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58 The PCB Magazine • August 2014 Conversely, if there is a protrusion on the roll surface (e.g., caused by a particle adhering to the roll surface) this becomes a spot of high pressure, which causes resist thinning that can lead to a variety of defects. It is well understood that higher lamination pressure will result in improved dry film resist conformation to the board surface. However, there are practical limitations to increasing the lamination pressure due to the design of a par- ticular model of automatic cut sheet laminator. Even if a high pressure setting is compatible with the design of the laminator, it may be dif- ficult to take advantage of the higher pressure because the lamination rolls bend under the in- creased pressure. Roll bending results in an un- even roll footprint on the resist. The footprint is narrow at the center of the rolls and wider at the ends. This means that the pressure on the resist at the center is less than at the ends (Fig- ure 2). Laminator suppliers are aware of this phe- nomenon and try to build the rolls as sturdy as is practical. Residual roll bending can be com- pensated for with rolls which are "crowned." A crowned roll has a rubber covering which is thicker in the center than at the edges and changes gradually from center to edge, resulting in a curved profile (Figure 3). In our studies, we found that the correct amount of crown for an ASL-24 laminator was in the range of 3–4 mils (75–100 microns). Figure 4 shows the results of lamination test at two different pressure settings: with straight PRESSURE IN HOT ROLL LAMINATION OF DRy FILM PHOTORESIST continues figure 3: nip pressure profile with crowned rolls. figure 2: nip pressure profile with straight rolls.