Improved Design of Dry Film Resist Laminator for Optimal Transfer of Micron-sized Features


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Introduction

Since the inception of dry film lamination, the process of squeezing two hot rolls at their end axis points has caused roll bending. An innovative process has now been developed whereby the film is attached with a process that minimizes this issue and provides optimal, evenly dispersed heat and pressure across the entire surface of the panel. This simple and elegant design will ensure that the integrity of the resulting image will be ideal, as measured by the continuity of the resulting transferred traces and features.

Electronic printed circuit board packaging is reaching its limit because of micron requirements, defined here as features below 5 mils (0.005”), or below 125 microns.

Over the years, front-end activities in the PCB manufacturing shop have changed from manual artwork to digitized input to vector photoplotting to phototools generated from digital/raster input. Today, digital imaging produced by either laser or projection techniques has solved the micron situation with great success.

So with our front-end issues resolved, we move to the next and key intermediate process step. This is arguably one of the most important PCB processes: the application of dry film resist to the copper surface.

Historically, here is where we continue to see faulty transfer of the original image to the copper surface. The current (and old) way of pressing down the dry film is simply inadequate. Rolls bend, resulting in low pressure in the central areas of the copper-clad panel. This can cause open circuit situations due to breaks in the copper images. These are, of course, most evident and critical on micro features (i.e., less than 0.005” [5 mil]).

The industry has made great strides in the past 20 years, but we still continue to look for ways to deal with the micron issue throughout the fabrication process. One at a time, we see these obstacles being corrected. This article highlights one of the most important steps to continue the goal of attaining optimal yield and success with the available tools.

The Innovation

Consider that we have just been handed an outstanding direct imaging database, almost perfect in every way with panel and feature dimensions meeting the most stringent micron requirements. Better yet, if environmental factors cause unexpected material movement, we can alter the data base with precise micron adjustments.

Our industry has been plodding along, not paying attention to a serious compromise since the inception of dry film lamination. From the 1950s–1980s this was not a problem, but with time, those micron requirements have been speeding our way.

While it appeared to be a perfectly adequate solution for applying dry film to copper, we have accepted the squeezing of two hot rolls at their axis ends to be our process standard (Figure 1).

Figure_1_FL.jpg 

Figure 1: Roll bends.

Figure_2_FL.jpg

Figure 2: Pressure comparisons.

Note: Figures 1 and 2 are historical data seen for several years in various publications. The information has always been acknowledged but never rectified—until now.

Figure 1 is an exaggerated graphic that shows how the rolls will bend no matter how much pressure is applied at the ends. It is pure physics, and in fact, the more pressure applied, the worse the bending. If one were to measure the pressure along the length of the roll, we would see high pressure being applied to the ends and the pressure would become less as we move toward the center. The higher pressure at the ends, combined with certain types of softer dry films, will most likely provide proper film adhesion with acceptable film conformance. However, moving toward the center of the rolls, and with certain types of films, there can be literally no pressure (Figure 2).

Again, this worked well when features were in the 10 mil (0.010”) range, but as features began to reach our micron status, dry film adherence to the copper was random and without conformity. More importantly, if the copper surface has small imperfections (pits and dents), dry film needs to be pressured into those areas with firm and predictable results, otherwise the resulting image on the dry film will not transfer properly, and in most inner layer cases, will cause open circuits.

Further, as equipment reaches the end of its useful life, machine tolerances begin to play havoc with our most important requirement—transferring micron features.

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