Multilayer Innerlayer processing

Multilayer and two-sided product mix Lamination presses and innerlayer processes are required. 

Table 9.4 lists several of the key variables that influence an overall via-to-innerlayer pad registration capability in the printed circuit manufacturing process. The values in the table represent individual process standard deviations using a specific multilayer circuit design. If we make the assumption that these processes are normally distributed and that they are centered on the desired nominal value, we can use additivity of variance to estimate an overall registration capability. In other words, taking the square root of the sum of the squares of each individual process standard deviation gives an overall process standard deviation that can be used to assess capability. 

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. 

Multilayer Lay-Up Prior to Lamination. In addition to the oxide and oxide alternative processing discussed in the previons section, the hold time between this process and multilayer lay-up and lamination is also important.This is due not only to the possibihty of oxidation of the innerlayer surfaces, but to the potential for moistnre absorption by the exposed resin surfaces on the innerlayer, for the reasons already discnssed. This hold time should be minimized, and the storage of the innerlayers prior to lay-np shonld be in a temperature- and hnmidity-controUed environment to rednce the likelihood of absorbing moisture. Typical environmental conditions for this storage are 68°F or 20°C, and a maximum of 50 percent relative hnmidity (RH), although lower humidity levels are recommended. 

Innerlayers of multilayer boards can shift during fabrication processes and create poor layer-to-layer alignment. During the artwork exposure or fabrication of the multilayer boards, the position of the copper shifts relative to the nominal stacked position. This can create breakout or missing of the copper pads when drUhng the interconnection holes. 

The multilayer process begins with the accumulation of the innerlayer clad dielectric laminate. 

Two-sided printed circuit boards are usually processed by first drilling and debarring, then metallizing the through hole. Multilayers require treatment prior to any metallization process. Resin smear must be removed from the innerlayers, and texturing of the epoxy surface enhances hole wall adhesion. The preprocessing can be categorized as either smear removal or etchback. 

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