Hi Yulia,
My opinions below. Steve Kelly

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Yulia Yishayahu
Sent: August-23-15 2:45 PM
To: [log in to unmask]
Subject: [TN] Design and manufacture of Flex-Rigid PCBs

Hello TechNetters,

I have some questions regarding design and manufacture of flex-rigid PCB.
IPC 2223 recommends to keep a clearance between PTH, placed at the rigid area, and rigid-to-flex interface. What about SMT pads? Should I consider the same clearance? Yes as much as possible
Is it possible that coverlay and no-flow prepreg, used in the middle of 8-layers board, may introduce non uniformity at the transition region and, therefore, affect the ability to manufacture good and reliable SMT pads? Not normally with proper design

Regarding the manufacturing process, I was told that the best way to make a flex-rigid build up is by joining a flex laminate to a one-sided rigid laminate by a no-flow prepreg and only after that I can use a copper foil. No - you do not have to do that
However, such a build up results in at least 6 mil dielectric thickness between "rigid" to "flex" layers and forces my laser micro via between these layers to be quite large. ( I have 8-layers pcb with a flexible laminate at layers 4 and 5; laser drills between L1-L2-L3-L4 and
L5-L6-L7-L8 and PTHs). Is it possible to manufacture a reliable flex-rigid PCB using only copper foils attached to the flex laminate by the prepreg, in order to reduse the dielectric thickness at the joint? In many cases you can build this as just a multi-layer flex with maybe some pre-pregs or bond ply's- maybe with some thicker polyimide. As components become smaller and lighter often you do not have to use the classic rigid flex. So you could as an example use copper/polyimide/copper for L1/L2 - then you can easily take your via to .004. 

Thank you,
Yulia.


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