Hi all,

We ran some 10 layer products with specified dielectrics as low as 3.1 and 4.1 and an overall finished thickness of .062. The specified impedance readings were 55 ohms +/- 10% on 4 of the innerlayers which had 1 oz copper. The outerlayers were 60 ohms. The supplied lines were .005 with spacings of .004. Although we have ran a significant amount of other products successfully we had some issues on this particular design. After reviewing all of our other products we determined that this was the only product with dielectrics specified this low.  We performed a study by pressing a full press opening ( 12 panels ) using an in house panel design that allowed us to test coupons all across the panel. 

Our results indicated that acceptance was very random. Coupons with readings out of spec where found throughout the test panel. Most rejected coupons were surrounded with good coupons. The interesting thing is we found less than .0003" variation on dielectrics from cross sections of accepted and rejected coupons and the line widths were all around .004-.0045.

My question is this.  Although our internal tolerances indicate we should be able to process within a 10% tolerance window for impedance,  should this tolerance increase as impedance values and dielectric requirements get smaller?

How is everyone else handling this?

Ed Cosper