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Testing the effectiveness of cleaning and washing flux in the microelectronics industry

17. 09. 2014

In the production of printed circuits boards it is necessary to ensure that all residues of soldering flux are removed effectively and carefully. Removal is carried out by washing and cleaning using spray at a high temperature (50 – 80 °C) in special liquids such as deionized water, organic solvents and other emulsion-type liquids intended for this purpose.

The effectiveness of this cleaning and washing process is evaluated using sample glass boards with stuck-on chips which are contaminated with soldering fluxes to a certain degree and in a certain composition. The boards then undergo a cleaning cycle along with boards fitted with printed circuits.

Due to the fact that chips are normally stuck onto the smooth surface of a glass board, in these tests they tend to become unstuck and fall off. This is undesirable – it means increased testing costs and lost time. This problem is resolved to some degree by etching the surface with hydrofluoric acid before the chips are stuck on; however, this is not an adequate solution since the board becomes opaque. Such opaque boards can no longer be evaluated optically.

A solution has now been provided by a method and device for surface treatment, developed at the Institute of Microelectronics (by Ing. Martin Buršík, Ing. Jaroslav Jankovský and Ing. Michal Řezníček). The method consists in a surface treatment in which an interlayer of glass paste is applied before the chip is stuck on. Before it has had a chance to dry, the layer is powdered with a special dust of heat-resistant material using a spraying device. Drying and firing at a high temperature follow, during which the dust is trapped on the required parts, where it forms an opaque surface. Residual non-trapped dust is washed off, so that mechanical cleaning does not cause the surface to frost up and thus prevent optical testing.

schema testovaciho substratu

Figure: Scheme of the testing substrate
1 glass substrate
2 glass paste in the form of sleepers
3 chip

This method and device will guarantee up to ten times better adhesion for chips stuck onto the testing substrate, and thus allows tests to be repeated several times, which means both financial and time savings.

Both the method and device are protected:

Source of figure and photo: Institute of Microelectronics, Faculty of Electrical Engineering and Communication Technologies BUT

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