PCB007 Magazine

PCB-Mar2015

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74 The PCB Magazine • March 2015 relative position on the opposite sides of the PCB. Pressure that is applied to the squeegees from both sides is identical. After screen print- ing, there will be more ink in the vias than is noted with either spray or curtain coating of soldermasks. While additional ink in the holes is a given with double-sided screen coating technology, excessive tack dry time and temperature is of equal concern. By over-drying, the ink has a much greater chance of solidifying in the via, making the ink plug more difficult to develop away. Subsequently, that brings us to the issue of phototool quality and the exposure/developing process steps. The phototool has two particu- lar features that should be evident. There is the clear area that allows UV light to pass through and the opaque or darkened areas designed to prevent the light from passing. Where the light is able to pass, the ink will undergo polymer- ization. And that's where the concept of D min and D max show up: the optical density of the clear and opaque areas of the phototool. The reader should also familiarize herself with the term densitometer. Proper mastery of this instrument will help to ensure optimum quality of the phototool. D-min is a measure of the tool to transmit light. The lower the num- ber, the greater the transmission. And that is what the engineer wants, maximum transmis- sion of the UV through clear area of the tool. D- max, on the other hand, is a measure of image area on the tool and its ability to block the UV light. The densitometer, thus, is an instrument that is used to measure and track the D-min and D-max attributes of the phototool. This con- cept is introduced here for one reason; as the D-max decreases (or is not high enough from the beginning) there is a greater chance for UV light to penetrate the tool and cause partial or complete exposure of the ink. This would then cause difficulty when trying to develop out the ink from the holes. There is also the chance that some of the ink may be exposed on the pads, which can then lead to non-conformance. The best advice is to use the densitometer to moni- tor the D-min and D-max of the tool. After re- peated exposures, the D-min will increase and the D-max will decrease. As a rule of thumb, a newly produced phototool should have a D-min between 0.08–0.11 and a D-max of 4.0 or more. When the D-min increases above 0.20 and the D-max decreases to a measured value below 3.8, figure 2: Double-sided screen printing. (source: Lackwerke peters GmBh) trouble in your tank MORE PESky SOLDERMASk PROBLEMS: PLUGGED VIA LEADING TO SkIP PLATING continues

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