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Lead-free: Soldering at the End of the Line
December 31, 1969 |Estimated reading time: 7 minutes
The end of the line is where rework and repair technicians ply their trade. To prevent this hallowed area from becoming the “bitter end” during the lead-free transition, we should consider proactive tactics to prepare technicians to phase into the lead-free zone.
By Charley Dennehy
Unlike soldering machines, which can be bought and programmed to handle increased thermal loads of lead-free solders, technicians make their living with their hands and eyes - requiring instruction and assistance to acclimate to lead-free soldering. Make no mistake, hand soldering with lead-free is different than soldering with leaded, eutectic solders (Figure 1).
Figure 1. Soldering comparison on a large OSP surface mount pad. The left shows SnPb eutectic; the right shows SAC 305 lead-free. Both alloys were hand soldered with an iron set at 316°C.
It may be heartening to know that (in a round-about way) the lead-free transition has been underway for some time. Many engineers and technicians have been exposed to a range of board finishes that are used with both SnPb (tin/lead) and lead-free processes. Immersion tin, OSP (Organic Solderability Preservative), ENIG (Electroless Nickel/Immersion Gold) and immersion silver are familiar to many. Components with tin-coated (not tin/lead) legs have been surreptitiously slipping into processes over the past several years, causing few rework hiccups (except BGA components, which are another story). It’s the addition of higher-temperature lead-free joining material that trips the latch into a new world of rework soldering.
It’s impossible to predict which lead-free solder a reader may use (and there are plenty of choices). As a baseline material, we’ll refer to the SAC variety. While SAC alloy combinations are abundant, most people have settled on a few closely related formulas that perform in roughly the same fashion when considering their impact on the rework-and-repair technician. For the sake of discussion, we’ll focus on SAC 305 (96.5% tin, 3% silver, 0.5% copper).
This solder flows at 217°C, or 34°C hotter than standard eutectic tin/lead solders. What does that mean to a rework technician? It depends on the application. The good news is that for medium-sized components placed on locations that don’t carry away a great deal of heat, or present difficult access due to super-fine pitch, adjustments in tooling, flux and iron temperatures are minimal. Once the new conditions are explained, most technicians (when asked to react to the difference in comparative soldering) shrug their shoulders, indicating the difference isn’t appreciable. On the other hand, increase the area of reflow, make the pitch microscopic, add heavy ground planes or rework BGAs, and that 34°C-temperature rise can loom like an insurmountable hurdle when using techniques formerly successful with tin/lead solder. Area reflow (selective soldering), BGA rework and thick thru-hole point-to-point work, all require a cautious approach. Not only should technicians concern themselves with the flow characteristics of lead-free solder, but also how increased temperatures of pre-heat and reflow may deleteriously effect surrounding components, solder mask or board material. This is especially true in this transition period between tin/lead and lead-free when printed circuits most likely will be assembled with components having a wide range of heat sensitivity.
Less-than-routine Rework
One of the main challenges of hand soldering is delivering adequate heat at the point of attack; right where the tip, solder and component lead connect. This has become more difficult as components have become smaller and boards denser. The increased thermal demands of SAC solders may require tips with higher temperatures and more instances where auxiliary heat (additional irons, convection heat, pre-heat or hot plates) will be required to provide the thermal power necessary for timely reflow. One also may find that lighter duty, no-clean fluxes are not up to some challenging applications. If the flux is decomposing routinely before joints are reflowed, it may be time to call your supplier for advice. They may propose alternative materials.
There has been a great deal of press on differences between the flow characteristics and cosmetic appearance of tin/lead and lead-free solders. After sifting through all the verbiage and pictures of lead-free soldering, nothing compares to having technicians handle the material themselves. The touch and feel is what counts for them, as well as for the people who inspect the work. After all, what does “grainier” appearance look like on your boards? You’ll find, depending on the application and the solder, instances where lead-free appearance is indistinguishable from tin/lead. In other cases, it will be remarkably less spread, feathered and smooth. It takes some time to develop a common understanding of what is acceptable in your company. Getting samples of projected materials, and permitting technicians and inspectors to test and discuss new materials with engineers, are inexpensive and timesaving introductions. Once technicians are comfortable with the basics, introduce them to fine pitch, providing an opportunity to work through the hand-eye coordination and tooling adjustments. Getting a proper heel fillet on fine-pitch gull-wing components is a developed skill with any solder.
Anyone practicing area reflow (selective soldering of large components with mini-waves or solder pots) occasionally is brought to the limits of their ability by the existing thermal challenges of tin/lead soldering. Components and connectors can be ungainly, and boards are thick and dense. Pre-heating and auxiliary heating required for successful reflow compels operators to wear protective gear to prevent from being scorched by the heat. Not only will solder pots require upgrades to resist corrosive affects of lead-free materials, but the nature of this rework requires rethinking to prevent board damage and technician injury. Technicians should become familiar with the tapes and temporary masks available to protect unaffected board material in the vicinity of rework from discoloring, warping or burning.
One of the most uncertain elements of the lead-free transition has been the unannounced arrival of BGA components with lead-free balls; lead-free BGA components provided for placement on boards originally fabricated and assembled for, and with, tin/lead solder. Often, these components arrive without any indication that the balls are lead-free, and it’s discovered that they are not SnPb when they don’t flow at eutectic tin/lead reflow temperatures.
This is vexing, but here to stay. There are some quick tests to determine if there is lead in a component; however, establishing the percentage requires more elaborate analysis. In any event, once the metallurgy of incoming parts is understood and we feel we must mix a lead-free component with a tin/lead board, there are several paths that can be taken. You can reball the component with SnPb balls, place lead-free balls using tin/lead paste and place the lead-free balls with lead-free paste or flux only. There’s a lot left to understand regarding this issue of “backward compatibility” in lead-free BGA components. Questions regarding the long-term impact of mixing various chemistries have yet to be answered. It’s hard to say which way you should go without knowing all the details of the particular case. My inclination, for the time being, is not to mix metals, and we generally reball the components, but that is by no means a law. For lead-free BGA rework, some feel it is best to use nitrogen in this process - providing lead-free BGA with the most suitable microenvironment for complete and consistent reflow.
As for repair (fixing broken pads, etches, boards) much of the above applies, as the same operations (soldering/de-soldering) also are the fundaments of repair. On the other hand, there have been a few trends that developed as to the type of repair showing up in facilities. With more ENIG boards being constructed, gold plating, which had been diminishing as a requested repair, has regained popularity. Solderability issues from new coatings can cause opens at BGA sites, requiring reconditioning at the site. Companies shifting to lead-free assembly also are having some difficulty avoiding the marring of mask and burning/delamination of base laminate during rework. Technicians obviously are adjusting to higher temperatures. Furthermore, we see more problems with solder lifting off pads during the re-hardening of reflowed lead-free solder - a phenomena requiring further examination.
Conclusion
In the end, it’s understandable to feel victimized by the change to lead-free solders and the fact that metallurgy choices may not be yours to make. The change most likely will come suddenly, inconveniently and piecemeal, making the management of separate materials a headache. There is abundant lead-free information online and quality support available to educate technicians and engineers for the change. When it comes to rework and repair, there are economical and simple steps that can be taken to permit a smooth phase-in to the lead-free zone. Go forth boldly and prosper.
Charley Dennehy, president, Circuit Technology Center, Inc., may be contacted at (978) 374-5000, ext. 12; e-mail: cdennehy@circuitnet.com.