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Fighting solder beads

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#40512

Fighting solder beads | 18 March, 2006

Good morning,

Once again a met annoying solder beading issue while producing board with PLL (leadless, contacts from the bottom side of the package). Board has very high placement density and beads appear near PLL package only. This package is finepitch one (pitch = 0.5mm). Please find geometry and process parameters below:

Pad size: 0.508mm x 0.254mm Aperture size: 0.482mm x 0.229mm (12.5um reduction from each side) Solder paste type: type 4, particle size: 20um-38um Squeegee pressure: 5.4kg Squeegee speed: 50 mm/sec Separation speed: 20 mm/sec Stencil type: nickel, electroformed Stencil thickness: 125um Board finish: Ni/Au

Fist of all I checked screen printing process. I found only one issue with screen printing. Aperture becomes totally blocked after first-second print stroke. Amount of deposited paste is enough for proper reflow, but due to this fast pint form deviates from ideal brick. I found no issues with board to stencil matching. Squeegee pressure, squeegee speed and separation speed are adjusted correctly. Decreasing separation speed makes print quality the same or worse, apertures at that still remain blocked.

One more thing I noted is solder mask opening. It is rather small (slightly larger that pad) and I suspect that solder paste can easily get to solder mask causing beading. Am I right? I need your feedback. Thanks in advance.

BR, Pavel

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#40516

Fighting solder beads | 19 March, 2006

hi

same sort of broblem v r getting after reflow. i am not sure about the solderbeads how u can reduce, but the other issue u r talking about is apeture blocking.... u can reduce it by reducing the seperation spead. reduce it as low as 1-2mm/sec. this may reduce atleast your this problem.

thanks

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#40517

Fighting solder beads | 20 March, 2006

Good morning,

In this case separation speed doesn�t affect print quality. Separation speed decrease makes print quality the same or worse. Apertures still remain blocked. Any ideas? Thanks.

BR, Pavel

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#40530

Fighting solder beads | 20 March, 2006

Pavel

Point: You suspect that solder paste can easily get to solder mask causing beading, because the aperture is larger than the component pad. Comment: Probably, this is incorrect. We overprint on solder mask all day long when doing paste-in-hole reflow soldering.

P: Your apertures still remain blocked. C: Consider checking the aspect / area ratios of your apertures for this component.

P: You met an annoying solder beading issue while producing a board with PLL. C: We suspect this is a reflow recipe issue as much as it is an excess paste issue. If you spike too sharply, it can leave small portions of solder on the edge of the solder paste brick seperate from the majority of the solder mass when the mass reflows. Some pastes are better at leaving solder beads at the periphery than others. [you can compare the tendancy of pastes to bead by printing a disk of paste, maybe 0.25 inch, on thermal glass and the reflowing the paste.]

Excessive pick and place pressure can schmush the paste between pads and cause bridging and beading is another possibility.

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#40531

Fighting solder beads | 21 March, 2006

Good morning Dave,

Here are some of my assumptions. Please correct me if I am wrong.

1. Mask opening. Due to mask to PCB misregistration mask is not always centered relative to pad center how is should be. Sometimes edge of the mask matches with edge of pad causing printing paste on mask. In worst case paste brick has 25-50um shift. PLL has leads from bottom side only. Hence, during placement paste can easily get to PCB mask forming bead during reflow (I suppose that paste can not get back to pad due to segregation).

2. Stencil design. As mentioned above, aperture has next geometry: 482um x 229um (electroformed nickel stencil, 125um).

Area ratio = LxW/2x(L+W)xT = 482x229/2x(482+229)x125 = 0.62 Aspect ratio = W/T = 229/125 = 1.83

If aspect ratio and area ratio are not violated paste brick must have shape close to �ideal� one. Am I right?

3. Reflow recipe. Beads are localized near PLL package only. Hence I don�t think that reflow recipe can cause beading. Other finepith components and chip components do not have beads. Spike ramp is 1 deg/C.

4. Placement pressure. Last thing to check.

Due to aperture blockage paste bricks have irregular form. Probably paste bricks are even higher than 125um, but I can�t check it (have no measurement tools). Can uneven brick shape contribute to beading?

Thanks in advance.

BR, Pavel

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#40534

Fighting solder beads | 21 March, 2006

If you usually get good release on other products with your paste, I would try a different batch of paste. The batch you are using may have a problem that contributes to the beading. Jerry

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GS

#40538

Fighting solder beads | 21 March, 2006

Ahoj Pavele,

di you considered to try a Type-3 solder paste ?

Best Regards...........GS

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#40542

Fighting solder beads | 21 March, 2006

1 Chatter about mask opening � No BIG problems with these statements. It�s just that you may need to tailor your reflow recipe to minimize these escapees.

2 Stencil design - Any area ratio below 0.66 is going to very challenging to print, as you can certify.

3 Reflow recipe � We hear you, but your PLL is a mess, unlike your other fine pitch stuff.

Other: Can uneven brick shape contribute to beading? - Sure. Protrusions of paste from the brick will tend to reflow earlier than the brick [because of lower mass, more surface area, etc] and tend to pull away from the brick.

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#40561

Fighting solder beads | 23 March, 2006

Good afternoon,

We can use type-3 solder paste, but what for? Type-3 solder particles are larger than type-4. Probably this will cause even worse release.

BR, Pavel

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Joe

#40565

Fighting solder beads | 23 March, 2006

Check placement pressure. I agree with the "smushing" theory. If you can place the component with less pressure, I'd do that first. A switch to Type three paste is also a good idea, as long as your minimum aperture dimension remains at 0.009" as you state, this dimension is fine for type three. But you will still see aperture clogging. Stick with the e-formed stencil. For better release, I would go to an 0.004" thick stencil (yours is 0.005"). If solder volume is an issue with the thinner stencil, move your aps back out to the full width of the pad. Type three is always preferred to type four if you can get away with it because type three has less particle suface area/volume, ie: less surface oxidation/volume. Surface oxidation inhibits the paste from coalescing at reflow.

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