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Volume of paste deposited with respect to board thickness

Alistair MacAdam

#28842

Volume of paste deposited with respect to board thickness | 1 June, 2004

I am currently looking at the volume of paste being deposited on a PWB 2.4mm thick with a 0.25mm tolerance. I am looking for information on PWB variation and a MPM AP25 series printer, and can the volume of paste be controlled on varing PWB thicknesses & if so how. The other end of this is does anyone have info or comment on how the varying thickness of a populated PWB, will affect the boards thermal profile.

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#28845

Volume of paste deposited with respect to board thickness | 1 June, 2004

Alistair,

your paste deposit is controlled by a few things: Stencil thickness Squeegee pressure Squeegee speed Print gap in 99 out of 100 applications for paste printing you would have a print gap of 0 i.e. on contact printing where the PCB is in full contact with the stencil. Your 0.25mm variation in PCB thickness would introduce a print gap of 0.25 at it's highest so you would potentially get that as additional height. In reality you probably wont, as your squeegee moves across the stencil you will press it against the PCB and shear it while the stencil is still against the PCB so again your basically on contact. What you will definately get is a feathered print where the edges are poorly defined and tailing at the front and back of the paste block that will apear like your scooping paste out of the block. you could try setting your PCB thickness to the nominal value of your range and limit the issue like that. In term's of the variation in thickness of the PCB vs the thermal profile, for my money I don't think so because it's the total thermal mass that counts here. The PCB variation is probably not totally due to variations in laminate thickness but variations in the bonding or layup process so you will still have the same amount of material it's just not as compressed as the next one.

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#28849

Volume of paste deposited with respect to board thickness | 1 June, 2004

We agree with John about his comments on the controlable factors relating to your printer, but we'd add that paste formulation and mesh are also contributors to nice looking bricks.

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#28852

Volume of paste deposited with respect to board thickness | 1 June, 2004

AP25 can handle warped boards to a degree. I also depends on the board clamping system on the machine. There are three avbl. First is Universal Vacuum box, second is dedicated tooling with vacuum and third is Y Snugger tooling that does not use vacuum. In general, all systems will accept boards that are warped within the IPC spec of 0.005" (0.127mm)per inch, which is what I believe is stated in IPC-600. Vacuum systems handle warped boards best.

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