Gene:  You left out something:

"We have just received a batch of boards from the assembly shop which have
cracked ___________when the connectors were inserted."  Fill in the space...

Describe in detail,  what you are seeing../happening

Does the board have Ni/gold for the finish ontop of the complete board
including the press fit holes?

Bob Z

> -----Original Message-----
> From: Gene Holmes [SMTP:[log in to unmask]]
> Sent: Thursday, September 02, 1999 6:35 PM
> To:   [log in to unmask]
> Subject:      [TN] Cracking PCB's
>
> 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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