We are experiencing a high degree of warpage on several different 
multi-layer PWBs. The PWBs are typically 8 layer, .063 thick GFN material 
approximate size of 8"x10".  Warpage is on the order of > .20" across a 13" 
diagonal. The PWBs have 1" wide break-aways on two opposite outside edges. 
Break-aways are GFN with no copper layers. The copper layers in the PWB are 
1/2 or 1 oz and are fairly well balanced around the mid-plane of the board. 
Numerous interstitial vias are used and all are located in the bottom half 
layers of the PWBs.

Not all PWBs we receive are warped. Recent runs show a 10% reject rate for 
warpage. It appears that some boards also warp after we receive them (i.e. 
Manufacturer says they past his inspection.) Some boards warp after reflow.

We are looking for any possible causes of the warpage. There is some thought 
that the unbalanced interstitial vias may be causing the warpage. Any 
suggestions as to cause of corrective action would be appreciated.

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