PCB007 Magazine

PCB-Jun2018

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38 PCB007 MAGAZINE I JUNE 2018 impellers, which move the solution in a very gentle way. The traditional pump is like an out- board motor on a boat—it is very effective at moving liquid, but it also causes quite a lot of froth and disturbance. In comparison, the re- vised design is more like a paddle. In the right hands it can create a gentle liquid movement with hardly a stir being seen on the surface. This gentle movement is ideal for immersion tin and this process development has the po- tential to reduce downtime, increase chemical life, and even improve the rate of deposition. This machine is also the first time I have used remote water heating to control the tem- perature of the working solutions. Again, the advantage of this process is that the surface temperature at the heater can be much more carefully controlled so there is less heat shock which can also damage the chemistry. I can only say I was impressed with this and the accuracy of control was good. It is something I would recom- mend to enhance the control and effect of the heating if there is room in the factory to accommodate it. The work- ing temperature of the water heating system does not need to be much high- er than the highest temperature for the heated solutions. In most cases, 10 de- grees difference should be more than enough so there do not have to be su- per-heated pipes running around the factory. The initial tests of the process are very positive and from what I can see it offers a real step forwards in a pro- cess that has remained relatively un- changed for a long time. When you are dealing with process- es which may have been established for a number of years, it is not always obvious how to adapt them to suit cur- rent product needs and requirements. Hole sizes get smaller, blind holes are more prevalent, and aspect ratios are increasing, all of which make it tough to get a good effect of chemistry both on the surface and inside the hole. Of- ten, these two requirements conflict with one another. Most fabricators don't have the lux- ury of updating equipment as often as their customers update their prod- uct designs. This makes it critical that when you do replace equipment, it has the right set of process enhancements to fit your predicted plans for the next few years. For this reason, it is impor- Figure 1: Horizontal immersion tin process. All chemical tin solution movement is driven by low impact propeller pumps.

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