In a message dated 02/20/97 2:48:56 AM, you 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]  

Your question does not have a direct answer--it depends on the solder assembly
 processes used as well as on the severity of the use environment. IPC-D-279,
'Design Guidelines for Reliable Surface Mount Technology Printed Board
Assemblies' deals with this issue in Appendix B. I also will give the
workshop described below on this subject at NEPCON West (2/25) and IPC EXPO
(3/9). 

"Design for Reliability and Quality Manufacturing of Plated-Through-Holes and
Vias"
Course Content:
For high-density electronic assemblies the diameters of plated-through-holes
(PTH)/vias are becoming progressively smaller, while board thicknesses might
increase.  This has led to PTH/via failures after assembly or in the field,
because the standard manufacturing processes can be inadequate to produce
high quality and yields.  High yields and long-term reliability can only be
assured by a deliberate design effort that considers the specific damage
mechanisms and stresses, resulting from further processing and operational
use, threatening the integrity of the electronic assembly.  Knowledge of the
material properties and fatigue behavior of the PTH/via copper barrels, and
assembly processes and use environments are necessary for the successful
'Design for Reliability' of high-performance, high-density electronic
assemblies.  
The design information presented has been included in the industry document
IPC-D-279, 'Design Guidelines for Reliable Surface Mount Technology Printed
Board Assemblies'. 

Werner Engelmaier
Engelmaier Associates, Inc.
Electronic Packaging, Interconnection and Reliability Consulting
23 Gunther Street
Mendham, NJ  07945  USA
Phone & Fax: 201-543-2747
E-mail: [log in to unmask] 

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