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 .

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 .

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 ?

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 ,
_______________________________________________________
VISWANATHAN A/L NARAYANA IYER  aka Vishy
Staff Engineer - AMT/SMT  Department,
Energy Systems Group,ACCES
Motorola Technology Sdn.Bhd.,
Penang  , Malaysia
Phone / Voice Mail : (604) 8503566
Fax    : (604) 6442277
Paging center : (604) 8504223  - Pager: 381
> mailto:[log in to unmask]
_______________________________________________________

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