How do you figure out how much paste you need?

morganmorgan wrote 07/31/2019 at 01:03 • 1 min read • Like

So I'm trying to figure out how much paste I need to order for an upcoming run and have just run up against a lot of sciencey sounding math and formulas that require info the datasheet doesn't provide. I made some assumptions, like solder paste having a Specific Gravity of ~4.14g/cm3. I then calculated the area of solder paste on my board, ~353mm2. And the volume for a 0.12mm thick screen, giving me 42.5cm/3. area / SG and this calls for ~85 grams of solder paste. Does this sound remotely reasonable?

In addition to this, how much extra should I account for? It comes in 50g or 250g and if two 50g will do I will be happy (and have ~$40 more on hand)

For reference here is the board, the highlighted part is an 0603 resistor with a 1.05x0.95 pad.



Andrey V ( wrote 08/06/2019 at 05:37 point

1mm = 10^-3m so, let's calculate paste volume for 1x1mm pad and standart 0.1 mm stencil. 10^-3 x 10^-3 x 10^-4 = 10^-10 m^3. One m^3 of pure lead has weight around 10^4 kg, so you will need 10^-10 x 10^4 = 10^-6 kg of lead for one pad. or 10^-3 gram for one pad. so with one gram of paste you can solder 1000/2=500 0603 smd resistors in theory)

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a.a.benn wrote 08/02/2019 at 05:26 point

No that sounds completely unreasonable. A tub of 250g solder paste lasts me hundreds of boards and months. You inadvertently converted between mm2 x mm = cm3. By your own calculations you need 42mm3 which is 1 gram. Under a gram, but you are going to waste/spill etc.

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