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56 The PCB Magazine • August 2014 by Karl Dietz Karl dIetZ consultIng llc Pressure in Hot Roll Lamination of Dry Film Photoresist c o l u M n karl's tech talk Lamination Pressure In lamination, we are trying to achieve good contact between the resist and the substrate sur- face by making the resist flow to conform to the surface topography. Flow is achieved by lower- ing the resist viscosity through heat, and by ap- plying a pressure differential for a certain time to cause the flow. The the photoresist is exposed to these conditions depends on the rpm of the hot rolls, the hot roll diameter, and to some ex- tent on the pressure as it affects the footprint width in the nip. The pressure may be transmit- ted to the lamination rolls pneumatically, hy- draulically, mechanically, or a combination of these means. This column focuses on pressure as an important variable that needs to be con- trolled and optimized. Non-uniform pressure can cause a variety of problems such as dry film resist wrinkles shown in Figure 1. If the hot roll surface has a localized recessed area (e.g., a cut caused by a knife during man- ual trimming of film), this will be a spot of low pressure, resulting in poor resist conformation to the board surface, which in turn can cause opens in a print and etch process. figure 1: lamination wrinkles pointing to one side.