Jim, The nickel dissolves comparatively slowly, so it is left behind. The inter-metallic formed is Ni3Sn4. > -----Original Message----- > From: Jim Kittel [SMTP:[log in to unmask]] > Sent: Wednesday, May 03, 2000 12:32 PM > To: [log in to unmask] > Subject: Re: [TN] Electroless/imersion Ni/Au Vs electrolitic Ni/Au > > Dennis, > > One more question please. When the assembler applies tin/lead paste to > the > electroless nickel/immersion gold surface does the gold and nickel both > dissolve into the solder during reflow or does the nickel remain behind as > a > barrier over the copper; and then what kind of inter-metallic would be > formed with the tin/lead? > > Jim Kittel > L-3 Communications > > -----Original Message----- > From: <Dennis Fritz> [SMTP:[log in to unmask]] > Sent: Wednesday, May 03, 2000 12:13 PM > To: [log in to unmask] > Subject: Re: [TN] Electroless/imersion Ni/Au Vs > electrolitic > Ni/Au > > Dear CL Chong: > > You have some of the answers already about electroless nickel > gold, > and you > have some misinformation: > > Immersion gold plating occurs when a small amount of the > electroless nickel > is dissolved into the gold solution, and gold is deposited in > place > of the > nickel (oxidation - reduction reaction). 4-10 microinches are > normal deposit > thicknesses, but not much more can be deposited since nickel stops > dissolving > through the gold layer. > > Yes, electroless nickel is required as a barrier for the gold (as > in > any > copper/nickel/gold construction since copper and gold migrate > rapidly into > each other). Also, electroless nickel is a convenient souce of > nickel ions > to reduce the gold in solution. Electroless plating coats all the > copper > traces - top and sides, without continuous conductor attachement > as > in > electroplate. Gold is most commonly electroplated when the > original > copper > foil gives conductive continuity, or when the circuits are > "bussed" > together > as in connector tabs. > > I am very surprised that 20 microinches of gold gave you > embrittlement as > gold is thought to be compatible in solder to 3 or 4%. That means > that you > were putting down less than 600 or 800 microinches of solder paste > - > true? > Anyway, the thinner immersion gold plate will give you less gold > in > your > solder joints. > > Ohter points: > Most LPI type solder masks are now compatible with electroless > nickel/immersion gold processing. That is feature most mask > suppliers have > been working on. > > Yes, phosphorus is co-deposited with the electroless nickel. But, > running > the process by the seller's instructions makes the phosphorus > deposit > consistant with bath age, bath loading, etc. Most currnent > thought > about > solderability of electroless nickel/immersion gold does not focus > on > the > phosphorus content of the deposit, but on the corrosion rate of > caused by the > immersion gold bath. Run properly by the manufacturers > instructions, there > should be a uniform corrosion of the nickel/phosphorus, and a > uniform deposit > of the immersion gold. > > 4 microinches of immersion gold will have some porosity and lower > shelf life. > Deposits nearer the 10 microinch thickness should store and > solder > well for > months. > > Please contact me off line if you want more specific information - > > Dennis Fritz > MacDermid. Inc > > ############################################################## > TechNet Mail List provided as a free service by IPC using LISTSERV > 1.8c > ############################################################## > To subscribe/unsubscribe, send a message to [log in to unmask] with > following text in > the body: > To subscribe: SUBSCRIBE TECHNET <your full name> > To unsubscribe: SIGNOFF TECHNET > ############################################################## > Please visit IPC web site (http://www.ipc.org/html/forum.htm) for > additional > information. > If you need assistance - 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 1.8c > ############################################################## > To subscribe/unsubscribe, send a message to [log in to unmask] with > following text in > the body: > To subscribe: SUBSCRIBE TECHNET <your full name> > To unsubscribe: SIGNOFF TECHNET > ############################################################## > Please visit IPC web site (http://www.ipc.org/html/forum.htm) for > additional > information. > If you need assistance - 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 1.8c ############################################################## To subscribe/unsubscribe, send a message to [log in to unmask] with following text in the body: To subscribe: SUBSCRIBE TECHNET <your full name> To unsubscribe: SIGNOFF TECHNET ############################################################## Please visit IPC web site (http://www.ipc.org/html/forum.htm) for additional information. 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