Vishy,
I have had success mounting single-sided flexible circuits on FR-4 using 3M
high-temperature acrylic double-sided tape. The OVERSIZED flex substrate is
placed face-down on a smooth table and stretched taut and smooth, holding
the edges with single-sided polyester tape (or similar). A strip (0.5" to 1"
wide) of the 3M acrylic double-sided tape is placed around the edge of the
FR-4 board, and the board is placed on the back of the circuit. A sharp
blade is then used to cut around the perimeter of the board, releasing the
circuit from the table. The circuit is now held taut to the FR-4 board by
the strip of 3M acrylic tape around the outer edges of the board. In this
way, no adhesive residue contaminates the backside of the circuit. After
processing, the circuit is removed from the carrier by cutting within the
tape perimeter.
As you can imagine, this process is not suitable for high-volume
applications, but it works well for low volume. I have successfully
performed SMT printing, placement and reflow with circuits mounted in this
fashion. The 3M tape is very aggressive and has no problem taking the reflow
temperature.
Good luck,
Kent
-----Original Message-----
From: Narayana Vishy-CVN002 <[log in to unmask]>
To: [log in to unmask] <[log in to unmask]>
Date: Thursday, March 11, 1999 11:17 PM
Subject: [TN] Flex circuit flatness - Is the solution out there ?
>Hi Technet pals :
>
>Flexible circuits have this natural tendency to be flexible . They tend to
>springup , bend ,kink & warp due to natural flexibility & handling .
>
>This behaviour is a major headache for the manufacturing engineers during
>the assembly process of these circuits - Defects & more defects !
>The non-flat nature of these circuits is the root cause of numerous defects
>like misaligned , unflush and unsoldered components . Processes such as
>solder paste printing,component placement & reflow oven are not very
>resistant to such flexible , warped & bent surfaces . Keeping these ciruits
>perfectly flat (like a PCB ) during these processes is key to eliminating
>many of the defects mentioned.
>
>Not to mention that much of conventional automation strategies are not
>easily applicable on flex ciruit assembly manufacturing - For example,
>automated vision isnpection is an entirely different ball game when it
comes
>to flexible circuits.
>
>Now the questions :
>
>1) Is there anybody out there who knows techniques of holding the flexible
>circuits down real flat ?
>2) Are there technologies/vendors in the market who have adhesives
>(sheets/sprays/liquids) which can :
> a. hold down flexible circuit panels flat on a solid pallet
>(carrier)
> b. withstand the reflow oven temperature (max 290 deg
>centigrade)
> c. be peelable/detachable from the flexible circuit panel w/o
>causing cosmetic defects on the flex circuit after the reflow process .
>
>Any clues ?
>
>
>Thanks in advance,
>Vishy.
>
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