Rigid-Flex PCB Fabrication: Window opening vs slot opening

Question

We had issue with one of the rigid-flex design cracking at the joint of Rigid and flex intersection. It is important to note that this is second batch of same PCB and PCB was not cracking in first batch.

So we reached out to manufacturer and asked for root cause.

They said reason incorrect laser parameters of their machine and also said that they will change the PP design from window opening to Slot opening which will eliminate need of laser all together.

My question is what exactly is changing? What does Window or slot opening actually achieve?

Answer 1

It looks like the terminology is not yet settled. In this article the groove cutting by a laser refers to the "positive and negative depth control method".

I think that your slot method based on gluing the finished flex layer to rigid layers. And your window method based on pressing layers all together, followed by contour milling and window cutting.

It seems that combining separated finished layers is more expencive than joint processing but can be more quality.

Answer 2

It seems that the improvement action is just an excuse to customers. The PP is only laminated as "after " image. I think the root cause may in the laination porcess. If they use uncorrect prameter in lamination may cause crack problem.

Answer 3

If the corners are cracking, then remove the corners. Make them curved outward, like a teardrop connection of sorts.
Also, you can add a bead of epoxy at the connection interface as a strain relief.

I recommend checking out the Flex PCB design guide from Sierra Express.

Also, another very good guide is from Wurth Electronics. Section 6 has an image of recommendations for transitioning from flex to rigid.