Hi Danny,
I guess it depends on the silver finish used and the peripheral chemicals
used with it. However in general, unless you need the extra solder save your
money.
As to paste, you may find that a 63/37 alloy paste will exhibit a rougher
surface finish due to silver 'contamination' from the finish knocking the alloy
off eutectic and creating differential freezing. That will depend on the trace
elements in alloy and will be real hard to determine from paste supplier to
supplier regardless of any commitments to purity, quality etc.... there are
certain constraints on all manufacturers of alloy, solder powder and hence
paste. It will also depend on the quantity of silver put down on the boards,
again like a lot of processes, it's difficult to maintain really controlled
deposition and really easy to screw it up, at least enough that you'd notice a
difference from one batch or supplier to another.
If you use 62/36/2 alloy paste then the effect of the silver
'contaminant' won't be noticed.
With regard to the paste and more importantly the flux base used, you'll
notice differences with different products from the different suppliers.
Some will solder with no issue others may leave an unsoldered area the
ubiquitous copper ring around the pad periphery.
If you need to put down more solder use a thicker stencil or open up the
apertures for certain or all pads to a 1:1 ratio.
There may be a protective organic layer (something
akin to benzotriazole is a favourite) over the silver finish and that should
give at least 6 months possibly even longer protection. I think Brian Ellis
summed it up extremely well in his last email regarding silver finish tarnish on
a capacitor (was it a capacitor?).
With a sulphur based pollutants around almost everywhere the tarnish I've come
across tends to be of silver sulphide so store your boards in good atmosphere.
Also watch out against storing boards in paper containing chlorides (nearly all
paper is loaded with chloride from the bleaching process). Years ago I worked on the mysteriously exploding
electrolytic capacitors, the ones that were allowed to 'dry off' on the blue
paper towelling before assembly and sealing. The paper with all the
chlorides.
Hope it helps,
Andrew Hoggan
BBA Associates Ltd
Hello
Everyone,
I'm needing information
on PCB's with silver finish. We are using HASL but having problems with our
proto fab shop. They have a very hard time keeping the HASL somewhat smooth on
the fine pitch parts. They sent us a board with silver finish and we ran it
using our same process. Appeared to make a pretty good solder connection, just
not as much solder on the pads now without the extra solder from the HASL. Are
there special requirements on which paste I should use.. Will I have to use a
thicker stencil... What is the shelf life before it tarnishes and can you
solder if it is tarnished.
The
information will be appreciated....
Thanks,
Danny
CSI