|Ask the Experts|
December 18, 2006
OSP Board rework and cleaning
What is the correct procedure for OSP board rework for boards populated with SMD and through hole components? Is there any specific temperature change required? Also, what is the best cleaning solvent for the OSP boards after rework?
|Expert Panel Responses|
You have a number of reasonable choices for your OSP rework. But before mentioning specific names, let me suggest a series of "filters" or qualifications that I would use when selecting a cleaner. The most important factors when selecting a cleaner, in order of importance, would be these: Effectiveness... Find a cleaner that can remove your contamination quickly, easily, without rinsing; then... Safety... Your best choices will be nonflammable azeotropes with higher TLV ratings; then... Handling... Find cleaners with little or no aroma and that dry fast, and then... Environmental Requirements... The issues of ozone-depletion, low-altitude smog, and global warming are rapidly eliminating many previously popular choices. Find cleaners that meet tomorrow's regulations today, or you simply will be going through this dance again in a couple of years... Lastly... Affordability... Find the least expensive choice that makes it through the other four filters. If you rate products on a scale of 0-5 across 5 factors, you could have a "desireability index" that scales from 0 to 25; the higher the number the better the choice. Let's see how the choices rate: =-=-=- HFC Solvents -=-=-=-=- In terms of general precision defluxing after rework and repair, nothing can beat HFC-43-10 solvent mixed with transdichloroethane ("trans") and HFC-365. One example is Micro Care's Flux Remover C; versions also are available from Chemtronics. It is functionally equivalent to the old "Freon(r) TMS" or Genesolv(r) 2004. Flux Remover C dries fast, offers powerful cleaning on most fluxes, it's nonflammable, it has a very low aroma, and is easy to use and to store. While it is most often used in the aerosol form, many customers like to use Flux Remover C in a vapor degreaser for quick cleaning. It's a great choice. It's ratings would be: Effective.....5 Safety........5 Handling......4 Environmental.4 Affordability.3 For a total of 21 points, which is very good. =-=-=- Siloxane Solvents =-=-=- A completely different type of cleaner comes from Dow-Corning with their siloxane-based cleaners. Marketed by Micro Care and Dow-Corning, these cleaners are an almost perfect choice. They are moderately powerful, yet plastic-safe. They are very good in terms of toxicity but they are flammable. They dry fast with almost no aroma. They are ozone-safe, low global warming and VOC exempt. And, they are highly affordable. So the siloxanes would be rated like this: Effective.....4 (slightly milder) Safety........3 (flammable) Handling......5 Environmental.5 Affordability.5 For a total of 23 points, which is the best rating of any of the choices. =-=-=- Hydrocarbon Solvents =-=-=- An older answer is Axarel(r) 2200. This is a hydrocarbon solvent, which means (in unsophisticated terms which will make all the Micro Care chemical engineers cringe) that it is derived from petroleum. This formula delivers moderately powerful cleaning and is generally plastic-safe, but it is slow drying and has some aroma. It also is combustible (but not flammable). It represents a reasonable combination of characteristics that make it one of Micro Care's best selling choices. A number of different hydrocarbon cleaners are available from Micro Care, Chemtronics, Petroferm, Kyzen, Zestron and other companies. Typical ratings might be: Effective.....4 Safety........3 (combustible) Handling......2 (because of slow drying) Environmental.3 (100% VOC) Affordability.4 For a total of 16 points, which is very reasonable. =-=-=- HCFC-225 =-=-=- Another choice, this one from Japan, is HCFC-225 from Asahi. Marketed in the US by TechSpray and others, HCFC-225 (in our opinion) is not a great choice except in California. It's moderately powerful but not as aggressive on plastics as HCFC-141b. It is nonflammable. Perhaps most importantly, it is not a Volatile Organic Compound (VOC). But HCFC-225 is Class II Ozone-Depleting Substance under the U.S. Clean Air Act and must be phased out at the end of 2010. It's ratings would be: Effective.....5 Safety........4 Handling......4 Environmental.2 (Ozone) Affordability.3 For a total of 18 points. =-=-=- HFE Solvents =-=-=- The last big choice is any cleaner containing HFE solvents and trans. These are a very interesting choice. Marketed by 3M Corp. under the Novec(r) brand name, the HFE solvents are accepted by many customers. They are nonflammable, low aroma, relatively non-toxic materials which can be used safely and effectively on the benchtop. The HFE cleaners have a low global warming index and have reduced VOC content. But in general the HFE solvents are much more expensive than the HFC cleaners. It's ratings would be: Effective.....4 Safety........5 Handling......4 Environmental.5 Affordability.1 For a total of 19 points. So while the HFEs are good, the HFC solvents from DuPont and the siloxane cleaners from Dow-Corning are clearly the best choices on the rework bench.
Boards or assemblies with OSP coatings are treated the same as any other solderable coating applied to printed circuit boards. If the product was soldered with 63/37 or 60/40 type of alloys then this is the alloy to be used to removed and re-solder those components in place. The procedure to be used is dependent upon the type of component being removed and replaced and IPC 7711/7721 discusses many type of techniques and tools to be used, whether they are either hot gas systems, conduction type tools like soldering irons or localized solder flow systems. The flux to be used should have been selected based upon some investigation and experimentation to verify its compatibility with the product, be it either a Low Solid content flux, (i.e. no clean flux) or an activated type of flux. If the flux selected is a flux which needs to be cleaned, then a cleaning process must be in place to remove the residual flux residues from the surfaces of the product. As to whether or not temperature changes are required, this is also based upon the type of solder being used to fabricate the original product. The question does not mention Lead-Free solder, so I'm going with the belief that the solder used was a eutectic material, such as 63/37, Sn/Pb and the rework temperatures should be the same as they are for the assembly process. Additionally although the finished plated through hole solder joints has more mass then the individual lead and the barrel of the initial joint, extra heat is not necessarily needed, but the time on the joint may be extended a couple of extra seconds. This should not impact the product in any detrimental fashion.
Vice President, Technical Director
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