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Reflow For Rigid Flex

Reflow For Rigid Flex
We have a new PCB assembly that includes a section of flex cable and FR4 circuit board. What are the reflow rules I need to consider? Jim Hall and Phil Zarrow, the Assembly Brothers, share their own insight and experience.
Board Talk


And welcome to Board Talk with Phil Zarrow and Jim Hall, the Assembly Brothers, here to solve, or add more confusion, to whatever you are pondering in the area of surface mount technology.

Today we have a question from S.J.  We have a new PCB assembly that includes a section of flex cable and FR4 circuit board.  What are the reflow rules I need to consider while processing a mix, FR4 and flex cable assembly through the reflow process.

Well, first of all I believe that's known in the vernacular of our industry as rigid flex, clever name don't you think? That's and interesting question with regard to reflow. Reflow is reflow. You follow all the other rules that you need driven by the specs of solder paste and of course your most vulnerable component on the board.

Your flex cable might become your most vulnerable component.

You may be fixed ring so remember when you do profile to use the fixture.

I want to go over my brother's head and say you should be fixturing. You really should very carefully look at fixturing, keeping that thing level, particular if there are surface mount parts on the flex section of your assembly.

You want to make sure that's not moving around and flexing during reflow and moving your components.

Generally most people build the entire assembly, either reflex or rigid flex in a fixture, right through printing, pick and place, and reflow. So they avoid disturbing stuff through handling.

But with regard to the reflow question I think that's straightforward, but the thought I am wondering about is with regard to more important issue particularly with rigid flex is storage and handling.

And why would you worry about storage and handling?

Because rigid flex boards tend to have an interesting moisture absorption aspect and sometimes we get involved with bake out.

Flex materials tend to absorb moisture much more quickly and so proper storage and perhaps baking before reflows is a significant consideration.

So that would probably be the main concern. There are some pieces of wisdom coming out of IPC on handling flex and rigid flex, I believe it's 1601 on the circuit board handling and storage specification. 

So anyway beyond whatever you do, don't solder like my brother.

Don't solder like my brother. 

IPC 1601 Printed Board Handling and Storage


Baking flex-rigids including Polyimide (normal stuff) is necessary, but remember that PI picks up moisture quickly once again, so solder immediately.
Graham Farmer, GTS Flexible Materials Ltd
Flex-rigids should be throughly dried before reflow. With very hot and long assemblies it should be a common practice to build a "cap" over the flexes to avoid heating them too much. The adhesive used for the coverlay is acrylic based and its Tg is too low.
Roberto Tulman, Eltek, Israel

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