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Solder bridges on TQFP 100 part

Views: 629

#83185

Solder bridges on TQFP 100 part | 7 August, 2019

Hi I am having trouble getting solder bridges on TQFP-100 (0.5mm pitch) components.

The boards are pasted on semi-auto printer using laser-cut 0.12mm stencil and SN63 NC257-2 T4 paste.

The components are placed using Samsung CP40LV and tghen reflowed in Heller 1707 EXL oven.

Thanks

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#83201

Solder bridges on TQFP 100 part | 7 August, 2019

Are the apertures on the stencil reduced in width or 1:1 with the pads?

It depends on the pad width and foil thickness but we usually reduce aperture width for any part that results in an aperture wall spacing of less than 8.5 mils.

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#83209

Solder bridges on TQFP 100 part | 7 August, 2019

The aperture over the pads is set to 95% coverage of the pad. The stencil is 0.12 mm thick.

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#83210

Solder bridges on TQFP 100 part | 7 August, 2019

You should be well within area and aspect ration guidelines with that. Are you printing and placing close to center on the pads?

Sometimes operators will "bump" the parts to center them on the pads if they're not placed correctly, and if they've been placed to one side of the paste bricks they'll produce bad bridging.

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#83211

Solder bridges on TQFP 100 part | 7 August, 2019

I am doing another run today. I'll check how central the machine is placing the components onto the pads.

I wonder if the part is off slightly and smudges past across the pins?

What about if the part is being placed too deep into the paste?

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#83213

Solder bridges on TQFP 100 part | 7 August, 2019

Normally if the package is defined properly I would expect your machine to place it correctly but we have some parts pushed right down to the pads.

If the paste is centered, and the parts are centered, and you have obvious bridging potential coming out of the placement machine (paste from adjacent pads touching or nearly touching) then you either have too much paste thickness or the part placement height is squeezing it out from under the leads.

Are you seeing any coplanarity issues with these parts?

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#83214

Solder bridges on TQFP 100 part | 7 August, 2019

How would I measure/observe coplanarity issues ?

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#83218

Solder bridges on TQFP 100 part | 7 August, 2019

Most placement machines will do a check for that prior to placement and reject the parts.

I just wondered if that was something that you'd experienced while also seeing the bridges.

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#83226

Solder bridges on TQFP 100 part | 8 August, 2019

What is your pad width? What is lead width? Is the part gang masked, or has masking between the pads? Pads plated or HASL, if HASL how thick? What is board thickness and size? Does it need to be fixtured for support thru out the process. Semi auto screen printer, what's your registration method?

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#83229

Solder bridges on TQFP 100 part | 8 August, 2019

Did you solved this issue? I would recommend you track online station by station, until you detected the solder bridge, so you can narrow down the analysis scope and focus on potential causes.

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#83231

Solder bridges on TQFP 100 part | 8 August, 2019

The pick n place height for component was set to 1.4mm but I had not allowed for pin thickness. I added this (total height now 1.6mm) and I get very few solder bridges now. I might try 1.7 and see if it makes any further differnce

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