ControllingSize-ChangeofPhototoolin.doc

In the manufacture of multi-layer PCBs there are many causes of layer-to-layer mis-registration. One of the more common problems is phototools that are not the correct size. Although this is a common problem, most people dont understand the factors that contribute to the problem or how to control them. The key to understanding these issues is to understand what causes the films to change size and how the films respond to changes in these factors. For PCB artwork reproduction we typically use either silver halide or diazo films to produce the phototool. These films are composed of a 7-mil (178 micron) thick polyester substrate with one or more coated layers. As the polyester substrate makes up 90-95% of the total mass of the films, the reaction of these films to changes in the environment is dominated by the properties of the polyester. The coated layers on the diazo film use an acrylic binder, which doesnt alter the films response significantly. Silver halide films have coatings made of gelatin or polymer-substituted gelatin. The gelatin absorbs more water than polyester does and causes the film to respond more strongly to changes in humidity. Both film types will change size when exposed to changes in temperature or relative humidity in the environment in which they are handled and stored. This is true of unprocessed as well as processed film. Both types of film also exhibit different responses caused by the extreme changes in temperature and/or humidity the film is subjected to during processing. A review of the physical properties of the films will help provide an understanding of how these changes affect film size. This discussion will highlight the effects of environmental changes first, then processing effects later. Environmental Effects All phototooling films coated on 7-mil polyester have a coefficient of thermal expansion (CTE) = 18 ppm/oC. This means that for every 1oC change in temperature, the phototool will change size by 18 micron over 1 meter (0.018 mils over 1 inch), or in more common dimensions, 9 micron over 50 cm (0.36 mils over 20). The coefficient of hydroscopic (humidity) expansion (CHE) for 7-mil polyester is 8.5 ppm per 1% change in relative humidity (RH). The CHE of diazo films is nearly the same as polyester base (9 ppm/%RH) because the coated layers do not absorb (desorb) an appreciable amount of water when exposed to changes in relative humidity. Modern silver halide films utilizing polymer-substituted gelatin binders have a slightly higher CHE, generally 10-12 ppm/%RH. Older silver halide films have a CHE that is in the range of 12-15 ppm/%RH. So a typical diazo or modern silver halide phototool exposed to a 1% change in RH will change size by about 5 micron over 50 cm (0.2 mils over 20). Figure 1. Coefficient of Hydroscopic Expansion for different types of phototooling films.Knowing these characteristics, we can then calculate how much change in size can be expected from a given change in temperature and relative humidity. Or conversely, how much variation in temperature and relative humidity can be tolerated while trying to maintain the film size within a given tolerance. Table 2 shows the amount of size change that can be expected over 50 cm for various combinations of temperature change and relative humidity change. I have outlined the boundary for a size tolerance of +/-25 micron. As you can see, there are several combinations of temperature and humidity control that could be chosen. Generally, it is easier to control temperature tightly than humidity, therefore it may be less expensive to control to +/-1oC and +/-3% than to +/-1.5oC and +/-2%, even though the resulting size control is the same.Figure 2. Expected size change (in micron) for a silver halide phototool, length = 50 cm.The response of the film to changes in temperature and humidity are independent and additive. However, the film response to temperature change occurs quickly while the response to a change in relative humidity occurs over several hours. Figure 3 shows the size change versus time for a 50 cm film when the relative humidity is increased by 5%. This characteristic of the polyester to absorb or desorb moisture slowly causes the film to change size differently than we might expect. As an example, Table 2 would predict only +1 micron size change when moving a phototool from one room to another where the temperature is 3oC lower and the relative humidity is 5% higher. In fact, the film will shrink by about 25 micron within a few minutes as the film quickly reacts to the change in temperature, and then it will grow to its final size over the next 6-8 hours as it slowly reacts to the change in humidity.Also, special precautions must be taken to insure the film is at its final size when you start using it. Because it takes several hours for the film to reach its final size after it has been brought into your photolab, it should be allowed adequate time to come into equilibrium with the room conditions before it is used. This “pre-conditioning” should be done before the phototooling film is first imaged, and any time it is moved to a different environment. Pre-conditioning takes 6-8 hours when air is allowed to contact both sides of the film-such as on open shelving in a specially designed pre-conditioning cabinet. Where this is not possible, adequate equilibration can often be achieved by opening a box of film, removing the inner packaging material, fanning the sheets of film so as to allow air between them, and holding the film in the closed box for 24-48 hours before using it. The film should be fanned again at least once every 12 hours to insure adequate exposure to the room air. Once the film has been equilibrated to the room humidity, we can take advantage of its slow response to humidity changes to maintain proper size. If the humidity in the areas where the phototools are used is well controlled it will cycle in a regular period around some average. Under these conditions the phototools will change size less than when the humidity variations are random or uncontrolled. If the time required to pass through a humidity cycle is less than one hour, then the film will change size by about one-half the amount you would predict from the total humidity range.Figure 4. Cyclical control of relative humidity.Figure 4 illustrates how the film changes size as the humidity cycles with relatively short cycle time. Because of this behavior, we can specify humidity control with twice the variation that would be predicted from the table in Figure 2 while still maintaining the desired size tolerances. Given that very tight humidity control can be quite expensive, good cyclical control of relative humidity will reduce the cost of maintaining good size controls. It is not only important that the environment in each room is controlled sufficiently well so that the phototool remains within size tolerances while in each room, all the rooms must be set up so they at the same conditions. Moving the film between two rooms that are each well controlled, but at different aim points will result in a size change. Table 5 is an example from one board shop in the U.S. the size change shown in mils per 20. The area shaded in green shows the variation in temperature and humidity within the photolab and film storage area. The area shaded in yellow represents the conditions in the resist exposure area (the “Yellow Room”). Each of the rooms was well controlled and designed to maintain phototool size within +/-1 mil / 20 (+/-25 micron / 50 cm). However, when the phototools were moved from the photolab to the yellow room, they would increase size by 1.3 mils (30 micron) on average. Worse than that, the films would initially shrink by 0.4 mils (15 micron) and slowly grow to their final size.Figure 5. Conditions in Photolab and Yellow Room at board shop in USA.The yellow room engineer reported that the films were growing 1.6-1.7 mils because the measurements were started after the film had reached temperature equilibrium, but before the moisture equilibration had progressed very far. Figure 5 shows how the film size changed over time after it was moved between these two “well controlled” rooms.Processing Effects Diazo films are exposed to a large increase in temperature and a moderate increase in relative humidity for a short period of time during processing. The film responds by increasing size during development, but returns to its original size after a short period of re-equilibration with the room environment During processing, silver halide films are completely immersed in aqueous processing solutions and then passed through a hot air dryer. The film passes from room conditions to being completely wet for about three minutes, to being hot at very low moisture content, and back to room conditions. Films often exhibit a small change in size after processing due to a hysteresis effect seen when polyester is subjected to large changes in moisture content. This effect is much less pronounced in modern polymer-gel emulsions than it is in older films. It remains important, however, to maintain consistent drying conditions for each film, and insure that all the films are completely dried. The amount of drying that a phototool receives determines the amount of moisture remaining in it after processing, and its final size. Varying parameters of the other processing steps has no effect on the after-processing size, except that changing the development time will also change the drying time in most processors. If the films are properly dried, they should have lower moisture content when they exit the dryer than the air in the room and they will be slightly small. Then they will increase in size as they come into moisture with the room air. This is a process that takes 30-45 minutes. If the processing conditions are consistent, any residual size change will be the same on every phototool and an appropriate compensation can be set into the plotter scale factor. Controlling Size Change Films can be used with satisfactory results in even the most demanding applications, if the environment is appropriate and the artwork is handled properly. To obtain optimum control of the size of your polyester based artwork, follow these handling recommendations: Insure that all the rooms (and exposure equipment) where the film is used are at the same temperature and relative humidity. This is especially true of the pre-conditioning, phototool plotting, and resist exposure rooms. Maintain the temperature and relative humidity in all rooms where the artwork is handled within the limits required for your size tolerances. Holding +