Multilayer Innerlayer materials

Structure. The structure of DYCOstrate PERL is shown in Fig. 23.28. The core material can be standard FR-4 multilayer innerlayers or a through-hole-plated two-sided or multilayer board. Multiple layers can be built up to increase density with the resulting buried and bhnd vias. Current production ranges from 4 layers to 12 layers with various buried board and buried via constructions. As with DYCOstrate, many materials are plasma-etchable and the resin butter-coated copper foils come in many thicknesses and resin types, such as BT, cyanate ester, and polyphenelene-ether (PPE). 

Time to delamination, or T260 time, which is the time it takes for a sample to delaminate when held at an isotherm temperature of 260°C, is not a fundamental base material property, but is a functional test related to the other material properties. In multilayer PCBs, this property can be affected by the quahty of the innerlayer copper surface preparation and multilayer lamination quality, among other things. 

The potential for moisture absorption in these processes is the other critical consideration. Oxide and oxide alternative treatments can be compromised if exposed to high temperatnres, or even moderate temperatures for extended times. Oxidation of these snrfaces dnring postdry-rng or baking can lead to adhesion or chemical resistance problems in snbsequent processes. On the other hand, moisture absorbed by the laminate materials dnring these processes can inpact the quahty of multilayer lamination and snbsequent performance dnring thermal cycles. In short, it is important to dry the innerlayers as mnch as possible after these processes while staying within the guidelines provided by the oxide and oxide replacement chemistry snppUer. 

FIGURE 13.9 Cross section of 14-Iayer multilayer printed wiring board, showing a typical inner layer and prepreg material relationship. In this case, to reduce z-axis expansion, the innerlayers are polyimide, while the prepreg material is semicured polyimide. Typical signal, power, and ground layers are also indicated, as well as the thickness of the copper foil for each layer. 

Aramid-reinforced laminate and prepreg allow fast microvia hole formation and at the same time maintain the performance characteristic of a smooth surface for fine-line conductor imaging. The ablation speed of non-woven (aramid) laminates and prepregs is close to that achieved when using nonreinforced materials such as resin-coated foil, dry film, or liquid dielectrics. Since aramid laminates are very stable, they allow the fabrication of doublesided, very thin, etched innerlayers, which are then pressed to a multilayer package in a single... 

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