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72 The PCB Magazine • September 2014 by Michael Carano omg eleCtroniC ChemiCals Root Cause of Failures in PWB Lamination C o l u m n trouble iN your taNk Introduction Understanding the interactions of the mate- rials, oxide treatment and the lamination pro- cess 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/metal- lization) will depend on several factors that will now be presented. Interlaminar Bond Strength There are several quality aspects of a multi- layer 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 delami- nation. Figure 1 shows an example of delamina- tion. In general, heat excursions increased the stress within the bond and that will lead to fail- ures. So the bond between the copper and the resin needs to be as robust as possible. Other defects are shown in Figure 1. While additional work-up is required, there is a line of demarcation between the plated copper and the copper interconnect. 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 misin - terpreted 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 T g s in the 250°C range. PTFE (polytetrafluoroethylene) and ceram- ic-filled materials are used for microwave ap- plications and have significantly different pro- cessing conditions than the other resins. But more on this in a future column. Factors Leading to Delamination Delamination of the type shown in Figure 1 can be caused by several factors. Some of those factors are mechanical (press cycles, pre-preg types, multilayer stack-ups, etc.) as well as the chemical factors (oxide and oxide alternatives). figure 1: delamination.

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