Mr. Khan,

The main problem with relatively fine lines and spaces via screened
images is in edge definition. 

The ink edges tend to be "shallow", without a side-wall to define the
edges of plated structures.  The copper will plate in any direction
available, including over the edges of the resist, resulting in poor
control of line width and spacing.

The resulting structure can also pose problems in strip/etch processes,
and provide a barrier to  full edge coverage if reflowed solder is used.

Incomplete strip for re-work also causes problems ranging from pits to
step-plate to etch-outs.  The residues remaining after stripping inks can
be difficult to detect.

The practicality of this sort of process would depend on your customer's
willingness to accept such variations and defects vs. their need for
lower prices. 

Regards,

Paul Galatis
Electrochemicals

********************************************************

On Thu, 29 May 1997 07:14:38 +0530 Anil K Singh <[log in to unmask]>
writes:
>In today's era of aggressive pricing there is a need to give a relook 
>to
>certain discarded processes which were quite cost effective compared 
>to
>their successors. Reverting to Screen Printing for Double Sided PTH
>Boards having 0.2mm traces and 0.2mm( 8 mil) spacing appears feasible.
>Would someone with experience in this field address the following
>queries:
>
>1. Choice of Screen Mesh & Grade ? SS or monofilament? HD or T ?
>
>2. Type of Screen Frame- construction? Optimum screen tension?
>
>3. Hardness of squeegee rubber & squeegee angle for printing?
>
>4. Screen height (gap) from the plane of the panel while printing?
>
>5. Type of Stencil Film and proper procedure for adhering it to the
>screen mesh?
>   Direct or Indirect?
>
>6. Ink rheology- viscosity? Thinner proportion? Any Brand preference?
>
>7. Any 'pre-baking' or 'air drying' required before pattern plating?
>
>8. Stripping medium ? Concentration and Temperature of the solution
>before    etching?
>
>9. Rework process? What if the boards require strip[ping and 
>reprinting?
>
>10 Any adverse effect in Copper Plating or Tin Lead Plating ?
>
>Many thanks in anticipation.
>
>P.V. Khan
>Chief Quality Manager
>Linaks Microelectronics Limited 
>Fax:+91-522-222061, 315978  E-Mail: [log in to unmask]
>
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