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Part sliding in reflow?

Kris W

#5988

Part sliding in reflow? | 17 April, 2001

Howdy all!

Nothin' up my sleeve!!! While attempting to reflow an RF antennae on std FR4, HASL coated, using no clean paste, in a Research Thermaflo 10, the part sometimes slides on one of the pads (as much as 0.125"). There is a slight amount of warp in the PCB during reflow. Every array is inspected via AOI before entering the oven.

In a related note... there is a small diode placed near the antenna that disappears during reflow. I assumed that the diode (SOT-23 style leads) did not have enough surface contact with the solder paste to withstand the force of the convection blowers. With that in mind I have tried reducing the fan speeds on the oven to reduce the velocity of the convection air.

The antenna is a u-shaped, blunt cut copper antenna plated with tin-lead. It is approximately 1.25" long, 0.25" wide and about 0.030" thick. I would estimate 1 of every 10 antenna placements moves during reflow.

Do you think that altering placement force to push the part further into the paste might help? I'm looking for anything that may help with this issue!

Thanks in advance!

Kris W.

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#5993

Part sliding in reflow? | 17 April, 2001

So, we�re talkin� �bout the smaller end of a "large" paper clip after it�s unfolded once, eh?

In keeping with your "nothin� up the sleeves" full disclosure, I�ll bet you�re feelin� a little sheepish about not tellin� us about: * Antenna pad dimensions, quantity, and location [relative to the antenna] * Results of the reduced blower volume on the SOT23 near the bent-up paper clip thingie * Effect on reflowing other SOT23 packages * Reflow profile, other than letting us fantasize what it would be using a NC paste

I hear you talkin� �bout the blowers pushin� the antenna off its pads and that makes sense [even tho you didn�t tell us, the bent-up paper clip thingie has to be soldered with two pads at the top of the U shape, because the pads have to be small, the U would the relatively long, the U would not be secured at the bottom of the U] because:

1 Bottom of the U acts like a long lever that pivots at the top of the U.

2 Alternately, the "arms" of the upper part of the U could be spreading apart during heating and then closing after the reflowed solder has cooled.

But then I hear you talkin� �bout the blowers pushin� every tenth antenna off its pads and I wonder why every tenth? Why not all of them? Yano, �cause the fans are turned-on, crankin� and doin their thing aren�t they? Could it be that

1 Blowers don�t blow at a constant volume, even though they are set at a constant rate, because they get behind [or ahead] on cooling , then need to crank up [or down] the volume every ten boards? [Didn�t someone like Glenn Campbell do a song about "cranking-up the volume"?] So as the oven compensates by firing-up the blower ever ten boards, it then blows the antenna from its pads. Coo, eh?

2 This could still be explained by the thermal expansion theory [#2 above] if the oven gets real hot to spread the legs [hey, hey, this is a family show, watch it!!!] of the U, get poor reflow bla, bla, bla AND just after that the fan kicks it up a notch [as Emeril says BAM!!!], but not such a high notch to dislodge the paper clip thingie on the next board.

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MW

#5997

Part sliding in reflow? | 18 April, 2001

I some something like this. My package was not flat the belly of the body rested on the PCB. We found that every 4-5 part would wander to the side. What was puzzling is that the blowers would have blown it the opposite side. ultumately we proved the flux was getting under the belly and moved the part as the flux spread. We eventually fixed the problem by changing the part.

Good Luck

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Kris W

#6034

Part sliding in reflow? | 20 April, 2001

Hi Dave,

Good to "talk" to ya again!! I have attached a photo of the antenna. Hopefully it will work its way through the system. The pictured antenna meets all our criteria. The defective antennae all seem to slide toward the center of the PCB (toward the viewer) at the leg shown on the right. Actually, I can't figure out how to insert a photo, so I'll e-mail it to you.

This morning, the issue came up and affected as many as ten arrays in a row (4-up in-line array with the antennae at the fixed rail side). We've tried rotating them 180 degrees with no visible improvement. I reduced the fan speeds on the top blowers an additional 20% which netted 20 arrays in a row without the defect. Profile still looks really good. Antennae are reaching 219C and are above liquidous for ~52 seconds.

I have identical lines elsewhere in the facility that are running very similar products with the same model numbers for the reflow equipment, and have never seen this sort of defect.

Once again, thanks in advance!!

Kris

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#6039

Part sliding in reflow? | 20 April, 2001

Good to talk with you too.

Oh, just the way I had it pictured!!! Yeh, right!!! [http://davesmt.50megs.com/pictures select kristenna.jpg ]

Which part of the antenna "sees" heat first [Are all legs (vertical part), two legs, or one leg of the antenna is on the leading edge of the board?]? When you say "with the antennae at the fixed rail side", what part of the antenna move parallel to the rail as the board are conveyed? Are the part on all boards in a panel symetrical? Or are some mirrored?

When you say "I reduced the fan speeds on the top blowers an additional 20% which netted 20 arrays in a row without the defect." ... Does that mean everything is okeedokeefine, er do we still have an ugly problem? Er, are you tryin to crank-up the line, er what?

From which rail do yer blowers blow?

What do you mean when you say "I have identical lines elsewhere in the facility that are running very similar products with the same model numbers for the reflow equipment, and have never seen this sort of defect."

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reflow oven profiler

Reflow Oven