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Imm Silver & Screen Printing

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Chunks

#43088

Imm Silver & Screen Printing | 31 July, 2006

I am printing convention leaded paste on onto Immersion Silver board. Board has 16mil fine pitch devices. I am having a problem getting the print to stick to board. Or I am having a problem with the stencil. Never experienced this with jobs before, but this is a new job, new board and a new stencil (and new stencil house!). My question is, do you guys do anything different to your stencils when printing on Imm. Silver? Stencil house is suggesting a different metal foil besides stainless.

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#43091

Imm Silver & Screen Printing | 31 July, 2006

Nothing like making a handful of changes all at once to confuse an issue, huh?

Did you (or whomever ordered the stencil) spec. the same stencil design parameters that you would normally have used?

We've never done fine pitch of any sort on silver so I can't help on that end, but the one board we've built on silver didn't seem to exhibit any less of a tendency to pull solder out of the apertures. I'm just wondering if you've got traps, electropolish, a decent aspect ratio, etc.

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RDR

#43092

Imm Silver & Screen Printing | 31 July, 2006

Never changed anytihgn when we switched to Immersion silver. Have run approximatelky 10,000 16mil pitch devices to date with immersion Silver PCB, stainless, chempolished 5 mil stencil, and pbfree paste (OM338).

Russ

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Chunks

#43094

Imm Silver & Screen Printing | 31 July, 2006

Hi Steve,

Yeah outta my control, till NPI tosses it over the wall.

Yes, everything was kept the same but we did go to a new stencil house. I'll try an ego busting move and ask if we can order a stencil from were we normally do if/when we respin the board (bat the eyes, show the rack, shake the butt).

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Chunks

#43095

Imm Silver & Screen Printing | 31 July, 2006

Hi Russ,

Wow, just Chempolished ah? These stencils are electropolished, so I would think they would release pretty easy. Ours are 6 mils, ut I don't think that would make a big deal though. I'm talking a nite and day diff between jobs. I guess a direct comparison between stencil mfgers is in order. Thanks for your replies gentlemen.

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#43113

Imm Silver & Screen Printing | 1 August, 2006

We never changed any thing after moving to imm silver.

"Poor release" (paste hanging-up in the stencil holes) could have three main (or combination of) causes: 1 Equipment * Proper aperture area ratio provide better release * Laser cut release better than chemical etched stencil 2 Process * Snap-off distance is too small. * Temperature in your screen printer needs to stable. Paste suppliers say that 1*C change can affect paste viscosity by about 10%. * Paste needs to be worked * Paste needs to be at room temperature * Stencil needs to be clean. * Fast separtion speed is best [you have good vacuum tooling to hold the PCB down, otherwise it will stick to the stencil] * Decrease print head speed to get the paste to stick to pads 3 Material * Solder paste viscosity is too high * Solder paste is too tacky * Solder paste particle size is too large * Some pastes stick more than others * Some flux formulations are very sensitive to humidity

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#43114

Imm Silver & Screen Printing | 1 August, 2006

Chunks, Make sure your aspect ratio is OK. Normally for an application like this, I would use a .005 foil. The foil material would be invar.The invar has better release characteristics.

Crappy solder paste can also cause release issues. I am not sure what type of printer you are using , but you can try to slow the release speed to aleviate the problem until you figure it out.

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#43115

Imm Silver & Screen Printing | 1 August, 2006

Hi Davef,

Could you elaborate more on faster separation speed is better of paste release. Any DOE have you performed before to prove that. My current paste printing practice is to make separation speed slow. Please advise.. rgds

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#43116

Imm Silver & Screen Printing | 1 August, 2006

It's been a while since I've built with 16 mil pitch, but I have to second the recommendation to go to 5 mil stencils. Night and day as far as release characteristics. The increase in aspect ratio opens up the window a lot.

(read = covers up a multitude of sins)

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#43121

Imm Silver & Screen Printing | 1 August, 2006

I would go with a 5 mil stencil as well, we experienced some similar issues with micro BGA's and the area ratio was suspect as well as the MESH size of the paste. We went to Type IV paste and verified our area ratio and everything ran beautifully. Pay close attention to your stencil and make sure when the stencil completes the stroke that the stencil is clean if not you need to change some parameters on your printer.

Good luck

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#43122

Imm Silver & Screen Printing | 1 August, 2006

I'm sure it'll work for someone's ego.

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RDR

#43123

Imm Silver & Screen Printing | 1 August, 2006

i'm not Dave F by any means but here is the deal with seperation speed.

Totally dependent upon paste formula.

I have two formulas of paste here (WS lead,NC lead and pbfee) and one we seperate as fast as machine wil allow (WS), while the other (NC) requires it slowed down significantly. They are not interchangeable and we must adjust for each type.

Russ

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G.P.

#43130

Imm Silver & Screen Printing | 1 August, 2006

I'm in 100% agreement with the 5 mils foil. In applications where there is less than 20 mil pitch actives and less than 0603 discretes 5 mils does great and increases your operating window by a ton it's less sensitive to release and print accuraccy. for Chunk's situation if the powers above let him purchase and spec out the new screen thatd be the way to go.

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