# How do you figure out how much paste you need?

morgan 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.

## Discussions

steverddrf wrote 09/18/2019 at 22:13 point

This sounds more like a college question than a practical hobby issue.

I would do it the easy way and import the BOM into a spreadsheet, set the dimensions for each type of pad per component and total the area column, multiply by the  stencil thickness to get the volume per board. A quick measure of a solder paste tube will give you the volume or you could simply fill an empty tube with water to the the level when it was full and weigh the water as 1L = 1kg and drug dealers type digital scales are dirt cheap on ebay / amazon.

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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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