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Reflow Profile Design

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Hi, This is a little embarrassing asking this question, a... - Jun 15, 2007 by Grant

What is the board finish? Did you profile built assy and ge... - Jun 15, 2007 by cman

Hi Grant, 1st off solder paste data sheets are just "guid... - Jun 15, 2007 by Dirk Nuendyke

Hi, Thanks for the reply guys, and this is interesting in... - Jun 15, 2007 by Grant

Hi Grant, it seams your PCBs are ENIG (Gold)finished, cor... - Jun 16, 2007 by GSx

Hi, We are using emerson gold, and I have not seen any ev... - Jun 16, 2007 by Grant

Grant, i have heard of a few cases where the PCB is "rewo... - Jun 18, 2007 by Dirk Nuendyke

If you are using a fixture/carrier in the oven, make sure th... - Jun 18, 2007 by SWAG

Hi, The problem seemed to be across multiple batches of b... - Jun 18, 2007 by Grant

I have heard that with lead free soldering, the cooldown pha... - Jun 18, 2007 by Dirk Nuendyke

Hi folks. If the solder is on the BGA but is not soldered a... - Jun 19, 2007 by Manuel_R

Grant

#50684

Reflow Profile Design | 15 June, 2007

Hi,

This is a little embarrassing asking this question, as I think I am profiling our oven incorrectly.

I have been using the profile based on the solder paste specification, however we have been getting a large BGA chip coming more unreliable, and breaking off the board after shipment. It's a clean break, and often there is clean hold pads under the part after it's broken off, so the solder does not look like its adhering to the pads correctly, even though the product tests ok.

We have run our profile specification window 6 degrees hotter in the reflow part of the profile, above what the solder paste specification has, and it seems to have fixed the issue.

However I assumed if I probed the parts, and they achieved the temps based on the solder paste I am using, it should result in good joints. Am I wrong here, as that does not seem the case, and there must be another reason the joints are not as good.

I even purchased two profilers to ensure we had something to compare against, and they are within 1 dec/c of each other, and we are using high temperature thermocouples. The paste is Koki which has given very good results in the past.

However for our most complex products, it seems we need to juggle the profile to get lower failure rates, independently of the paste specification.

Is this normal, am I doing this right, as if this is normal then it would be good to know what would cause a profile that's done according to the paste specification not to solder correctly. I seem to be missing some kind of knowledge here, which is freaking me out a little. One of the limits of learning all this by myself I guess. It leave gaps sometimes.

Any advice would be great.

Regards,

Grant

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cman

#50685

Reflow Profile Design | 15 June, 2007

What is the board finish? Did you profile built assy and get the temps on the board with a t/c in the board? DId you drill the BGA and get the t/c at the ball? When the parts came off the board did all of hte solder come off the pads and stay with the part? Did the BGA ball colaps? Is this a ROHS appliction? if not is the part ROHS.

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Dirk Nuendyke

#50686

Reflow Profile Design | 15 June, 2007

Hi Grant,

1st off solder paste data sheets are just "guidelines". You always need to set your specs. based on the needs of your own process and product. Don't ever follow your solder-paste data sheet to the tee, or you're severely limiting yourself.

2nd, you're on the right track increasing your peak temperature in the reflow zone. If you have a backward compatability scenario ( SAC or RoHS BGA in a tin-lead process), you definitely need to run hotter regardless, especially your time-above-liquidus and peak temperatures. Search the SMTNet archives, keywords "hybrid profile BGA's". There's lots of good information in there.

3rd, the most accurate way to profile a BGA is right at the ball. If you can sacrifice a PCB, drill a hole right at the pad of the BGA in a couple of places (middle of the device and edge), place a BGA using your BGA rework machine, and then when you're ready to profile, insert your T/C directly into the hole and cover up the hole with a small dab of SMT glue or aluminum and Kapton tape.

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Grant

#50694

Reflow Profile Design | 15 June, 2007

Hi,

Thanks for the reply guys, and this is interesting information. I had assumed that if we had a paste spec, and this was the spec for the solder to reflow correctly, then if I achieved that spec, the solder should perform correctly. However this seems not the case. Oops!

For some more info, we are using SAC solder, lead free and the BGA is very large with balls around the outside only, which makes it less mechanically strong, and prone to high stress forces around the BGA edge when the PCB is warped in any way.

We are profiling by using high temperature soldering of the thermocouple to the BGA ball via a hole in from the bottom side of the PCB. I am not sure if we have covered up the hole on the rear and I am going to check that. (I am wondering if this could be showing the thermocouple hotter than it is by heat energy leaking in via the hole from the back side.)

We have a whole panel of product we ran and then thermocoupled that board, so we should have an accurate simulation of a real product running. Everything is RoHS.

The failure was a clean snap of the BGA part off the PCB, with all the solder on the BGA itself with the pad on the PCB clean (gold) without any solder on it. This is what made me think we must be too cool on the profile, as the gold should be consumed in the joint, and to see the gold pads exposed like that worried me.

However we have been distracted and thought there was other causes of the failures because when I looked into the problem the guys said they had dropped the profiler in the oven at one time previously, and did not know if it was still accurate. It was annoying to find that out just when you needed it to solve a problem.

We then questioned the accuracy of the profiler. So to eliminate this from ever being a problem again, I purchased another identical model profiler, so we would have a "standard" one, and the one that's used. Then we could check them any time. However this was not an issue, as they were both matched within 1 deg/c, so we knew we had an accurate profiler.

We also have only a 5 zone oven with passive cooling. This was really struggling with RoHS and the boards we do. Some boards have over 2500 parts per board in a very small space.

We have been working that problem for about 8 months building a new factory, so there was a thinking that things would just become better once we had the new oven up and running. We are just about to move into a new factory on Monday that's got the space for the shiny new 10 zone oven with active cooling.

However when I saw the way the BGA's were breaking off in customer returned product, I realized this was not the cause, as we should not be getting the clean break like this if the solder had melted into the PCB pad correctly, and I realized something else was wrong with our profile itself.

It's funny how sometimes these issues can be caused by multiple problems at the same time, and these issues were important, but distracting us from a more simple and obvious problem.

It was simply our procedures, and my personal knowledge of how to reflow profile a product. I embarrassingly realized I had something wrong with the profile itself. I don't have a process engineer, so have to do all this myself, and sometimes I miss things because I have to run the company, and cannot spend enough time thinking about the issues in production as I should.

This is why after experiencing this, I though it was a good idea to post on SMTnet, so I could get peoples advice because in these things I am a bit isolated. I also have questions about reflow profile development, as I am obviously missing some knowledge here.

I will check out the archives for Hybrid Profile and see what shows up. Thanks for the guidance.

Regards,

Grant

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GSx

#50696

Reflow Profile Design | 16 June, 2007

Hi Grant,

it seams your PCBs are ENIG (Gold)finished, correct?

If yes, did you considered the quality of PCB, for istance are you sure are there any black pad evidence on BGA pads?

When BGA drops away like that, one of the potential cause can be also the quality of ENIG finisching .

GSx

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Grant

#50697

Reflow Profile Design | 16 June, 2007

Hi,

We are using emerson gold, and I have not seen any evidence of black pad, as the pads are clean and gold colored when the part comes off. That is what worried me, as I would have thought that if the solder melted correctly, it would have stuck to the pad better than this, and taken the gold with it if the part was sheered off?

Regards,

Grant

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Dirk Nuendyke

#50718

Reflow Profile Design | 18 June, 2007

Grant,

i have heard of a few cases where the PCB is "reworked" by the PCB house, where the immersion silver that did not wet to the base copper was reworked by just putting immersion silver on top of the non-wetted immersion silver.

I know your PCB is ENiG, but this may be a possible scenario as well.

just throwing it out there.

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SWAG

#50719

Reflow Profile Design | 18 June, 2007

If you are using a fixture/carrier in the oven, make sure that it is not too restrictive on the perimeter of the board/panel. A carrier intended to limit warpage can actually cause warpage if the expanding PCB material has no place to go during reflow.

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Grant

#50722

Reflow Profile Design | 18 June, 2007

Hi,

The problem seemed to be across multiple batches of boards, so I think they were made correctly, and we don't use a carrier in the reflow oven.

However we just moved our equipment to the new factory yesterday, and connect and start turning on things tomorrow. It's exciting!

The new oven we have haas active cooling, compared to passive cooling for the old oven. Does anyone have any suggestions on how our profiling process would change? Do you just cool has fast as you can, or is there a recommended cooling speed for a product?

Cannot wait to turn everything back on again in the new building!

Regards,

Grant

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Dirk Nuendyke

#50724

Reflow Profile Design | 18 June, 2007

I have heard that with lead free soldering, the cooldown phase is NOT as critical as it is for Tin-Lead.

-2�C per second cooldown is good enough to form a good solder joint with good grain structure.

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Manuel_R

#50749

Reflow Profile Design | 19 June, 2007

Hi folks. If the solder is on the BGA but is not soldered at all on the pads, then you can discard BGA solder issues, by the other hand; you might be experiencing wicking due to heat is being absorbed on component so the balls on BGA are hotter than PCB pads. I know that is hard to measure temperature on lower side and higher side of the BGA ball, maybe you can try increasing bottom side temperatures on the zones that are affecting preheat+soak (below 217�C) to try to get more temperature to pads below BGA. The lower the Cooling, the higher the intermetallic layer is resulting in brittle solder joints, if a fracture become you can observe solder in the pads and also solder in the BGA balls.

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