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I am using the smallest sized readily available lead paste I could find (15-25 micron size) and printing using a metal stencil with the smallest pitch being .5mm pitch qfns. Reflow is with a SMTmax oven, with max temp around 240C.

The problem I am having is leftover solder bb's, which can be washed off with denatured alcohol, albeit with difficulty. The joints seems secure, even though they don't look right. Adding flux and reflowing with my benchtop reflow doesn't reflow these bb's, even getting it much hotter than the oven.

Could this just be a bad batch of solder?

Is this typical of fine screen solder paste?

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    \$\begingroup\$ Smells like you got an old batch. \$\endgroup\$ – Ignacio Vazquez-Abrams Jun 26 '13 at 1:42
  • \$\begingroup\$ My experience with chipquick is the low temp rework stuff. I didn't know they made paste. I don't trust those joints at all. Rule 1 of soldering is that if it doesn't look right, it isn't. \$\endgroup\$ – Scott Seidman Jun 26 '13 at 1:44
  • \$\begingroup\$ I don't think they actually make it, but its semi-rebranded stuff from amtech solder. I know its not right, its annoying. Coarser grained stuff doesn't print right either. \$\endgroup\$ – Erik Friesen Jun 26 '13 at 2:30
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    \$\begingroup\$ It looks to me like your flux has been activated, but the solder hasn't gone eutectic. Either your temp controller isn't right, its not lead-tin, or the flux has gone bad. Can you check with a thermistor? Has this oven ever worked for any paste? Do the joints improve if you touch them up with an iron? \$\endgroup\$ – Scott Seidman Jun 26 '13 at 3:37
  • \$\begingroup\$ I can't check with a thermistor, but I know its getting hot enough, I am having rated parts that are melting at 250C setting, and the joints look the same. I have had better luck with other paste. \$\endgroup\$ – Erik Friesen Jun 26 '13 at 11:29

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