The DSP project I'm working on uses an Analog Devices ADSP-2181 chip, housed in a 128-lead TQFP (thin plastic quad flat pack) surface-mount package. If you need one of these 16-bit devices, chances are you'll have to accept a Pb-free RoHS-compliant (Restriction of Hazardous Substances) metric version, as the earlier leaded-solder-suffix part numbers are now
unobtainium, unless you can find some new/old stock languishing on a distributor shelf somewhere.
Thankfully, the RoHS folks in Europe have saved me from poisoning. My ADSP-2181s are now Pb-free, and are essentially certified to be devoid of cadmium, mercury, hexavalent chromium, PBBs (polybrominated biphenyls), PBDE (polybrominated diphenyl ethers), antimony compounds, and bromine flame-retardants, too. Of course, these ICs do have quantities of iron, nickel, silver, gold, magnesium, palladium, and beryllium. Tin is also predominant as a finish plating. Knowing these precious metals are on and in my DSP makes me happy.
Soldering my packages manually in my modest lab is relatively easy. I don't worry about whiskering, those electrically conductive, crystalline structures of tin that sometimes grow from surfaces where electroplated tin is used as a final finish. Whiskers can sometimes bridge closely-spaced leads. Such potentially devastating failure modes supposedly crop up in time, and it’s easy to understand that high-volume applications subject to such failures in the field are in for trouble.
Interestingly, it seems nobody really knows why whiskers grow! However, NEMI (The National Electronics Manufacturing Initiative) conducted a Tin Whisker Accelerated Test Project to develop industry-standard test conditions for the evaluation of tin whisker growth.
NEMI researchers discovered that whiskers will grow at room ambient conditions (~25ºC at 50% to 80% relative humidity, but stop growing at temperatures above 90ºC. The team of NEMI metallurgists, material scientists, and chemists also found that so-called bright tin grows more whiskers, and grows them faster, than matte-finish tin. Finally, good old-fashioned lead-tin alloys prevent whisker eruptions.
Obviously, all of this is problematical for high-rel military and aerospace applications. However, help is available.
To my amazement, I discovered at least one company that can help mil-aero OEMs get around RoHS incompatibility difficulties. California-based
Interconnect Systems is actually offering a custom service that can flip the RoHS coin, converting your Pb-free ball-grid array packages to -- are you ready? -- leaded BGAs!
Imagine that. For a small fee, ISI will apply its high-volume ISO-9001-certified process, converting BGAs of any ball pitch and pin-count to leaded high-rel BGA packages. ISI says it has ramped up to do this for high volumes, converting RoHS-compliant devices into good old fashioned leaded solder-ball types.
The process is called re-balling. The technique removes those nasty non-lead balls, attaching leaded Sn63Pb37 eutectic spheres instead.
The company says its re-balling conversion ensures that the BGAs conform to all requisite dimensional and solderability specs, and backs its claim with stringent optical inspection and testing. In QC, ISI sets strict control parameters set for every phase of handling and production. The company has established controls for temperature, humidity, ESD protection, ball size and position, tolerances, cleanliness, and inspection, with military MSL (Moisture Sensitivity Level) requirements strictly observed. Controlled solder profiles are also established for zone-controlled re-flow soldering ovens.
Of course, no such exacting process would be complete without re-marking the packages, so ISI will do that per customer specifications for a fee. When everything is said and done, you get your updated BGAs in JEDEC trays or as tape-and-reel put-ups per EIA-481 (replete with nitrogen-purged dry-pack bags).
Now, there are companies that specialize in obsolete parts, such as germanium transistors, but re-engineering that which is already re-engineered is perversely charming in this era of devout sensitivity to pollution. What do you think?
