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What caused this reflow issue?

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We had an order using a 2up PCB (same manufacturer) with sam... - Apr 14, 2008 by Mark M.  

deleted: ... - Apr 16, 2008 by Tony SMT  

#54375

What caused this reflow issue? | 14 April, 2008

We had an order using a 2up PCB (same manufacturer) with same components and same board layout without any problems. Perfect run.

The next order ran on the same day with same paste using same parts, same layout, same profile, but different 2up PCB had 4 - 15 bottom side transistors fall off during topside reflow (photo Q19 & Q21). This defect is present on every board of this work order, but not every transistor falls off. The location is random but only affects these transistor pads. There are other transistors of equal size on the board that do not fall off.

As you can see, the pads don�t have any reflow on them. When the parts fell off the board, the paste/solder went with the parts. The PCBs have several different lot codes, but every board has this defect. We thought it was a contamination issue on the boards but we ran several blank boards (no-pop) which had perfect solder reflow (photo Q27).

Note: No-pop units were ran as 1 of a 2up, the second unit being completely populated with same defects occurring.

The only apparent difference between the two jobs was the PCB.

However, when considering contamination, a no�pop board had perfect reflow. The profile was considered as well, but the bottom of the transistors had reflow and the other work order ran without defect. Another notable item is that the ground plane of this transistor and the pad it is reflowed to are about the same size. In other words you can not visually inspect for ideal solder fillets after reflow. We had also considered the paste was getting dry, but again, other transistor locations were fine and there were no solder balls anywhere on these boards.

Is it possible the PCBs are contaminated just enough that reflow is possible without a no-pop location as in Q27 but cause massive defects when there is a part above the pad? Keeping in consideration that the pad and the part landing are of equal size.

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#54377

What caused this reflow issue? | 14 April, 2008

I dont know if this is related, but there appears to be a blister/measle in the upper right corner of the Q19 large pad. (upper right of Q19 as you are facing the photo)

If your profile and paste is good, my bet would be that there is something wrong with the plating on the PCB. I do not see any evidence of wetting at all.

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#54378

What caused this reflow issue? | 14 April, 2008

Bottom-side SMT parts CAN and DO go liquidus during the 2nd reflow cycle. It could be that there was enough variability between the 2 different lot code PCBs to cause the parts to fall off. Remember wetting force determines surface tension (the ability for the parts to remain on the PCB during a 2nd reflow) and your wetting force is a function of the solderability of parts and boards, as well as the strength and activity of the flux in your paste.

Try turning down your bottom side heaters in the reflow zone by 10 - 20 degrees. Also, it'd be recommended to thermally profile the bottom side to see what's actually happening. Also, investigate whether the DPAKs exceed the weight in grams vs. solderable area (i doubt it...these looks pretty standard).

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#54382

What caused this reflow issue? | 14 April, 2008

Your problem is process you need to eliminate and there are so many causes to considered. 1.your solder paste 2.your PCB pads 3.your component 4.your profile 5.your printer 6.your placement. you need to isolate all these one of this is your problem.

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#54390

What caused this reflow issue? | 15 April, 2008

Contaniminates are always a issue.

What chemicals do you use?

Do you plasma clean?

Are the production personnel wereing finger cots to reduce added contaminates?

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#54391

What caused this reflow issue? | 15 April, 2008

If the solder had formed an intermetallic then it couldn't have pulled off completely like it did, it's obvious that the boards have a problem with their finish.

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#54394

What caused this reflow issue? | 15 April, 2008

We find it curious that: * Heat slug pad for Q19 didn't take solder, indicating a pad [board] solderability issue. * Signal termination pads for Q19 didn't flow well, indicating a poor thermal recipe or a pad [board] solderability issue. * Signal termination pads for the two resistors next to Q19 didn't flow well, indicating a poor thermal recipe or a pad [board] solderability issue. * 4 of 6 transistors show poor solder flow on the heat slugs, indicating poor solderability on the component. * It's a little more difficult to see, but we seculate that very few of the signal leads on the transistors took solder, indicating poor solderability on the component.

We think it unusual that both the pads and the component terminations have solderability issues. This seems to say: * Something is contaminating or cross-contaminating the pads and the terminations. * You are NOT telling us everything.

Have you compared the actual temperature on the parts that solder well and the parts that don't solder well?

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#54402

What caused this reflow issue? | 16 April, 2008

One issue stated incorrectly was that the work order that had no problems, with the same layout and same parts, is a one up at SMT. The problomatic units are two ups.

I know the first instinct is to say the profile, but we have run this assembly many many times without fault.

Becuase it was a bottom side, and the parts did not fall off until topside, we couldn't check the profile for bottom side.

Also, becuase the SMT lead was in a meeting the operators chose to run continuously while tweaking the topside reflow profile. They should have stopped but becuase of the missing SMT lead and mixed communnication they ran every board before the right people found out.

Wish I had more info but we are dealing with second hand knowledge of the specific details. The operators swear they ran the right profile.

I will get more photos for you davef. I'll get you good board and bad board photos. From each I'll get similar transistors that did not have reflow problems as well as a few random reflow joints from the affected areas.

I'll post more photos later today.

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#54403

What caused this reflow issue? | 16 April, 2008

Do you specify copper weight on your boards? This could be the difference between the the boards. Also the finish will be different between the boards as well. These two factors are enough to cause bottom side parts to reflow and fall off.

In either case, you can over come this issue by running lower bottom temps on your second pass reflow as CK has already said. I'd bet the house this will work.

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#54405

What caused this reflow issue? | 16 April, 2008

The problem is there wasn't any reflow when the board went through on the bottom side. Even if they didn't fall off becuase the temp was set low enough, while running the top side, the fact still remains that there wasn't reflow on the PCB pads to begin with.

So in the end I would have all my parts still on the baord but also still lacking reflow to the PCB but not to the compoenets.

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#54407

What caused this reflow issue? | 16 April, 2008

Per Tony SMT

Here are more photos:

Q15, Q16, Q17 represent good transistors on a 1 up board with the transistor locations that are in question.

Q13 is a good transistor on a bad 2up board. Right next to it is Q14 a bad transistor on a bad 2 up board.

The transistors equate to a 14 user part across 60+ boards with massive defect amounts around 35 to 45% across every board.

Now here is a kicker. When the reowrk was being preformed today an operator found one Q44 location of the 60 bad boards that fell off.

This one is a different transistor about the same size. What I don't know is if this was an issue of the operators tweaking the profile on the run or if this is encompassed into the problem at hand. 1/60 seems to advise me that this component is not part of the issue and is most likely someone "tweaking" something in the process.

NOTE: The product was run on different reflow ovens during the mayhem to ensure that the boards had the same defects on either oven.

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#54408

What caused this reflow issue? | 16 April, 2008

What board finish are you using? Is it the same on both boards? Were both boards manufactured at the same plant?

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#54414

What caused this reflow issue? | 16 April, 2008

And your screen print looks good?

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#54416

What caused this reflow issue? | 16 April, 2008

Are there any heat sink issues?

Re-version of boards with manufacture, miscommunication of boards bieng delivered?

Component manufacture defect allowing additional contaminate?

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#54417

What caused this reflow issue? | 16 April, 2008

This looks like it didn't solder the bottom side completely. The recipe for the reflow oven was not right and you did not hit a high enough temp at these locations to get a good solder joint.

Then you saw the problem when you soldered the topside. Changes were made then but the problem was already there. All of the changes made could not fix the initial problem which was on the bottom side.

Jerry

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#54419

What caused this reflow issue? | 16 April, 2008

Just a thought: if nothing else makes sense, you might want to watch the action of the edge-hold chains of the reflow oven, are they moving smoothly and uniformly around the drive gears and idlers? I've actually seen the situation where the chains were so dry and stiff that they would periodically bind up and jerk. A slightly raised pin would then physically 'grab' the trailing edge of the PCB and snap it forward hard enough to lose larger bottom-side components.

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#54420

What caused this reflow issue? | 16 April, 2008

[quote]Just a thought: if nothing else makes sense, you might want to watch the action of the edge-hold chains of the reflow oven, are they moving smoothly and uniformly around the drive gears and idlers?[/quote]

As a PM provider and Man. Tech. for our company, this is a valid point. The machine itself can be the problem.

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#54421

What caused this reflow issue? | 16 April, 2008

I think you are close to the problem when you expect the pcb having a problem. When I look at your different pics I saw that your via holes looks like this pcb has a hasl finish... and when i look at Q 14 this picture tells me that the pcb is not an hasl but it looks like an immertion white tin.

If the pcb is an immertion tin type of finish then the problem you have is that pcb has less 40 micro inches of white tin as specified in IPC 4554. This problem leads to copper migration and mixing with surface you end up with wetting problems at the surface.

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#54423

What caused this reflow issue? | 17 April, 2008

I have seen similar things like this before with Solder Resist redepositig back on the pads, this is a clear residue very hard to see. Solder resist will obviously resist solder adhering to it. If you rework the boards then the heat of the iron will help shift it. The Paste probably falls away with the component as this is the path of least resistance and wets much easier to the SMD than PCB with resist residues on the pad. It has a better chance of 'sticking' to a non placed pad as it has a greater wetting action to try to wet the pad and has to physically fall off which is difficult. This occurs when PCB manufacturer has poor ventilation during curing and the resist vapour redeposits back on the circuit. The bleeding back onto the pads at the edges of the wells where you see dewetting is similar but this is due to the residue from the resist liquifying in the HASL process and dragging back over the pads when being lifted out of the solder pot. This is especially bad when used with Lead Free HASL finish due to temperature of dwell and time in pot. Trying prewiping the PCB with Stencil Cleaning Solvent to see if this removes the problem Hope it helps Greg

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#54427

What caused this reflow issue? | 17 April, 2008

Our quality types wouldn't allow: Q16 on "1up good board2.jpg", Q17 on "1up good board.jpg", or Q15 on "1up good board3.jpg". They'd probably pass Q13 on "Q13 next to Q14 no-reflow.jpg", if it wasn't part of a bigger issue.

Again, you appear to have solderability issues with both a component and a board, which is peculiar. * Do bare boards at incoming solder well? * Do parts at incoming solder well?

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#54430

What caused this reflow issue? | 17 April, 2008

I'd suspect contamination also, possibly from the board supplier. Sr. Tech's post mentioned what looks like a blister on the board, in the upper right of the Q19 photo. This could very well be a contaminate from the PWB fab shop. I've seen similar problems caused by the PWB lamination process. Once the boards are subjected to the reflow process they want to outgas any solvents or uncured laminate resins that may not have fully cured during the fab process.

I'd also suspect other contaminates possibly from operators or inspectors. You may want to clean these PWBs before applying solder paste. The fact that there is no evidence of solder wetting or even flux residue on or around the Q19 heat sink / ground pad is a pretty good sign that the pad was isolated so that solder couldn't adhere to it. This is assuming that the board has not been cleaned. There appears to be a small amount of flux residue on the top right Q19 pad.

Another unknown is the board finish. Photos sometimes make it hard to determine what finish is being used. It almost looks like white tin which is notorious for oxidation in the right conditions. This will prevent the pads from wetting also.

Solder reflowing in other areas and to the parts are a pretty safe indication that the profile is probably not the culprit. Even if solder does not fully wet to the pad edges as in some of the photos that can be a result of pad contamination, paste volume and flux chemistry. Molten solder is just like electricity. It will take the least path of resistance so if the surface tension is high wetting across the entire pad surface can be minimal but still acceptable.

If available you may check with your QA folks to see if this PWB supplier has any history of similar problems.

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