Edwards, Ted A (AZ75) wrote:
> 
>      We are redoing our specification and are wondering if anyone has design
> guidelines that take into account either aspect ratio (or drilled hole size)
> and choice of material into account.  For instance on a 0.125" board with a
> 0.0135" hole( ratio 9.4), or a 0.093" board with a 0.0135"  mil hole(ratio
> 6.9) do you specify to the designer use of a higher glass transition
> material that you do on the same hole size on a 0.062"        ( ratio4.6)?
>  An example would be polyimide for the 0.125 thickness, multifunctional for
> the 0.093 thickness and standard FR-4 for the 0.062 thickness or a chart
> that shows increasing thickness or aspect ratio versus what material should
> be used.  Anybody have any thoughts on this or comments?  Or is there any
> fabrication industry consensus?
>           Thanks in advance for your inputs;
>                [log in to unmask]
> 
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Ted, these type of guidelines are mostly internal to each company. There
is little public information on such matters. However, you can generate
this information easily. Calculate the stress in FR-4 PTH taking its AR
into account. Since that hole is reliable, the stress developed in
copper is not producing cracks. Now you can repeat the similar stress
calculation for other AR and materials. For a specific AR of PTH and
material properties, make sure that stress in plated copper does not
exceed the one you calculated for FR4 PTH. It will get you the answer
you are looking for. It may not be exact solution but very close to the
real world situation.


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