nicely explained, thank you
Dennis Saputelli
Harry Selfridge wrote:
>
> Nothing evil about gold for soldered connections - as long as the thickness
> is kept within IPC-2221 guidelines of less than 0.8um (about
> 31microinches). Where you get into trouble is with thicker layers of
> gold. Tin-lead solder normally used in electronics is not fully compatible
> with gold plating. Gold readily alloys with tin-lead, but when the gold
> concentration exceeds a few percent the alloy becomes weak during thermal
> cycling. This process is referred to in the literature as gold
> embrittlement. As long as the gold plating thickness is small (20-30 �in)
> this is not considered a problem. In thick gold layers, the only bonds that
> are formed when soldering are the brittle high percentage gold
> alloys. When you have a brittle alloy over a soft all-gold underlying
> layer, naturally something is going to give under stress.
>
> When the gold layer is kept thin enough, the soldering process penetrates
> to the nickel and forms a more stable and durable transition layer with the
> nickel. The gold is distributed and diluted (alloyed) throughout the
> entire cross section of the joint and the percentage of gold in the alloy
> at the joint is relatively low.
>
> The nickel is there for two reasons when you gold plate a board: first as a
> barrier metal to prevent gold-copper transition alloys that are weak
> alloys, and second as a hard underlay for the soft gold. Normally you are
> looking for at least 100 microinches of nickel under gold for mechanical
> connections, and that works just fine as the barrier/transition for
> soldered joints as well.
>
> Short story - make sure the barrier nickel is thick enough, and the gold is
> thin enough. All will be well with soldered joints.
>
> At 11:34 AM 4/23/02 +1000, you wrote:
> >Greetings all,
> >
> > I have been following the recent thread dealing with gold plating
> > (WANTED: PCB Expert) where the subject of embrittlement was raised. The
> > original focus of this thread - PCB warping - is not a problem for us.
> > The comment made by Brad Verlander got me thinking and a little worried:
> > (In the extreme cases I have been told of boards having significant
> > numbers of their components drop off the boards when the board is
> > manually flexed.)
> >
> > This has raised some issues with a couple of the boards we make here
> > at Zener. The story goes like this...
> >
> >We have a number of PCBs designed to accept terminations made via spring
> >loaded contacts. We have had reliability problems with straight tinned
> >copper for these terminations, mainly due to surface irregularities of the
> >solder film. A decision was made to try gold plating to alleviate the
> >problems. This has proved successful but the PCB manufacturer has gold
> >plated the entire PCB not selectively in the areas of concern. After
> >reading the above mentioned thread enquiries were made regarding the
> >nature of the gold plating, the reply being Au (2 microinches) over Ni
> >(100 microinches).
> >
> > Is this good, bad or indifferent? Do these figures indicate possible
> > embrittlement problems in the future?
> >
> > Can anyone point me towards further reading on this topic - books,
> > articles, web sites etc.?
> >
> > Thanks in advance for any input.
> >
> >
> >Best Regards,
> >
> >LINDEN DOYLE
> >Product Development Engineer
> >Zener Electric Pty Ltd
> >
> >[EMAIL PROTECTED]
> >Ph: +61 2 9795 3600
> >Fax: +61 2 9795 3611
> snip
--
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