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Understanding the interactions of the materials, oxide treatment, and the lamination process will help you get to the root cause failures in multilayer fabrication.
When troubleshooting multilayer defects, it is necessary to again understand the effect certain process parameters have on quality and reliability. Truly, the quality of a multilayer printed circuit board (prior to desmear/metallization) will depend on several factors that will now be presented.
Interlaminar Bond Strength
There are several quality aspects of a multilayer PCB that should be measured on a regular basis. One key determinant of the reliability of the multilayer package is the interlaminar bond strength. The interlaminar bond strength is the strength of the heat-resistant bond between the pre-preg and the copper foil. Ideally, one strives for optimum resin flow encapsulation of the pre-preg with the treated copper innerlayer. The stronger the bond between the pre-preg and treated copper, the lower the chance of delamination. Figure 1 shows an example of delamination. In general, heat excursions increased the stress within the bond and that will lead to failures. So the bond between the copper and the resin needs to be as robust as possible.
The simple definition of delamination is, “a separation between plies within a base material, between a base material and a conductive foil, or any other planar separation within a printed board.” Again, we are referring to a separation. (More on blister and laminate voids in another column.) It is a huge concern that separation of the pre-preg from the copper foil is often misinterpreted for a blister. Indeed it is more serious than that. As an example, higher temperature resin systems may require more adjustments to the printed circuit processes such as: lamination cycle, baking, hole cleaning, drilling and routing. Polyimide resin and cyanate ester are the most commonly used high-temperature resin systems. These resins have Tgs in the 250°C range.
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Editor's Note: This column originally appeared in the September 2014 issue of The PCB Magazine.