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In lamination, we are trying to achieve good contact between the resist and the substrate surface by making the resist flow to conform to the surface topography. Flow is achieved by lowering the resist viscosity through heat, and by applying a pressure differential for a certain time to cause the flow. The photoresist is exposed to these conditions depends on the rpm of the hot rolls, the hot roll diameter, and to some extent on the pressure as it affects the footprint width in the nip. The pressure may be transmitted to the lamination rolls pneumatically, hydraulically, mechanically, or a combination of these means. This column focuses on pressure as an important variable that needs to be controlled 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 manual 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.
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 particular model of automatic cut sheet laminator. Even if a high-pressure setting is compatible with the design of the laminator, it may be difficult to take advantage of the higher pressure because the lamination rolls bend under the increased pressure. Roll bending results in an uneven 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 (Figure 2).
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Editor's Note: This column originally appeared in the August 2014 issue of The PCB Magazine.