Hi Blue Smurf - G. Franck Jr.'s answer cover's 99% of the basics (a wonderfully complete and detailed answer too!). As he said, copper on the surface is BAD. Both the Cu3Sn and Cu6Sn5 intermetallic oxide species require very active fluxes to permit soldering. Test data has shown that the intermetallics will oxidize within ten minutes of reexposure to the atmosphere if they start out oxide free. With that in mind, any amount of either Cu3Sn or Cu6Sn5 intermetallic is going to cause problems. A secondary issue is if the intermetallic gets too thick it can be a source for crack initiation leading to cracked solder joints. How do you not get copper/tin intermetallics? - keep you solder finish thick enough that the copper can not diffuse to the surface. I have seen some data that suggests 60 microinches minimum. For more intermetallic info take a look at figure 6-10, page 178, "The Mechanics of Solder Alloy Wetting and Spreading", ISBN 0-442-01752-9 (the whole book is pretty good).One additional chunk of data - the tin preferentially oxides over the lead in a tin lead alloy thus making soldering even tougher for surfaces that are old (SnO2 is the main culprit- I have some SERA curves that show that point). Dave Hillman Rockwell Collins [log in to unmask] ____________________________ Reply Separator _________________________________ Subject: Re: ASSY/FAB: SnPb Oxidation Author: [log in to unmask] at ccmgw1 Date: 11/18/96 2:50 PM I still have some questions about this response: 1. What if the Cu3Sn is too thick? 2. How thick is too thick? (2 microns??) 3. How can you get rid of it? To my knowledge, the thickness of Intermetallic Compound (IMC) formation is poportional to the square root of time. (Considering constant temperature) Blue Smurf ---------- From: TechNet-request To: John_Gulley; technet Subject: Re: ASSY/FAB: SnPb Oxidation Date: Sunday, November 17, 1996 2:12PM John, A quick response to oxidation/solderability The Lead oxide is not a problem, the tin oxides are the ones we are dealing with. Now, simple tin oxide is not a problem for most fluxes, if the oxide is not TOO thick. But you probably do not have simple tin oxide. While the solder is sitting on the surface of the copper, copper atoms will migrate/diffuse from the copper layer into the solder, and tin will migrate the other way into the copper. The lead, being physically much larger, just kinda gets into the way. What happens when copper starts showing up on/near the surface? Glad you asked. It oxidizes, just like the tin. HOWEVER, the tin and copper start making interesting alloys. And it is the oxide of the alloy which needs to be stripped. Without getting into metallurgy (which I cant even spell correctly) and all the different alloys, just remember that the higher the copper content at the surface, the harder it is to remove the oxide. OK John, on your boards, enough copper has made its way to the surface to create a copper/tin alloy which you can not solder to. What happens if you remove the top layer of oxide and metal? Answer: You expose a layer of tin/copper/lead which is slightly higher in copper than the one on the surface. In other words, a layer slightly harder to solder to. So, to your question, how do I remove Oxide from solder? Well, you can use Thiourea Hydrochloride, often found in Fab shop solder brightners, (and I think Wal-mart sells it as Tarn-ex, next to Cascade in the PWB section) but what have you accomplished? You will very quickly form another even harder to solder to oxide coating. Summary: Copper is a major contributor to solderability problems. Copper is at its lowest concentration on the surface of the solder. Therefore, the solder which is easiest to solder is on the surface of the solder. If the surface solder is not solderable, none of the solder is solderable. Then your only hope it to a) re-HASL or b) scrap and reorder. There is a relationship which goes something like this: PWB solderability shelf life (Time) =3D solder thickness (L) / Copper diffusion rate (L/Time) (The diffusion rate in a function of Temperature, and alloy.) There are other factors in the equation, but this is first order. In my experience, this is the major problem of HASL boards that are 9+ months old. There are other causes for solderability problems. May none of them be as bad or prevalent as this one. George Franck Jr Raytheon E-Systems Product Assurance Engineer =46alls Church Va *************************************************************************** * TechNet mail list is provided as a service by IPC using SmartList v3.05 * *************************************************************************** * To unsubscribe from this list at any time, send a message to: * * [log in to unmask] with <subject: unsubscribe> and no text. * *************************************************************************** *************************************************************************** * TechNet mail list is provided as a service by IPC using SmartList v3.05 * *************************************************************************** * To unsubscribe from this list at any time, send a message to: * * [log in to unmask] with <subject: unsubscribe> and no text. * *************************************************************************** *************************************************************************** * TechNet mail list is provided as a service by IPC using SmartList v3.05 * *************************************************************************** * To unsubscribe from this list at any time, send a message to: * * [log in to unmask] with <subject: unsubscribe> and no text. * ***************************************************************************