Sorry to dump such a long list on you all at the end of the week, but I'm
kinda desperate for some answers.
I know some of these questions have been asked before, so I apologise if I
am revisiting old ground here.
Over the past few months we have had a series of boards made which have a
variety of BGAs and fine pitch devices on them as well as press fit
backplane connectors. The boards are 1.6mm FR4 with an immersion gold
finish to get the required flatness for the BGAs and fine pitch components,
the connectors are from ERNI.
We have just received a batch of boards from the assembly shop which have
cracked when the connectors were inserted. This is really bad news as these
babies cost in excess of 4000 dollars each. This has prompted an inquiry
which has raised more questions than I can answer confidently.There are a
lot of questions that I need both the fab and assembly houses who carried
out  the work to answer in due course. In the meantime I'm hoping you guys
can dump a little of your vast knowledge on me - sorry for the long list.
Here goes...
1. The spec sheet for the connectors call for a drill of 0.007in with a
finished size of 0.006in Does the use of gold mean a thinner finish and
perhaps the fab shop are drilling smaller to accommodate this and
subsequently cause the pin of the connector to try and cut through the
board material rather than swage through the metal ?

2.As the gold finish is harder than tin lead would this cause insertion
forces to increase to the extent that the board could crack?

3.This is one for the assemblers out there. Do you measure the insertion
force when fitting press fit devices?

4.Not wanting to forget the chemists. The material the fab house is using
has an Er of 3.8 does this mean more epoxy less glass and possibly more
prone to cracking.

5.What finish are other people specifying for this kind of assembly?

6.Finally,I have also heard of possible reliability issues when press fit
connectors are combined with gold finish. What they, if any, and how do I
deal with them.

Sorry once again for the long list, but it's kinda worrying, sitting here
looking at a potential 20000 dollar pile of scrap.. oh happy days

Cheers
        Gene


Gene Holmes
HCD Ltd
www.hcduk.com

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