** See comments below **

Narayana Vishy-CVN002 wrote:

> Hi Andy,
>  Recently,  I posted this query on the forum. Any comments /leads ?
>
> Vishy.
> ----------------------------------------------------------------------------
> -------------------
> Hi TechNet pals ,
>
>  I just joined this mail forum today -  Thanks to Joe Fjelstad for letting
> me know about this forum at the recent PCB ASIA lectures in Penang - Hi Joe
> !  ;
>
> Based on the understanding I have of the flexible circuits manufacturing
> processes , I have the following comments/queries : I hope these have not
> been discussed in this forum before !
>
> Pad registration accuracy on flex circuits has always been an issue. Pads
> are often defined by the coverlay opening (COVERLAY DEFINED PADS ) rather
> than by the pad itself  (RESIST/METAL DEFINED PADS).Coverlay defined pads
> are necessary to hold down the pads to avoid the peeling-off effect.
>
> For a pad which is defined by the coverlay opening , accuracy of the EXPOSED
> pad location with respect to the fiducials is lost due to the running out of
> the coverlay opening.
>
> Coverlay hole punching errors , as I understand , are due to
> a) punching tool accuracy
> b) stack up tolerance errors as the punch moves along punching the various
> pad locations .

** There are also dimensional stability, CTE and CHE effects to be considered.
(Mechanical, Thermal and Hygroscopic) **

>
>
> The resultant effect is that the paste is printed as per the gerber file
> (ACTUAL METAL LOCATION WITH RESPECT TO FIDUCIALS) while the actual EXPOSED
> pad location is off due to the shifted coverlay opening.This will cause
> errors in the relative location of the EXPOSED pad,the part and the paste -
> resulting in tombstone / solder defects.
> The above effect magnifies as we go to small parts such as 0603 (or 30x60)
> and 0402 (or 20x40) parts .

** The etched circuit pattern and the coverfilm punched pattern can be
compensated to provide a finished circuit with nominal pad locations. This can
be "dialed in" during the pre-production phase, and additional image
compensation adjustments can be made for each material lot if very tight
tolerances are required. **

>
>
> Based on the above experience,following are my queries :
>
> 1) Any comments on the above problem ?
>
> 2) As a flex manufacturer, what is the BEST pad location accuracy with
> respect to the fiducial can one offer for COVERLAY DEFINED PADS ? How ? What
> are the current capabilites in the market ? Where are these capabilites
> available ?

** Our etched pad to fiducial location accuracy was +-0.0005". Our coverfilm
registration was +-0.003" and we required a coverfilm annular ring design of
0.008" to insure good adhesion. Punched features were held to +-0.002". **

>
>
> 3)  Have any of you had good/bad experience of assembling 0603 & 0402 size
> components on flexible circuits ? If yes, would you share the following
> details ;
> - Pad dimensions & geometry for such parts
> - Flex pad design rules/requirements for such pads for a satisfactory
> soldering of such parts.
>
> 4) Are there any IPC standards for the above ?
>
> 5) As a flex assembler , would  process engineers like to share their
> experience of assembling such parts ?
>
> Thanks in advance ,

Andy Magee
Flex Guru - Consulting
[log in to unmask]
(937) 435-3629

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