PCB007 Magazine

PCB007-Dec2020

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78 PCB007 MAGAZINE I DECEMBER 2020 While frustrating as this situation is, a process engineer and skilled troubleshooter can look downstream to find the root cause and take cor- rective actions. Case in point is the resist lam- ination process. Assuming one has achieved a clean copper foil surface with a reasonable de- gree of surface roughness, the mechanics of the resist lamination process takes center stage. There are several critical variables in the re- sist lamination process that are keys to suc- cess. In hot roll lamination, heat is applied to the rolls and transferred through the polyester film coversheet and the resist to the resist/cop- per interface to achieve good resist-to-copper conformation and adhesion [1–2] . The rolls are heated by a variety of methods. Check with the equipment supplier in this case. While temperature is important, pressure, as supplied by the rollers, is a significantly more important variable with respect to resist con- formity. Regardless, temperature plays an im- portant role in reducing the resist viscosity for improved flow. For the hot roll laminator, Table 1 shows what these values are. While these parameters are critical to qual- ity, one must not ignore the condition of the rollers that make the contact between the cop- per layers and resist. For optimum resist con- formity, the rollers must be properly main- tained and changed after so many uses. These rollers eventually can bow and receive scratch- es and gouges after repeated use. These condi- tions are deleterious and negatively impact re- sist adhesion (Figure 4). As shown in Figure 4, the condition of the latter two sets of rollers requires attention. Otherwise, resist conformity and adhesion is compromised, including sporadic resist lifting, neckdowns, line width reduction, ragged trac- es, and opens caused by etch-outs. Note that the rollers are made of rubber. The pliability of the rubber helps to make intimate contact with the resist-to-board interface. Increased usage, time, and temperature will increase the hard- ness of the rubber coating, thus reducing the ability of the rolls to make intimate contact. Finally, the actual temperature at the resist/ copper interface is the key deliverable with re- spect to resist adhesion and conformity. This temperature depends on the contact time of the resist with the heat as supplied by the rollers, the temperature of the heat source, and the ther- mal mass and temperature of the board. The contact time, in turn, is a function of the lami- Table 1: Automatic cut sheet laminator and manual hot roll laminator. Figure 4: A schematic of the rolls (top and bottom). The top shows good condition. The middle and bottom pairs show bow and other deterioration. (Source: Tim Blair)

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