Well Jan ,
By now you've read it all , likely still confused , with applications
for solder ; finger or switch pads mixed up , not mentioning the
battered calculator crunching (my was, in disbelief) all the microns &
microinches conversions, applications & ancient stds.
Mind you most of the standards simply don't keep up with , with IPC's
being actually most agile (the IEC's are straight joke) .
After I went through all of the reading ; recommendations ; selective
platings ; costs , trials ; cycling , shearing , etc. :
The conclusion was to minimize the problem :
We started with 1.5 um (micron = 1/1000 of mm , comparing to microinch =
1/1000 of " ; on the 25.4 conversion) to cover :
TH solder joints ; SMT solder joints ; split lands for carbon pills on
switching (1M cycles) ; and test fingers (say up to dozen insertions &
pulls) with the ITRI (check: http://www.itri.co.uk/pubs.htm#Soldering
;we found even Winchester nontoxic bullets in = you get shot leadless)
recommendation of 1.5 um .
Worked fine still with dull joints (as explained elsewhere) ; we dropped
down to 0.8 um of Au on Ni ; getting better ; now we cycle in chamber
0.3 (!, yes, it's very pale, ) um Au ; and literally we can not break
the joints in telecom category of IPC-SM -785 reliability test , after
almost 4K of 0-100'C cycles we did not crack a joint (being nervous
about what the hell is happening !) yet .
Than you get as clear joints (no clean) as on tin ; without silver
pastes , elevated and prolonged temps , save on the paste before you get
yourself into balls and , finally, yes the embrittlement is an issue .
Quite an interesting one as for example on identical pad, paste and
component with one on 0.3 um we've got 40% less strength than on 0.8um
with elevated temp (10'C) , while only some 5% less with the same temp .
To put in other words , the increased strength ratio to us indicated
increased hardness and brittleness and decreased plasticity ;
therefore reliability is certainly related in very much measurable way
to amount of plating .
Cheaply & efficiently : set yourself with one paste ; one artwork ; one
batch of components ; one profile ; one stencil (or dispenser program) ,
send the artwork around = order all the assessed suppliers ; than sit
behind your shear tool and the softest one (complying on visuals with
610 or J stds) should give you the best reliability factor on life
cycling .
That is this part of it .
On the question of porosity talk to fabricating fellows ; there are ways
to make even thin plate fine enough to cover the Ni well ;
we have on those 0.3 um validated switch pads on one million cycles
(started this minimalistic approach on telephones) with no change, = no
abrasion or oxide .
The boards are plastic wrapped (sometimes vacuum, sometimes paper
interleaved; sometimes silica) ;they live a year without any degradation
on shaded shelf .
See you
Paul Klasek
http://www.resmed.com
PS
Tones more on this subject in IPC archives
> ----------
> From: Jan Satterfield[SMTP:[log in to unmask]]
> Sent: Wednesday, 9 September 1998 3:03
> To: [log in to unmask]
> Subject: [TN] Soldering to Gold Plated Pads
>
> I have just spent the last 2 hours going through the TechNet archives
> reading correspondance regarding to soldering to gold plated pads. I
> am
> now more confused then when I began. We have just finished reflowing
> and wave soldering our first gold plated pwb (mixed technology). It
> looks terrible. Talk about dull and grainy solder connections! The
> wave soldered connections look worse than the reflowed connections.
>
> The suggestions I've read in the archives include:
> Altering reflow profile (longer/hotter).
> SEM Analysis and microsection. (we are only building 5 CCA's at this
> time).
> "Quit looking at the solder joint appearance".
> A remedy of better preheat, slower conveyor speed and higher wave
> solder
> temperatures.
> Use 2% silver in your paste.
> Use more solder paste.
>
> Are these dull connections acceptable? What constitutes true gold
> embrittlement? Is gold embrittlement a legitimate defect? How are
> other companies dealing with gold plated pads. Do you need to change
> your process?
>
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