Eric: I am also interested in a response to Eric's second to last question regarding the rate of tin rich IMC converting to copper rich IMC. To Dave and George: in addition to Eric's question, do you have info you can share regarding the effect (mechanism) of lead in inhibiting the rate and/or thickness of tin/copper IMC growth? George, your point on lead atomic radii inhibiting its migration caught my attention. Are there other factors aside from size encumbrance that inhibit migration (ie. - mechanism whereby nickel prevents/inhibits gold/copper migration)? Thanks for the great discussion, Ted Stern Eric Dawson wrote: > Hi George, > One pertinent point here is that the intermetallic layers are compounds not > alloys. Thus their oxidising and other chemical reactions will probably be > different from those of their constituent metals. > The surface activation energies may be the key, can you give me references > please George? I would like to follow them up. (See my other reply!) > One point though, I would expect the copper rich compound to lie adjacent to > the copper interface and the copper deficient one next to the solder > interface. My microsections after solder etching showed that the normally > visible two layers were still present, so I must have been soldering the > copper deficient layer, which, as you say, is the easiest one to solder. > Also, since the melting points of these compounds are higher than soldering > temperatures we would always be soldering to the copper deficient layer. Or > are we? So, open question, can the copper deficient, aka easily solderable, > compound turn into a copper rich layer. Has anybody reading this discussion > got access to a well equipped laboratory? > Regards > Eric Dawson > > -----Original Message----- > > From: Franck, George [SMTP:[log in to unmask]] > > Sent: Tuesday, November 13, 2001 7:20 PM > > To: [log in to unmask] > > Subject: Re: [TN] Intermetallic Layers > > > > During my PWB education I learned that there are at least three "alloys" > > of > > copper and tin that are formed as the Copper diffuses into the solder. > > The > > first alloy formed (and I don't remember the ratios) has the highest tin > > to > > copper ratio, and has surface activation energies within the range of most > > fluxes. Therefore, this first tin copper alloy, or IMC, is solderable. > > All > > the others alloys, with higher amounts of copper in them, are not. > > > > Activation energies reflect how strongly attached the Oxygen atom are on > > the > > surface of the metal. The more copper on the surface alloy, the tighter > > the > > oxygen is attached, the stronger the flux needed to break the oxygen from > > the surface. > > > > While the copper is diffusing into the Solder, the Tin is also diffusing > > into the copper surface, lowering the % tin in the solder alloy, which > > raises the melt temperature of the remaining solder. It is all quite an > > interesting solid state diffusion system. By the way, for all intents and > > purposes, the lead is just too big (physical size of the atom) to > > participate in this diffusion system much, except to get in the way. > > > > It is possible that Eric only exposed the "solderable" tin-copper > > intermetallic in his tests, i.e. the one with the least amount of copper > > in > > it. > > > > What was the magic chemical I played with in my old PWB shop to make > > boards > > solderable, Thiourea hydrochloride? Wasn't that the active chemical in > > Tarnex also? > > > > George Franck, CID+ > > who recalls his "PWB as Black Magic" roots. > > > > -------------------- > > Disclaimers: > > 1) If Dave Hillman says I am wrong, then I probably am. > > 2) My company will neither confirm nor deny the existence of any of my > > opinions. > > 3) Tin-copper alloys are commonly referred to as Bronze. > > > > -----Original Message----- > > From: David Hillman [mailto:[log in to unmask]] > > Sent: Tuesday, November 13, 2001 12:25 PM > > To: [log in to unmask] > > Subject: Re: [TN] Intermetallic Layers > > > > > > Hi Eric! Sorry but I humbly disagree. There is an abundance of published > > industry data which shows that oxidized Sn/Cu intermetallic is one of the > > more unsolderable surfaces encountered on an assembly. Problems with poor > > plated thru hole fill due to "weak knees" (thin solder causes the Sn/Cu > > intermetallic at the hole rim to become oxidized) and thin HASL coatings > > are just two examples of everyday industry solderability issues associated > > with Sn/Cu IMCs. What type of flux did you use in your testing? Very > > aggressive flux chemistries will allow the soldering of Sn/Cu IMCs but a > > fair majority of the industry is moving away from these types of flux > > formulations. With all that being said, without the Sn/Cu IMC we would > > have > > no solder joints so it is a critical part of a solder joint. > > > > Dave Hillman > > Rockwell Collins > > [log in to unmask] > > > > > > > > > > Eric Dawson <[log in to unmask]>@ipc.org> on 11/12/2001 07:49:56 AM > > > > Please respond to "TechNet E-Mail Forum." <[log in to unmask]> > > > > Sent by: TechNet <[log in to unmask]> > > > > > > To: [log in to unmask] > > cc: > > > > Subject: [TN] Intermetallic Layers > > > > > > Hi All, > > > > I notice that a few new references have been made to the above subject and > > thought I would share some of my early work with you. > > > > In my first post as a graduate, I researched the causes of dewetting on > > circuit boards. I came across a solution, the make-up of which I have > > since > > lost, which dissolved the tin lead alloy but left the two intermetallic > > layers. > > > > I produced a number of one inch square coupons which I soldered using > > 63/37 > > alloy, RMA flux and a Tri Moore Rotary Dip Tester. > > > > These were divided into three categories: > > > > 1 Control > > 2 Remove tin lead and re test in the Rotary Dip. > > 3 Remove tin lead, Several hours steam ageing and re test. > > > > Microsections showed that I had taken off the alloy successfully and left > > the IMC.s. > > > > All of the re tested coupons soldered successfully, including the steam > > aged > > ones. > > > > This was some years ago so I cannot remember the etchant nor the hours and > > conditions in the steam ageing equipment. I do remember that the IMC > > soldered very well so I am always dubious when somebody blames the > > intermetallic for poor soldering. > > > > I seem to be alone in my faith that the IMC solders but this was the > > result > > of my very own work and I stand by it. > > > > Regards > > Eric Dawson, Defender of the IMC. > > > > -------------------------------------------------------------------------- > > -- > > ----- > > > > Technet Mail List provided as a free service by IPC using LISTSERV 1.8d > > To unsubscribe, send a message to [log in to unmask] with following text in > > the BODY (NOT the subject field): SIGNOFF Technet > > To temporarily halt delivery of Technet send the following message: SET > > Technet NOMAIL > > Search previous postings at: www.ipc.org > On-Line Resources & Databases > > > E-mail Archives > > Please visit IPC web site (http://www.ipc.org/html/forum.htm) for > > additional > > information, or contact Keach Sasamori at [log in to unmask] or 847-509-9700 > > ext.5315 > > -------------------------------------------------------------------------- > > -- > > ----- > > > > -------------------------------------------------------------------------- > > -- > > ----- > > Technet Mail List provided as a free service by IPC using LISTSERV 1.8d > > To unsubscribe, send a message to [log in to unmask] with following text in > > the BODY (NOT the subject field): SIGNOFF Technet > > To temporarily halt delivery of Technet send the following message: SET > > Technet NOMAIL > > Search previous postings at: www.ipc.org > On-Line Resources & Databases > > > E-mail Archives > > Please visit IPC web site (http://www.ipc.org/html/forum.htm) for > > additional > > information, or contact Keach Sasamori at [log in to unmask] or 847-509-9700 > > ext.5315 > > -------------------------------------------------------------------------- > > -- > > ----- > > > > -------------------------------------------------------------------------- > > ------- > > Technet Mail List provided as a free service by IPC using LISTSERV 1.8d > > To unsubscribe, send a message to [log in to unmask] with following text in > > the BODY (NOT the subject field): SIGNOFF Technet > > To temporarily halt delivery of Technet send the following message: SET > > Technet NOMAIL > > Search previous postings at: www.ipc.org > On-Line Resources & Databases > > > E-mail Archives > > Please visit IPC web site (http://www.ipc.org/html/forum.htm) for > > additional > > information, or contact Keach Sasamori at [log in to unmask] or 847-509-9700 > > ext.5315 > > -------------------------------------------------------------------------- > > ------- > > --------------------------------------------------------------------------------- > Technet Mail List provided as a free service by IPC using LISTSERV 1.8d > To unsubscribe, send a message to [log in to unmask] with following text in > the BODY (NOT the subject field): SIGNOFF Technet > To temporarily halt delivery of Technet send the following message: SET Technet NOMAIL > Search previous postings at: www.ipc.org > On-Line Resources & Databases > E-mail Archives > Please visit IPC web site (http://www.ipc.org/html/forum.htm) for additional > information, or contact Keach Sasamori at [log in to unmask] or 847-509-9700 ext.5315 > --------------------------------------------------------------------------------- --------------------------------------------------------------------------------- Technet Mail List provided as a free service by IPC using LISTSERV 1.8d To unsubscribe, send a message to [log in to unmask] with following text in the BODY (NOT the subject field): SIGNOFF Technet To temporarily halt delivery of Technet send the following message: SET Technet NOMAIL Search previous postings at: www.ipc.org > On-Line Resources & Databases > E-mail Archives Please visit IPC web site (http://www.ipc.org/html/forum.htm) for additional information, or contact Keach Sasamori at [log in to unmask] or 847-509-9700 ext.5315 ---------------------------------------------------------------------------------