Hello Michael,

We produce a lot of press fit backplanes with both Reflow and HASL finish. If 
the sectioning shows irregular solder build up, this does not sound like reflow 
which tends to be more regular than HASL. There is always a tendency for the 
solder to thicken at the centre of the hole due to surface tension and it is 
important to control the thickness at plating. 0.0004" (10 microns)is usually 
ideal. We generally find that reflow is more consistent for maintaining hole 
size as HASL can leave more solder in some holes than others. Reflow is also the 
cheapest finish and provides the best protection for the traces if the solder 
mask should get damaged in the field.

On your numbered points:-
1) I believe Ni/Au could be a problem because solder acts as a lubricant and gas 
tight seal for press fit and electroless nickel is brittle and could break-up; I 
would like to hear from any experts in the field about this aspect.
2) Impedance on inner layer striplines will not be affected by surface finish 
but there could be a small effect on surface microstrips between Reflow and 
SMOBC. Has anyone made any measurements of this?
3) There is nothing wrong with your spec and it may be that the hole size 
problem has arisen from using an inappropriate drill size, or poor plating 
distribution. Copper plate thickness is more commonly the cause coupled with odd 
finished hole sizes and tolerances for which appropriate drill sizes are not 
available. 
4) Hole size tolerance would be standard at +/- 0.002" and should be no problem.

Final thoughts, if you are checking hole sizes with pin gauges you cannot use  a 
pin gauge which is at the bottom limit. I would suggest 0.5 thou" below bottom 
limit as a go/no-go.
Are you sure your press-fit pins have not changed or are out of tolerance? We 
found this happened on more than one occassion.

Paul Gould
Teknacron Circuits,UK
E-mail [log in to unmask]

In your message dated Thursday 21, November 1996 you wrote :
> Our principle product uses a 14"x17", .125 thick, 8 layer backplane with 48
> press fit connectors. Its currently specified as SM over reflow solder. Our
> PCB supplier has performed wonderfully in the past, but we are now receiving
> some boards with out of spec finished holes (spec .033 +.002/-.003). Cross
> sectioning shows the problem to be extra, irregular solder buildup in the
> holes. They are suggesting we should spec SMOBC to help the problem. Another
> new vendor we are talking with is recommending a Ni/Au process. My feeling
> is the original vendor produced very satisfactory product for 3 yrs, and
> should be able to continue to do so. However, I'd like to take this
> opportunity to do some investigation. I have a few questions:
> 
> 1. Is anyone out there using Ni/Au on boards with press-fit connectors?
> 2. Would anyone venture an opinion on impedance differences that would exist
> between the three processes? 
> 3. Does our current spec seem unreasonable or difficult to manufacture? 
> 4. What about if the hole was specified as .033 +/- .002?
> 
> Thanks!
> Michael Branning
> Avtec, Inc.
> 4335 Augusta Hwy
> Gilbert, SC 29054
> (803) 892-2181
> (803) 892-3715 FAX
> [log in to unmask]
> 

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