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QFP area solder paste shifted

Views: 2767

#51824

QFP area solder paste shifted | 18 September, 2007

Hi, good day to all. This is my 1st question post in this forum. Hope able to share some knowledge here. As we know QFP area like 0.4mm pitch was a very critical area to control especially those PCB using silkscreen printing process manufactured. Once during that PCB go through solder paste process, if the PCB pattern at QFP area was shifted around 0.1mm meaning that the solder paste almost cover half of the gap for the 0.4mm pitch.As a resulted the SMD component like IC after reflow was happend solder bridge. In this case, how do we able to control such kind of problem? or do anybody here face this kind of problem also before...

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#51829

QFP area solder paste shifted | 18 September, 2007

Welcome Jimmy Let's mince words to help focus on the issues. When you say "QFP area solder paste shifted," do you mean: * Paste printed on land pattern perfectly, but moves after printing. * Paste printed off the land pattern, even though the stencil was aligned perfectly with the pattern. * Paste printed off the land pattern, for someother reason, such as poor alignment between the stencil and the pads.

Next, what is the occurance of this off pad issue?

After that, tell us about the openings on your stencil for the QFP and the pads on the board for the QFP.

Finally, we're certain someone will want to know about your solder paste alloy and flux and the solderability protection on the pads of your board.

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#51832

QFP area solder paste shifted | 19 September, 2007

In addition to what DaveF said, I'd like to know if there are other parts on this pcb, and if the paste for those parts was printed accurately or not.

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#51834

QFP area solder paste shifted | 19 September, 2007

nevermind misread the post. But if the silkscreen is too thick, can you switch to another PCB manufacturer?

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RDR

#51836

QFP area solder paste shifted | 19 September, 2007

What is your alignment method for printing?

Vision?

Mechanical?

What is the advertised repeatability for your machine?

Russ

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#51846

QFP area solder paste shifted | 19 September, 2007

Hi,davef, thanks for your concern. The condition was the pasted printed off with around 0.8~0.9mm from the land pattern. This will easy observe at QFP area. At fiducial, the stencil seens aligned accurately.

Once solder bridge happend, its easy to repair by do some simple touch up.

Is it possible solder paste alloy and flux and the solderability protection on the pads contribute to this problem?

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#51847

QFP area solder paste shifted | 19 September, 2007

Those others parts also happend printed not accurate, but there is no solder problem. Only the QFP area looks bad condition.

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#51848

QFP area solder paste shifted | 19 September, 2007

Yupe, if condition keep worst, most probably will re-allocate the PCB manufacturer.

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#51849

QFP area solder paste shifted | 19 September, 2007

By vision. Could you elaborate the meaning for "the advertised repeatability for your machine"?

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#51850

QFP area solder paste shifted | 20 September, 2007

Jimmy So, it sounds like your stencil and board are made to dissimilar versions. Choices are: * Buy new stencil to match the pads on the board * Tweek position of stencil to better match-up with the pads on board, ignoring the fiducials. You may be able to do this by defining the pads a fiducials. * Do rework

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RDR

#51864

QFP area solder paste shifted | 21 September, 2007

this would be meaniung that somewhere in your documantation it should have spec called "repeatability"

what this means is that when the vision aligns the stencil/board, the board will always be within "x" amount of the njominal xy "T" position, some printers cannot hold and repeat to the .4mm pitch needs.

You need at least .05 min repeatability

Russ

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