Reflow TQFP without stencil?

[David]

what happen if its stored for a long time? is it evaporates? or become hard solid? cant it be diluted with some sort of alcohol?

It should be kept refrigerated to maintain its life. The shelf life is not particularly long and the flux will separate from the paste over time if left out.

[DJPhil]

what happen if its stored for a long time? is it evaporates? or become hard solid? cant it be diluted with some sort of alcohol?

It separates kind of like peanut butter that's been opened and left to sit too long. You can try to whip it back together, but it won't be the same.
Curious inventor has a good writeup with pics on using solder paste.

Here's a good shot of what the paste looks like when it's fresh. It's basically tiny beads of solder in a flux pudding.



Gravity and time will pull the beads down and filter the flux up, and if left open the flux will dry out. That basically leaves you with a tube of dried tree sap and metal. No good!
Refrigeration slows down both processes and, like peanut butter, can make an open container last several months.

Hope that helps, even though I'm just restating what others have already covered.

[mikeselectricstuff]

i use the smallest needle i can to dispense paste without damaging my hand or the syringe with the force needed to dispense it, and its usually slightly over enough.

You really need a compressed-air dispenser to avoid the whole finger-pressure  problem. Should be  foot-pedal operated - you could maybe even use a tyre foot pump as a low-tech version...

[charliex]

i use the smallest needle i can to dispense paste without damaging my hand or the syringe with the force needed to dispense it, and its usually slightly over enough.

You really need a compressed-air dispenser to avoid the whole finger-pressure  problem. Should be  foot-pedal operated - you could maybe even use a tyre foot pump as a low-tech version...

from the next line.
"I do have an automatic feeder that helps a lot"

http://www.zeph.com/zt-5100.htm
 

[.o:0|O|0:o.]

<snip> cant it be diluted with some sort of alcohol?

You could try acetone -- the industrial type, not nail polish remover -- which is also used for thinning two-part epoxy resins. Another option is to use epoxy resin thinner, a mixture of aromatic hydrocarbon solvents and aliphatic alcohols. Acetone is always good to have when working on PCBs.

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