At a gues, they probably like gold because it soaks up solder so easily (figuratively speaking that is...it doesn't tarnish, so provides a soldering surface with consistenly uniform properties) and so makes the initial soldering quicker than raw copper, ie, their minds are still working in the 1990s... As for embrittlement, I wonder if the effects aren't more pronounced because of the finer geometries used in dense situations like BGAs...
2c, aj >-----Original Message----- >From: [EMAIL PROTECTED] >[mailto:[EMAIL PROTECTED] On Behalf Of [EMAIL PROTECTED] >Sent: Monday, January 08, 2007 9:07 PM >To: [email protected] >Subject: Re: [PEDA] Making BGA PCB parts in Protel 98SE > >I've done both manual "dogboning" of via's to the BGA pad as >well as just let it do its thing... > >With BGA's, the denser and more rows of balls on the BGA the >more layers typically required... > >I really don't suggest solder mask defined pads... Altera and >Intel both have written tech sheets about this. Immersion gold >surfacing seem more prone to embrittlement issues even when >the electroless process is done correctly...and most CM's seem >to like imersion gold for fine BGA's. > >And with metal defined pads, make sure your solder mask is >tight enough to avoid exposing traces that go between >balls/pads on the top layer. Had many issues with fabs' near >their capability misregistering the mask and exposing copper >traces under the BGA - dead shorts during reflow were common. > > >In a message dated 1/8/2007 7:52:57 PM Eastern Standard Time, >[EMAIL PROTECTED] writes: > > >> >> --- Dennis Saputelli <[EMAIL PROTECTED]> wrote: >> >> > >> > i assume you meant '99SE' ? >> > >> Yes I did mean that as matter of fact. >> >> > the autorouter for it's time (which was not new even at that time) >> > actually wasn't too bad, but it certainly was not made to do BGAs >> > it did do a decent job at lined up rows of random wired DIPS :) >> > >> Well I have wired lots of dips with it, but it had quite a >few anoying >> bugs in it still. In any case I'm looking at surface mount devices >> instead. So how does one do a BGA since it's only contact >technically >> is the top or bottom layer. Does one employ vias near the pads or >> something? >> >> > i wouldn't get my hopes up about autorouting that thing >> > >> A 144 pin LFBGA is too much for it? >> >> > to do a solder mask opening smaller than the pad use a negative >> > expansion >> > >> That is definately helpful. >> >> > my brain is wired in mils so... >> > for a 40 mil pad with a 30 mil opening use >> > solder mask expansion of -5 mils >> > >> I'm so use to converting between the two I almost do it in my head >> these days. So that would be the override designation for the >> soldermask? (is this defined on the part or in the Design >Rules... 99SE >> is a bit of a maze it looks like someone kept adding features to >> something that started small). >> >> > what is driving the 'solder mask defined' ? >> > i have read both pros and cons about that but everyone i know does >> > not do it >> > >> You mean solder mask over pad instead of mask about pad? It was >> mentioned in the package JTAG information so I asked. >> >> > .125mm is 4.9 mils >> > 5 mil trace/space is almost ordinary these days >> > >> > for BGAs depending on the pitch we sometimes need to go to >4mil trace >> > >> > 4mil space AKA 4/4 >> > >> Right FBGA is 0.8 mm which is 31.4 mils. So it's not a big deal to >> have such small traces anymore then? >> >> Stephen >> >> Stephen R. Phillips was here >> Please be advised what was said may be absolutely wrong, and >hereby this >> disclaimer follows. I reserve the right to be wrong and >admit it in front of >> the entire world. >> >> __________________________________________________ >> Do You Yahoo!? >> Tired of spam? Yahoo! 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