Tag Archives: OSP

Revelations at ACI

Posted on by

Folks,

I’m taking a few moments from Wassail Weekend, held annually in my village, Woodstock, VT (“The prettiest small town in America”), to write a post about the recent workshops at ACI.

Indium colleague Ed Briggs and I gave a three-hour presentation on “Lead-Free Assembly for High Yields and Reliability.” I think Ed’s analyses of “graping” and the “head-in-pillow” defect are the best around.

There was quite a bit of discussion on the challenges faced by solder paste flux in the new world of lead-free solder paste and miniaturized components (i.e., very small solder paste deposits.) One of the hottest topics was nitrogen and lead-free SMT assembly. There seemed to be uniform agreement that solder paste users should be able to demand that their lead-free solder paste perform well with any PWB pad finish (e.g., OSP, immersion silver, electroless nickel-gold, etc.) without the use of nitrogen. Not only does using nitrogen cost money, but it will usually make tombstoning worse. However, in the opinion of most people, nitrogen is a must for wave soldering and, since it minimizes dross development, it likely pays for itself.

After Ed and I finished, Fred Dimock, of BTU, gave one of the best talks I have ever experienced on reflow soldering. He discussed thermal profiling in detail, including the importance of assuring that thermocouples are not oxidized (when oxidized they lose accuracy). He also discussed a reflow oven design that minimizes temperature overshoot during heating, and undershoot when the heater is off. Understanding these topics is critical with the tight temperature control that many lead-free assemblers face.

Fred Verdi of ACI finished the meeting with an excellent presentation on “Pb-free Electronics for Aerospace and Defense.” Fred’s talk discussed the work that went into the “Manhattan Project.” A free download of the entire project report is available.

There appears to be agreement that acceptable lead-free reliability has been established for consumer products with lifetimes of five years or so, but not for military/aerospace electronics where lifetimes can be up to 40 years and under harsh service conditions. These vast product lifetime and consequences of failure differences are depicted in Fred’s chart (see the pdf link). Commercial products are in quadrant A and military/aerospace products in quadrant D.

One of the greatest risks faced by quadrant D products is tin whiskers. Fred spent quite a bit of time discussing this interesting phenomenon. One of the challenges of this risk is that there is no way to accelerate it, so you can’t do an equivalent test to accelerated thermal cycling or drop shock. Fred mentioned that there have now been verified tin whisker fails, the Toyota accelerator mechanism being one.

In addition to tin whiskers, lead-free reliability for quadrant D products (with a service life of up to 40 years) in thermal cycle and other areas remains a concern.  I mention that tin pest was not on the list of issues for this quadrant.

Fred and the Manhattan Project Team have identified many “gaps” that need to be addressed to determine and mitigate the risk of lead-free assembly for quadrant D products.  They plan to start this approximately $100 million program in 2013.

For those that missed this free workshop, another is planned in about six months.

Cheers,

Dr. Ron

Which Surface is Best?

Posted on by

RoHS has been in effect since, when, 2006? Pretty close to five years now. It’s been around long enough that there’s even talk of follow-on legislation. All of the PCB fabricators have pretty much figured out how to deal with RoHS. There are Pb-free versions of every PCB finish at this point. But, we still get questions about the best choice of PCB finish.

I don’t think industry has selected one PCB finish as the “standard” preferred choice. A lot depends on the application and the componentry being used. For large parts, HASL, leaded or lead-free, is a good choice. It’s inexpensive and works well. For leaded work, HASL still seems to be the most common finish. We don’t see quite so much Pb -free HASL, though.

If you’re working with small geometry parts, then you really need to go to immersion silver or ENIG. The consistently flat surface of those finishes will help keep the small parts on the pads where they belong. The disadvantage of silver is that it requires a little more care in handling and storage. It can oxidize which will make soldering more difficult. ENIG is more expensive, but it tends to work real well and is easier to store. Fingerprints can be a problem though. We’ve seen the oils from a fingerprint essentially etch the gold surface off. Weird.

OSP becomes a viable choice with high-volume, cost critical applications. It used to be very sensitive to storage and handling, but has gotten a lot better over the last few years. We don’t see immersion tin much at all. It apparently is harsher on the environment to produce than other finishes.

All that makes it more understandable that we don’t have one preferred finish. It seems confusing, but really it’s not that different than any other product. There certainly isn’t just one preferred style of tire for all motor vehicles.

Duane Benson
Gotta have those monster truck tires if you live in Kelso

http://blog.screamingcircuits.com/