Printed Circuit Board Assembly & PCB Design SMT Electronics Assembly Manufacturing Forum

Printed Circuit Board Assembly & PCB Design Forum

SMT electronics assembly manufacturing forum.


Edmund

#9313

water soluble mask | 26 September, 1999

i had a problem here, during the wave soldering process, we apply a water soluble mask on the gold finger to protect it from contamination. But after washing, there is still some mask left on the gold finger. Can anyone pls advise is there any parameter in the aqueous cleaner that i left behind. Thank You.

Rdgs.....

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Earl Moon

#9314

Re: water soluble mask | 26 September, 1999

| i had a problem here, during the wave soldering process, we apply a water soluble mask on the gold finger to protect it from contamination. But after washing, there is still some mask left on the gold finger. | Can anyone pls advise is there any parameter in the aqueous cleaner that i left behind. | Thank You. | | Rdgs..... | | Just in case it's not the cleaner, it might be you over-dried the mask making it just effective enough to do its job, but too dry to be removed during cleaning.

Earl Moon

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Brian

#9315

Re: water soluble mask | 27 September, 1999

| i had a problem here, during the wave soldering process, we apply a water soluble mask on the gold finger to protect it from contamination. But after washing, there is still some mask left on the gold finger. | Can anyone pls advise is there any parameter in the aqueous cleaner that i left behind. | Thank You. | | Rdgs..... | | I have used many WS masks over the years. It is always a tight-rope exercise between adequate masking and removal. To understand this, consider that the mask must be polymerised fairly well to withstand the scrubbing action of a wave, yet remain water soluble. In reality, none of them are truly water soluble, they are water-washable, which is not the same thing. To achieve this, the mask is usually a mixture/suspension of an insoluble polymer, such as polyvinyl acetate, and a soluble gum, such as gum arabic, in a suitable solvent system. To withstand the wave, it has to be cured to provide a molecule size of sufficient toughness. The action of going through the wave tends to overcure it. When it is washed, the water temperature has to be high enough to soften the gum sufficiently that the water jets can dislodge the mask and its adherent residues, without polymerising it further. Many cleaning machines simply do not have sufficient kinetic energy built into the water jets to successfully clean off these residues. My experience is that you need a minimum pump power of about 1 - 1,5 kW for each metre of spray bar at 55�C, assuming solid jets that are coherent up to striking the board, preferably at an angle, and this may take from 2 - 5 minutes for complete removal. It is better not to have a high pressure, low volume system: a medium pressure (2 - 5 bars) system is far more efficient for cleaning because the volume is much greater for a given pump power.

There are therefore 2 types of machine in which difficulty may be experienced: small conveyorised machines and "dishwasher" type machines. The former, because the wash compartment is too short to allow the PCBs to remain in it for the required 2-5 minutes. The latter because a) the pump power is usually inadequate (typically 0.3 - 0.5 kW for an aggregate 80 cm of spray bars), b) some of the kinetic energy imparted to the water is wasted by being used to turn the spray bars and c) the spray bars are not solid jets and are a compromise between the requirements for cleaning and rinsing.

I know many successful users of WS masks, but each has chosen the type and process cycle very carefully after suitable qualification trials with at least half-a-dozen makes and most of them use adequate cleaning machines whose price is typically $50k upwards for batch machines or $300k upwards for conveyorised machines (in other words, fully professional machines with a high performance). Most guys I know with lower-performance machines use peelable (latex) masks with the peeling done BEFORE cleaning (many such masks leave ionisable residues).

Hope this helps

Brian

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