Printed Circuit Board Assembly & PCB Design SMT Electronics Assembly Manufacturing Forum

Printed Circuit Board Assembly & PCB Design Forum

SMT electronics assembly manufacturing forum.


Flex embedded into FR4

#24616

Flex embedded into FR4 | 28 May, 2003

This is our first experience with running embedded flexs. Our board supplier suggested that we prebake our assemblies before placing components. What does everyone else do? Is prebaking normal when a flex is embedded?

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#24622

Flex embedded into FR4 | 29 May, 2003

The permeability of polyamide film (Kapton) is 54 g/sq m/mil/day and the total absorption is 2.8 wt%. The absorption rate changes rapidly over time with a large percentage occurring in the first 5 minutes and nearly reaching completion in 30 min to an hour depending on the film type and thickness. Another polyamide film (Upilex) has only 1.7 g/sq m/mil/day permeability and 1.2 wt% absorption, so you must know what specific materials you're considering.

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#24626

Flex embedded into FR4 | 29 May, 2003

I hate to answer a question with a question but what are the particular effects of moisture absorption in a flex circuit?

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#24627

Flex embedded into FR4 | 29 May, 2003

In itself none, but heating moisture laden flex can cause the flex to delaminate.

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#24628

Flex embedded into FR4 | 29 May, 2003

So is it fair to say that on a moisture laden double sided flex circuit you could see an increase in tombstoning on the 2nd pass due to warpage created by delamination? In other words a 0603 for instance could tombstone due to the delaminated surface not allowing the part to lay flat on the surface of the flex.

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#24634

Flex embedded into FR4 | 29 May, 2003

This flex is .001" Kapton polyimide. The flex portion of this rigid flex consists of 3 layers of 1 oz copper with 5 layers of Kapton. Part of the flex is sandwiched in standard FR4 with two layer of copper on each side for a finished thickness of .062".

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