Is the ball are Sn plated and reflowed? If it is, you have surface migration via residual plating solution... oxidation/corrosion product on surface due to improper rinsing.... (haven't seen that for a long time...). jk (wild goose....) On Jan 18, 2008, at 8:36 AM, Bev Christian wrote: > Sigh. My replies below, again in BOLD. > > -----Original Message----- > From: TechNet [mailto:[log in to unmask]] On Behalf Of Wenger, George M. > Sent: Friday, January 18, 2008 7:25 AM > To: [log in to unmask] > Subject: Re: [TN] SV: [TN] SV: [TN] Solderability of Tin-Gold > Intermetallic > > I usually like to respond with answers not questions but un > Bev, > > I usually like to respond with answers not questions but until I > understand an issue it is hard to respond with answers. Your question > is still vague to me. Although you said that you are talking about > trying to make joints and not talking about joints already made, you > also said that what you are exploring is whether unusual amounts of > gold > tin intermetallics on the surface of the solder balls pried off of > boards that show head in pillow did not solder because of these > intermetallics. I view Head-in-a-Pillow solder joints as joints that > have not wet for some reason. > WELL I GUESS THIS IS A CASE OF SEMANTICS. I DON'T CONSIDER THAT A > JOINT > WAS EVER MADE. If I HAD BEEN TALKING ABOUT A FRACTURE BECAUSE OF HIGH > AMOUNTS OF GOLD in A SOLDER JOINT, ok, BUT THERE WAS NO METALLURIGICAL > BOND EVER MADE IN THE CASES I AM DISCUSSING. > > If that is the case then I assume the > gold-tin intermetallics you are talking about are from the > materials in > the BGA and not from the surface finish on your board. I'm having > difficulty believing that there is enough gold in under-bump > metallization to cause a gold-tin intermetallic issues. > TRUTHFULLY SO AM I, BUT WE ARE EXPLORING ALL AVENUES. > > You indicated > that you used XRF to measure the gold concentration on samples that > you > pried off. Ingemar anticipated that you meant the part of the ball > that > was soldered to the board pads. > NO. I MEANT THE BALL OF THE PART. > > However, from the way you worded your > inquiry I can't tell if you meant you looked at the board pad or > the BGA > ball. > THE LATTER. > > I assumed, maybe incorrectly, that you looked at the surface of > the ball after peeling the BGA off. > CORRECT, AFTER PEELING OFF THE BGA. WE MEASURED 1.28 +/- 0.15% [Au] > > If that is the case then I would > assume that you should be able to take a non-soldered BGA and measure > the gold in a BGA ball > CORRECT AND GOT 0.86 +/- 0.8% [Au] > > and then put the BGA though a reflow process > without any solder paste and re-measure the ball after reflow to > see if > the gold concentration changed. > ON THE LIST OF THINGS TO DO. > > I'm assuming that you're talking about > small size uBGA's with ball diameters in the 10 to 13 mil range. > THERE YOU AMERICANS GO AGAIN. I KEEP TELLING YOU THE METRIC SYSTEM IS > NOT A COMMIE PLOT. :) THE BALL DIAMETERS ARE 300 MICRONS. > > Since > the XRF penetration depth is typically between 1 and 2 mils you'll be > analyzing the gold in the top surface of the ball. I assume that > if you > don't see a high gold concentration in the un-reflowed solder ball > that > the gold-tin intermetallics you see after the reflow process > migrated to > the surface of the ball during the cooling process. If that is the > case > they probably aren't the reason for the Head-in-a-Pillow soldering > defects. > THAT'S WHAT ONE WOULD THINK. HOWEVER IF THE DIFFERENT BGA NEXT DOOR > SOLDERS PERFECTLY, YOU LOOK AT ALTERANTIVES. > > MAN, THE E-MAILS ARE COMING FASTER THAN I CAN ANSWER... AND I DO > HAVE A > JOB - OTHER THAN TECHNETTING. :) > > Regards, > George > George M. Wenger > CommScope / Andrew Wireless Solutions > Senior Principal FMA / Reliability Engineer > 40 Technology Drive, Warren, NJ 07059 > (908) 546-4531 [Office] (732) 309-8964 [Cell] > -----Original Message----- > From: TechNet [mailto:[log in to unmask]] On Behalf Of Hernefjord Ingemar > Sent: Friday, January 18, 2008 4:45 AM > To: [log in to unmask] > Subject: Re: [TN] SV: [TN] SV: [TN] Solderability of Tin-Gold > Intermetallic > > Finally, you disclosed what you really wanted to know. You pried off > BGAs and found unusual amount of AuSn platetlets on the surface of the > balls. This is my opinion: If the wetting on AuSn formations is > good or > not is not the first thing you ought to think of. When you say the > 'surface', I anticipate that you mean the part of the ball THAT WAS > SOLDERED TO THE BOARD PADS. If THAT surface has lots of AuSn > formations, > then I understand how the BGAs could be pried off = probably too much > gold from the ENIG pads = something wrong in the soldering process. IF > the solder joints were normal, you would have a real problem to get > the > BGAs off the board, because healthy solder joints are enormously > strong. > > As to your question, whether the solder wets to AuSn intermetallics, I > think all is depending on what atmosphere you had during the IMC > formation. AuSns are, as you know, not chemical bonds, but alloys with > its gold phases and its tin phases. Now, if the tin phases are not > oxidized, then you ought to get good wetting, and vice versa. In > practice, I think that there is a high risk of AuSn oxidation, that's > why you do AuSn soldering of RF details in vacuum or inert atmosphere. > > Inge > > -----Original Message----- > From: TechNet [mailto:[log in to unmask]] On Behalf Of Bev Christian > Sent: den 17 januari 2008 16:42 > To: [log in to unmask] > Subject: Re: [TN] SV: [TN] SV: [TN] Solderability of Tin-Gold > Intermetallic > > > Bingo! There is the crux of it. I am not talking about joints already > made, I am talking about TRYING to MAKE joints. > > I hope I don't get in trouble for this. > > My original, vague question was because of - head in pillow. Inge > guessed correctly. > > I don't mean to be rude, but we already have a very extensive fishbone > for this, so please don't come back with the usual reasons. They are > being addressed, trust me. > > What I am exploring is whether unusual amounts of gold tin > intermetallics on the surface of the solder balls pried off of boards > that show head in pillow did not solder because of these materials. > > Bev > RIM > > -----Original Message----- > From: TechNet [mailto:[log in to unmask]] On Behalf Of Werner > Engelmaier /* > Sent: Thursday, January 17, 2008 10:36 AM > To: [log in to unmask] > Subject: Re: [TN] SV: [TN] SV: [TN] Solderability of Tin-Gold > Intermetallic > > Hi All, > We know that Cu6Sn5 is non-solderable after oxidation. However, I > do not > know whether we really KNOW that it is solderable prior to that, > and we > know even less about the solderability of AuSn4, Ag3Sn, or for that > matter Sn3Ni4. > > > > Werner > > > > ************** > Start the year off right. 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