---------- Forwarded message ----------
Date: Thu, 29 Feb 1996 16:53:34 +0000
From: Dave Schmitz <[log in to unmask]>
To: Lisa Williams <[log in to unmask]>
Subject: Re: Surface Mount Considerations (fwd)

On Feb 29, 12:33pm, Lisa Williams wrote:
> Subject: Surface Mount Considerations (fwd)
>
>
> ---------- Forwarded message ----------
> Date: Thu, 29 Feb 96 10:16:00 CST
> From: Thorson, Kevin J @EAG <[log in to unmask]>
> To: 'IPC Technical Forum' <[log in to unmask]>
> Subject: Surface Mount Considerations
>
>
> I am working with a design that requires CTE management for solderjoint
> fatigue of a leadless ceramic component, and I am considering random fiber
> aramid pwb construction.  The problem I have is that the solder side copper
> heat sink modulus is almost 10X that of the pwb and the results of the
> composite heat sink assembly is of little benefit of CTE the component sees.
>
> Can anyone recommend a heat sink/pwb adhesive that is forgiving enough to
> allow the expansion differences while still having good properties of
> shear/peel strength and thermally conductive?
>
> Thank You in Advance,
> Kevin Thorson
>
>
>-- End of excerpt from Lisa Williams


I'm not sure this will answer your question but for large ceramic parts, I have
experienced that reflow soldering to bare copper (not a nickel/Gold finish) has
the best component adhesion.  Adding a few large holes under the ceramic device
and allowing the solder to fillet to the ceramic component during reflow
increases the joint strength even more acting as a rivit to the ceramic
substrate. ( I've used holes as large as .080 Dia.)  Some heat transfers
through these holes to the copper layers below but for more heat disapation use
many smaller vias without solder, copper is a better conductor. Also the
smaller holes tend to allow solder to bleed through the board which leaves a
bump on the second side causing a screening problem if you have double side
reflow.

I hope this helps.




-- 
Dave Schmitz          
Motorola
600 N. Hwy 45
Libertyville, IL 60048

phone: (847)523-5294   pager: 3189  Fax  : (847)523-2244