Dimensional Drift
Cumulative variation in the height of a partially assembled multi-layer board stackup during sequential lamination cycles leads to final board thickness errors. Sub-stack thickness drift represents the departure of the actual sub-stack height from its nominal calculated design thickness. This issue arises from variations in prepreg thickness, copper distribution, and resin flow during multiple press runs.
Boards built with sequential lamination steps are especially vulnerable to this effect. Tight control of each lamination phase minimizes overall deviation.
Cumulative Variation
Sequential lamination requires some layers to undergo the high-temperature press cycle multiple times, which continues to compress and squeeze out resin from those layers. This repeated heating and pressure causes the older layers to become thinner than predicted, while the newly added layers retain more of their original dimension. The resulting thickness difference shifts the reference planes of high-speed traces closer together, lowering the trace impedance.
It also causes mechanical problems during assembly when the total board thickness exceeds or falls below the tolerance limits of standard connectors. Designers must account for this shrinkage in their initial stackup calculations.
Inspection Threshold
In-line thickness gauges measure the sub-stack after each lamination step to catch deviations early. If the drift exceeds the allowed process margin, operators adjust subsequent prepreg selections or press cycles. Monitoring these trends keeps the build within specified physical tolerances.
Proper drift control ensures assembly compatibility.
