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Talk about paranoid. Our intrepid columnist tackles some real-life inanities.

"Tell me lies
Tell me sweet little lies" – Fleetwood Mac

We're one month past July 1. RoHS is in effect and the sky has not fallen - how about that? Since the EU has decided to continue this endeavor in spite of dissonance and other expressions (and expletives) of derision, some (but not all) of us are realizing that we ought to do something in the direction of RoHS compliance.

Of course, times of crisis - real or perceived - abound with rumors, allegations, false information, ad nauseum. This is "industry generated" hearsay, stuff that people take seriously. Let's look at some of these examples from the field - you cannot make this stuff up.

"The same printer cannot print leaded and Pb-free."

Surprisingly, this did not come from a printer salesperson but rather from high-rel OEMs (particularly Class 3 medical). For many assemblers the workload does not (yet) justify a dedicated Pb-free line. Can you use the same printer for leaded and unleaded? Let's think this through. First, you will be using different stencils (likely with different aperture guidelines) from one job to another. Solder paste should be thoroughly cleaned between jobs anyway. Maybe dedicate a squeegee for each and monitor materials in process. Put a placard on the machine: red when it is running SnPb and green when it is printing Pb-free. No, you do not need a dedicated printer for Pb-free.

"Will I need a new reflow oven?"

There is concern about lead particles floating in the recirculated air (or nitrogen, if you imbibe) and said particulates being deposited on a Pb-free assembly, thus rendering it noncompliant. Talk about paranoid. I suspect there might indeed be lead particles floating in there, but the amount would be so minute (let alone that which might end up on an assembly), it would likely be impossible to detect, let alone measure.

The real concern is the oven's ability to handle higher temperature excursions for ongoing periods of time as well as maintaining adequate throughput. Jim Hall (also of ITM Consulting) estimates that approximately 20% more heating capacity is required to attain a given cycle rate from SnPb to SAC. So those using a four-zone oven for SnPb should seriously consider a five-zone. If using a five-zone, to maintain the conveyor speed (because the output from pick-and-place mandates it), consider a seven-zone. To test your oven, take your most complex assembly (in terms of mass, surface geometry and component complexity) and profile it for a SAC alloy (full liquidus/minimum reflow temperature of 230°C). If the oven manufacturer is okay with running the new setpoints on a continuous duty cycle and you can live with the throughput, you are in the clear. Do not neglect cooling. Hall also estimates that for most SAC profiles up to 44% more cooling capacity will be required to achieve the same oven exit temperature as SnPb. Check with the oven manufacturer about retrofitting some extra cooling on that bad boy.

"I need to run a nitrogen atmosphere for reflow of Pb-free alloys."

This one most likely came from the nitrogen companies. You can process Pb-free in an ambient air environment. However, Soldering Technology International (solderingtech.com) demonstrated better wetting resulting in slightly better tensile strength with Pb-free joints reflowed in nitrogen. The folks at Auburn University also conclusively showed, by comparing the ratio of the diameter of printed paste to reflowed solder, better spread (wetting) on a variety of surface finishes when the SAC alloy was reflowed in nitrogen. But the results obtained in air were still more than adequate for the majority of applications.

"Vapor phase is the only way to reflow Pb-free."

Since most assemblers use convection dominant ovens, this statement is clearly wrong. To its credit, vapor phase is superior to the process of the early '80s. When it comes to reflow, particularly complex and heavy assemblies (backplanes, for example), vapor phase is easiest. Pop it into the vapor and hold it there until the assembly reaches the full liquidus/vapor temperature. No tedious profiling and virtually no gradient. But it is not inexpensive, and one saying goes, if vapor phase is the solution, what's the problem? But for complex or heavy assemblies, it is definitely the way to go.

"SAC alloys will be less reliable than SnPb."

Dr. Jean-Paul Clech of EPSI, one of my favorite solder joint reliability experts, collected every study he could find in the private and public domain concerning the reliability of SnPb and Pb-free solder joints. He found that the reliability of solder joints comprised of the respective alloys is application-specific.

So, as you plod your way through the controversial field of Pb-free, get the facts. In God we trust, all others bring data. Remember, we're all in this together.

Phil Zarrow is president and SMT process consultant with ITM Consulting (itmconsulting.org); itmconsulting@aol.com. He still bears the scars, physical and mental, of reflowing convection/IR ovens.

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