Hi Technetters:

We are having an outside lab do thermally stressed cross-sections on 5 layer
rigid-flex product and are finding that some fail due to delamination of the
coverfilm from the rigid layer and partial seperation on the internal pad.
However this conditions do not exist in the "as is" cross-sections prior to
thermal stress.

The rigid-flex is 5 layer 0.040" thick with 2 layers of flex in between the
rigid section. Layer 2 is the rigid solid ground. The delamination is only
noticed on this ground layer where the coverfilm that acts as a pouch seem
to seperate from the copper surface. The boards are Hot air levelled prior
to thermal stress. The thermal stress is performed per IPC procedure at 540F
for 10 sec. The drilled looks very good, plating is between 1.4 to 1.8 mils
in the holes. The smallest via is only 13.5 mil drilled and the etchback is
0.2 to 0.5 mil. The rigid-flex is made with modified acrylic adhesive system
for coverfilm and bonding.

Can the thermal stress procedure induce these defects?

Athough the boards have good plating and hole quality and pass as is
cross-sections, are they rejects or is there any other tests that can be
performed to prove that the boards are reliable?

Thanks in advance for your help.

Chetan Shah


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