Howard, I believe there are many pro's and con's to re-using solder paste. One of the biggest pro's are cost. Solder paste is expensive. Another pro is it's just plain easier to scrape the paste off of one stencil and slap it on another rather than taking the time to "auto dispense paste" , use a dispense gun, or scoop it out of a fresh jar. It's just one less step in setup of the printer you have to do. One of the biggest con's I think is the fact that no matter how long the paste is on the screen it will dry a little every second it is exposed to air. What would happen if an operator used 3/4 of a jar/tube of paste to run 100 boards with 50 components per board and it took them 7.5 hours? I think in cases like that you should definitely scrap the paste and start with fresh paste on the next run. The biggest problem is how do you monitor exposed paste. I strongly believe that four days is far too long to be re-using solder paste. My previous company I worked for stated in the "AI" that paste should only be re-used at a maximum of 72 hours. Three days was stretching it a little I thought, Would you eat cup cakes that the batter was exposed to air for three days before they were finally cooked? He, he, :). To make a long story short, if you can find a reliable way to monitor paste consistently in terms of how long it has been on the screen compared to how many prints were done. I would follow the manufactures recommendation, if it says eight hour stencil life, I would compare how long the paste was on the screen compared to how much fresh was added throughout the day. If very little fresh was added I would scrap the paste. As far as your mid-chip solder ball problem, I believe humidity is a minor problem that adds to a more wide spread situation. Something you should look into is the screen design. One thing to look at is the aspect ratio. I had the exact same problem at my previous employer, we had mid chip solder ball problems practically every day. What was strange was the problem began occurring just one day out of the blue. Nothing in our process or environmental conditions changed. Eventually I did some investigating and found that too much paste was getting deposited on the PCB. Another thing I noticed was the settings or parameters on the various SMT equipment we used. I found that for example, our "Quad" line, QSP-2 & IIC produced a much higher amount of solder balls. Reason is I believe was the Z-placement. Our other line, the MyData line, produced fewer if any solder ball problems. I found if you could get the "Z" set right on the quad line and the part thickness correct on the MyData line, mid-chip solder ball problems were far and few between. I hope some of this helps. If you have any other questions feel free to contact me offline. I am rather curious on how you determined 500 PPM post reflow. Bill Davis Project Engineer Thermo Assembly & Test 970-242-5593 --------------------------------------------------------------------------------- Technet Mail List provided as a free service by IPC using LISTSERV 1.8d To unsubscribe, send a message to [log in to unmask] with following text in the BODY (NOT the subject field): SIGNOFF Technet To temporarily halt delivery of Technet send the following message: SET Technet NOMAIL Search previous postings at: www.ipc.org > On-Line Resources & Databases > E-mail Archives Please visit IPC web site (http://www.ipc.org/html/forum.htm) for additional information, or contact Keach Sasamori at [log in to unmask] or 847-509-9700 ext.5315 ---------------------------------------------------------------------------------