Answer to your firstquestion, does 1ppm make a difference?
In my opinion probably not. I doubt that the published data is really
accurate to better than 1ppm. Anyway what do the manufacturers of the MCM
recommend or guarantee or have qualified their device to? What is the
environment the part is going to operate in and how big is it? If you know
the geometry of the pins you it's easy to calculate the maximum dimensional
mismatch at extremes of temperature which will give you a feel for whether
you have something to worry about.
Second question, does the whole board need to be of the same material. If
you build a board with layers of different CTE at best you'll induce lots of
internal stresses, at worth the board will warp horribly.
Regards,
Eric Christison
Mechanical Engineer
Consumer & Micro Group - Imaging Division
STMicroelectronics
33 Pinkhill
Edinburgh
EH12 7BF
Tel: +44 (0)131 336 6165
Fax: +44 (0)131 336 6001
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]] On Behalf Of Valerie St.Cyr
> Sent: Thursday, March 10, 2005 4:12 PM
> To: [log in to unmask]
> Subject: [TN] CTE matching
>
>
> I have a question for the forum:
>
> We have an engineer who will be using an MCM on his PCB, and
> he has said that the CTE is 11-12 ppm/C (I have no other
> information as yet on this MCM). He believes that because of
> this he needs to use a material with a closer x,y CTE. There
> is a material where the specs give the x,y CTE as up to 14,
> which is the material we would typically use; and there is an
> alternate material where the specs say the max CTE is 13. Of
> course the alternate material costs more.
>
> First, is there any practical information on matching MCM
> CTE? Would a 1 ppm per deg C make a difference?
>
> Second, I was under the impression that to match CTE we would
> only have to change the top layer, where the joint between
> the device and the board is formed. He believes that we would
> need to change all the cores and prepregs in the stackup.
> This will be somewhere over 24 layers (26, 28 , 30?? don't
> know just yet) so the cost delta for the material will be
> magnified if the entire stackup is converted.
>
> The alternative material which is proposed is not Thermount;
> but I did go out to Dupont's web pages and read all their
> papers on CTE matching. I sent those to the Engineer but it
> wasn't enough to convince him. Does someone have any
> alternative links to papers detailing real life projects for
> CTE matching and what they did and what was achieved, and so forth?
>
> Is there a board fabber on distribution with a lot of
> background/customer demand for this who would care to comment
> on my two questions?
>
> Thank you all,
>
> Valerie
>
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