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
www.bba-associates.com

  -----Original Message-----
  From: TechNet [mailto:[log in to unmask]]On Behalf Of Danny W. Harkins
  Sent: 26 July 2001 20:49
  To: [log in to unmask]
  Subject: [TN] Silver finish



  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