It is crucial not to combine lead-based solder with low-temperature bismuth-based solders due to significant differences in melting points, which can lead to catastrophic failures. The resulting joint becomes exceedingly fragile because of pronounced intermetallic growth, leading to breakage even under what could be deemed moderate pressure. For instance, utilizing the incorrect solder mixture along with a firm grip could allow me to detach SMD aluminum polymer capacitors from the board using just my hands.
In my memory, there’s an ancient IBM research paper that delved into the disastrous outcomes of Bi/Pb mixtures for temperature-sensitive components within a mainframe. Unfortunately, I am currently unable to locate that document.
Moreover, according to the following source,
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Caution is paramount when utilizing tin/bismuth alloys. Mixing tin/bismuth with lead-containing alloys poses a risk due to the potential formation of a low melting combination that liquefies around 95°C, risking failure of the solder joint from natural heat exposure during operation.
Tin/bismuth alloys can be safely combined with other lead-free tin-based alloys. In certain scenarios, surface-mount assemblies use SAC305 on one side and tin/bismuth on the other. The low reflow temperature associated with tin/bismuth inhibits intermetallic growth in the SAC305 solder joints, facilitating soldering of thermally sensitive components.
It’s imperative to avoid combining Pb solder with low-temperature bismuth-based solders because this can create a brittle joint due to excessive intermetallic growth. Despite moderate force, the joints are prone to failure. For instance, with a suitable solder mix and grip, I can effortlessly remove SMD aluminum polymer capacitors from the circuit board. I vaguely recall an ancient IBM article that analyzed this Bi/Pb mix for temperature-sensitive components and found it to be a complete failure. Sadly, that article eludes me at the moment.
Hot Air Solder Leveling (HASL) has remained a key choice for PCB surface finishes over the years. In the late 60s, the shift from the traditional 60/40 tin-lead reflow to HASL began. This time-tested, reliable finish is still prevalent across military, aerospace, medical, and various other applications.
Tracing the Development of Hot Air Solder Leveling
The historical reliance on HASL for providing an exceptional solderable finish for printed circuit boards has solidified its position in both domestic and international markets, despite lead presence.
Since the initiation of the EU’s RoHS directive in the mid-2000s, the realization that HASL would continue to be utilized became apparent. The combination of its durability and the trust of clients called for solutions to eliminate lead while maintaining the process.
Considering the ubiquity of PCBs across industries—from industrial machinery to toys—safety became a pressing concern. The recognized health risks associated with lead exposure prompted electronic manufacturers to prioritize removing lead from their products.
Initially, the goal was to eliminate lead entirely from consumer products.
However, as we progressed to today’s standards, it has become evident that achieving a 100% lead-free status across all products may be unattainable. Instead, selecting the most suitable PCB surface finish while ensuring environmental safety has emerged as a pragmatic approach.
What Is the Status of Lead-Free HASL Finish?
The Lead-Free HASL (LFH) variant became a prevalent alternative finish alongside immersion gold during the transition to safer options. So why hasn't it taken the lead as the industry standard? The complexity involved in applying LFH often necessitates manufacturers to outsource this process.
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PCBs featuring HASL and Lead-Free HASL Surface Finish
The chemical composition of LFH has transformed over the years alongside its applications. Both vertical and horizontal methods faced challenges similar to traditional HASL, including pooling, uneven finishes, and a cloudy appearance on PCB surfaces.
Initial tests with LFH resulted in subpar appearances and performance issues during assembly due to the amalgamation of tin, silver, and copper alloys. However, adjustments—like removing silver, altering tin-copper ratios, and refining the manufacturing approach—have led to improved smoother surface coatings. This advancement has escalated demand, bringing more LFH processes in-house and reducing delivery timelines for customers.
Challenges in Processing Pb-Free HASL
Applying LFH requires higher temperatures. The initial pass yields a rough, dull surface, but a second pass enhances the finish to a shiny, level, and smooth coat. Unfortunately, the heat exposure from dual dips in the molten bath results in copper residue on hole walls, which can compromise copper levels below the acceptable threshold set by IPC standards, necessitating further process modifications and possibly affecting the LFH finish.
After numerous adjustments to both chemistry and process, lead-free HASL has evolved into a stable surface finish for PCBs. Ongoing improvements have established a standardized approach to implementing lead-free HASL, significantly alleviating the challenges prevalent in PCB manufacturing.
The Decline of LFH in the PCB Sector
With all advancements made, why does LFH still appear to be the least adopted surface finish in today’s market?
ENIG (Electroless Nickel Immersion Gold), OSP (Organic Solderability Preservative), and even immersion tin and silver have gained dominance in the surface finish hierarchy. While Pb-Free HASL has become comparatively viable, it remains a more intricate process than other alternatives. As technology progresses and real estate on surfaces diminishes, it further complicates LFH's position as a go-to surface treatment.
Conclusion
Lead-Free HASL has noticeably enhanced its usability regarding processes, though it still has issues to address to compete effectively against other surface finishes. Nevertheless, this previously overlooked finish is gradually gaining traction among customers, leading manufacturers to reevaluate its position in comparison to immersion silver, OSP, and immersion tin.
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