Hi Earl,
I appreciate your response to the stiffener problem, and agree that the
CIC is a bitch in more ways than one. It is a problem in trying to plate to
especially the Invar, since every time you go into an acid bath that contains
copper it wants to immersion plate copper (which we call smut); this usually
has very poor adhesion and creates a problem with hole and interconnect
integrity. The other problem is that of differential CTE, which creates large
interfacial stress at the bond line, so you must have a good adhesive. That's
why I suggested the Hi-temp epoxy, as Polyimide is a very poor adhesive.
I must disagree with you on the CTE's of aluminum and copper
(depending on your definition of relative), as the aluminum has a CTE of about
24 PPM /deg. C. and Copper is about 18 PPM / deg. C. This makes the copper a
better CTE match with the board, as well as being a better thermal conductor. I
would expect that, depending on the degree of constraint necessary, that the
copper would be a better choice than CIC or Aluminum. Another note worthy of
mention, If you constrain the X-Y plane with CIC the Z axis expansion increases
dramatically. I hope SteveM picks up on this also.
Hey! it's still good to see your responses. Keep 'em Flying Earl,
Regards
Les
> From: [log in to unmask], on 1/5/99 2:05 PM:
> What a way to start a new year! Les said it right as it should be
> interesting how everyone responds
> to this loaded question.
>
> I love CIC, but it's a bitch and or bastard (depending on political
> correctness) to work with. I
> still love it, especially as a tailoring core as Les indicated, but the 10
> mil stuff is my
> favorite.
>
> No one really suggested matching your stiffner on both sides. I mean,
> everyone agrees a balanced
> construction is required. Of course, you probably don't have room on the
> top/bottom side for
> another stiffner.
>
> About Tg: Most all relatively conventional materials (again as everyone
> knows) has about the same Z
> axis expansion rate at between 50-60 ppm/degree C. no matter Tg. You're
> currently using polyimide
> so you can expect the same expansion - but it will go to a much higher
> temperature (250-270 Tg)
> depending on the press cycle and cure, before it splodes - determined using
> DSC or TMA.
>
> You could place the stiffner in the middle of the MLB no matter the material
> selected. Aluminum and
> copper expand at relatively the same rate in the X-Y so make your choice as
> copper because you can
> plate to it (easily) or through it and it can serve as a thermal transfer
> mechanism.
>
> As always, and as everyone else, I have some pretty/ugly pictures you are
> welcome to view without
> fear of disrupting family and friendly relationships. Also, constraint often
> is easier accomplished
> than compliance during the fabrication process.
>
> Earl Moon
>
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