Pb-free alloys exacerbate copper erosion and blowholes.

Ed.: This is a continuation from last month’s column.
 
Last month, we looked at the general fabrication issues to anticipate regardless of alloy. Some specific concerns we need to watch for in Pb-free processing include:

Copper erosion. This can be a minimal risk or a huge concern, depending on the PWB. We’ve still got a lot to learn about it. Essentially, the electrodeposited copper on the PWB dissolves more quickly in Pb-free alloy than in SnPb, and soldering cycles with long dwell times can remove enough copper to affect signal integrity or interconnect reliability. In a recent test of 10 fabricators, the erosion rates of various electrodeposited coppers varied by a factor of almost 2X. As an industry, we don’t yet understand why one plating bath can produce a copper with more or less erodibility than another bath, but we have documented the differences, and the investigations continue.

Blowholes. As I’ve mentioned, this seems to be one of the phenomena that just gets bigger and badder in the Pb-free world. Pb-free boards seem to blow out more frequently than do SnPb ones; the resulting PTH voids are bigger than in SnPb, and defects that used to look like pinholes in the otherwise normal SnPb solder fillets now resemble balloons – inflated radially, with a very thin shell of solder that can be broken just by poking it with a fingernail or small hand tool. Adding insult to injury, simple bake-outs do not seem to fully resolve the problems. We’ve documented several cases where a PWB produces blowholes on a Pb-free wave, but none on a SnPb wave, so copper erosion may play a part in blowhole formation as well. Again, we’ve got a lot to learn, and investigations continue.

It would be unfair to describe a number of potential problems without offering some practical preventions or remedies; I would expect to get my share of hate mail from the fabrication community if I fell short of offering some positive advice. I’ve learned a few tricks over the years to either prevent or work around fabrication issues on the assembly line, and I’ll be sharing them next month.

In the same vein, it would be unprofessional for me to make blanket generalizations that bash all board fabricators for lousy quality. Not all circuit cards carry issues that impact assembly yields. I’ve experienced an enormous performance spectrum in the market – especially during the transition – and some topnotch shops make great boards. It’s no wonder why they are often a little pricier than their competitors.

This month’s lesson learned is almost too cliché to even print, but it’s a truism we experience in many facets of our lives: You get what you pay for. In its quest to continually lower costs, our industry has attempted to commoditize many elements of the assembly process, judging them by acquisition cost alone rather than total cost of ownership. In many cases, we’ve forgotten saving a few pennies on the front end can cost us big dollars in rework at the backend. Sure, passive components or memory devices can be viewed easily as commodities, and changing suppliers typically has a negligible impact on an assembly operation. But not all constituents of the assembly process are interchangeable, and my experience with PWBs indicates they are one of the elements that should not be viewed as a commodity.

I am certainly no expert on the PWB fabrication process. Rather, I’m one of those people who knows just enough about it to be dangerous (and occasionally annoying to those who possess real expertise). But from the 30,000-ft. perspective, it does not substantially differ from assembly in the sense that successful quality requires an intricate balance of materials, equipment and process controls. That delicate balance is rooted in solid science, but much of it is achieved only through years of experience and sustained operations.

It’s time to take PWBs off the commodity list, begin appreciating good fabricators, and be willing to pay for the quality we demand. The alternative: We will never get what we want, only what we deserve. And we’ll continue to pay too much for our PWBs through the hidden costs of rework, repair and returns.

Chrys Shea is an R&D applications engineering manager at Cookson Electronics (cooksonelectronics.com); chrysshea@cooksonelectronics.com. Her column appears monthly.

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