paste *5 Atmos *6 EPM CIG 400 wave solder machine *7 Alpha NR310B no-clean flux *8 IBL vapor phase reflow oven *9 Galden LS-215 VF fluid *10 LPI soldermask *11 DEK 265 GSX screen printer *12 Siemens S20 and F4 assembly equipment

Steve Fraser, general manager, EPIC Technologies; and Chris Munroe, director of engineering, EPIC Technologies, may be contacted via e-mail: todd.baggett@epictech.com.