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Solder paste stencil life


Solder paste stencil life | 2 July, 2002

My question is refering to "stencil life". How accurate is this and is this something I can go by. The solder paste I am using is Kester R562, R562 specifications state that it has a 12 hour stencil life, optimal temp ranges 70-77 F, 35-65% RH. Our facility operates at a constant 75 F, 45% RH. I am running 5 machines with 1 automated screen printer, we are usually swapping stencils and scraping off the paste and reusing it. Are other people doing this as well or am I the only one? And at the end of the day, if there is any paste left over, is anyone out there saving this paste in a sealed jar and reusing it the next day? Another question is when a new solder paste tube is opened, how long before it goes bad? Is the stencil life basically the tube life after is has been opened?

Thanks in advance!!

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Solder paste stencil life | 2 July, 2002

FIrst, be a doubting Thomas and never trust supplier propaganda sheets, do your own testing. How often do you knead the paste? We have a standing requirement that if the paste printer is idle for 30 minutes, we have a minimum of 4 kneads before printing.

To test for stencil life in a recent solder paste evaluation, we let the paste sit idle on the screen for 0.5 and 1 hour. At those time intervals, 5 boards were printed and we checked solder volumes on the pads. All of the pastes needed at least 3 boards to get the volumes back up.

In the past we ran a study with paste left on the screen overnight, and with at least 5 kneads, we had no problems. Manufacturing didn't buy in to it especailly during the summer months (I'm in the sunny south with normal outside humidities of 90 - 95%). Some of the folks around here said in their previous jobs it was common practice to scrape off the stencil, put the paste in a jar, then mix it with new paste the next day.

Again, what works for us won't necessarily work for you. Set up a test matrix, execute it, and see what your results show you.

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Solder paste stencil life | 2 July, 2002

We normally disregard what the "tech" sheets say. A lot of it has to do with your printer settings, prints per hour, placement time, stencil cleaning procedures etc...

Just another wrench to throw in, We have noticed lot to lot paste differences also with three different suppliers. You need to do your own testing if you have the appropriate equipment. (volume measurement, etc...

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