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Reflow PBGA

Hi everyone. I am now faced with a problem on soldering PBGA... - Jun 25, 2002 by

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Steve L

#20487

Reflow PBGA | 25 June, 2002

Hi everyone. I am now faced with a problem on soldering PBGA. At first, I ran the board (FR4, 7" X 10" with 12 layer) at Peak Temp: 205 C and 54 seconds above 183 C. After reflow, I found several cold solder joints and one ball without second collapse. So I slowed down the belt speed to 36 and used same temperature setting (Peak temp: 208, 62 seconds above 183 C). After inspection, I found lots of BGA balls sitting on the pad - solder not reflowed. I applied flux to the PBGA and reflowed the board using the same profile. This time, the solder reflowed, but most solder ball looked Non-uniform, rough, and grainy. We have a 5 zone reflow oven (Forced Convection) from Heller. Could anyone be able to give me any suggestion on this PBGA soldering? Appreciate your help.

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Daan Terstegge

#20488

Reflow PBGA | 25 June, 2002

Hi Steve,

To my opinion your peak temperature is a little low. If you are sure that the BGA (especially the balls under it) reaches this 205 degrees then it should be OK, but if you measured somewhere else on the board then it's probably better to raise the peak temperature. I'd say you need about 235 degrees C on the hottest spot of the board, which equals to perhaps 210 degrees underneath your BGA. This delta T depends on the oven settings and the oven itself.

Daan Terstegge http://www.smtinfo.net

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Dason C

#20489

Reflow PBGA | 25 June, 2002

Is the paste fresh or not or exposed to the floor for a longer time. What kind of the paste which you are using?

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ianchan

#20493

Reflow PBGA | 26 June, 2002

Steve,

I'd second Daan's opinion.

BGA profile we use gets a peak of 210deg-C to 223deg-C (but that's using our WS paste unique characteristics) with reflow zone holding time of 45-60sec. We had 6 PBGA and 2 ceramic BGA on a 6 layer PCB.

typical profile we see gets a 230deg-C peak reflow temperature. Check with your paste supplier, for their lab qualification settings during the conceive period of your paste type's reflow profile.

Check whether your paste reflow can go to 230deg-C? that shd give your PBGA around an ideal 218+/-5 deg-C beneath it.(from our experience, go do your own tests)

your oven delta-T capability will also influence this reading, just as the connection of your (K-type?) thermocouple linked to the BGA balls you are profiling (you are profiling from the BGA central ball ain't you?).

Oh! Oh! and to sum it all up, go check the fine SMTnet archives.

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ianchan

#20494

Reflow PBGA | 26 June, 2002

Steve,

Dason has valid points here.

dun listen to what supplier's say about paste lying on the PCB pads able to be exposed at ambient temperatures for 8-24 hrs? that will never get approval for Class-3 type of customer auditors. (Would any serious process owner leave printed boards lying around in the shop-floor for that amount of time, and not process the boards thru the oven?)

Know your machines and know your materials, and all the poor soldering joints can be solved.

We control our WS paste no more than 6hrs exposure time in the stencil. Labels on the paste-Can, assist in the FIFO control and traceability if need be on the Can-opening time.

check your supplier for B&W instructional specs, not some verbal promise in performance.

Good Luck!

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Steve L

#20502

Reflow PBGA | 26 June, 2002

Thanks for all of your valuable inputs.

The paste we are useing is PWS0111Ae36 (Alpha MEtal), peak temp from 210 to 225 C. The paste was on the floor about 3 weeks.

ianchan, when you mentioned "no more than 6 hrs in the stencil", you mean no more than 6 hrs in stencil at one time or that 6 hrs is the total stencil life for paste.

According to our oven manual, The maximum differenctial betwee the highest and lowest temperature on the PCB should be less than 20 C. Is that the Delta-T capability? And forgive my limited knowledge, where is the fine SMTnet archives? http://www.SMTNET.COM?

Thanks again for your help.

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#20504

Reflow PBGA | 26 June, 2002

Go to the top of the page and enter your topic in the "Search SMTnet" window. It can take some tweaking to get it dialed in to the exact topic you want.

Do you mean the paste was on site for 3 weeks, or was it actually out of the fridge for 3 weeks? We try to keep our "out of fridge" paste quantities at about 2 days worth because the room temp shelf life is about 1/4 the refrigerated life.

I couldn't find anything on the Alphametals site on your paste, but if you leave containers out of the fridge for three weeks, or leave your containers open all day, or leave it on the stencil all day (or on boards without reflowing for that matter) I think you're asking for trouble.

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#20505

Reflow PBGA | 26 June, 2002

Steve,

We do not currently run BGAs, but I have been told by a CM we use that does BGAs, that our Heller 1500 (5 zone) oven has an insufficient number of zones to properly reflow a BGA. Anyone else have an opinion about this it would be appreciated as we are looking into processing BGAs in house.

James

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Dason C

#20506

Reflow PBGA | 26 June, 2002

Steve, I suggest to review some of the paste handlering procedure from some of the supplier. Aim look have a better procedure for our industrial but you need to check with your paste requriement. ie. Indium can store the paste @25C for 3 mths???

http://www.aimsolder.com/techarticles/Handling Solder Paste Article- manuscript.pdf

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Steve L

#20509

Reflow PBGA | 26 June, 2002

Thanks, guys.

The solder paste we are using is a kind of prototype from Alpha Metal - more aggressive, long floor life, etc.

The paste I used to solder PBGA was out of fridge for about 3 weeks. It was still good for our other Non-BGA products. Because of very small volume, we try to use the paste as long as possible.

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ianchan

#20514

Reflow PBGA | 26 June, 2002

Hi mate,

suggestion: consider "re-fridge" the paste after each Lot (small volume) batch run, instead of leaving it at ambient temperature for 3mths. follow FIFO control of such re-fridged paste. check with your supplier (in documented format reply) what is the procedure control for repeated use of the "out of fridge" paste?

Note: correct us if we have got your points wrong, thanx.

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ianchan

#20515

Reflow PBGA | 26 June, 2002

Hi mate,

"no more than 6hrs in the stencil" for us means:

1) we finish setup of reflow oven, P&P m/c, and lastly the printing machine(includes loading of stencil into printer machine).

2) once printer machine is setup, we rip-open the paste Can protective seal, and pour the paste onto the stencil. we record the "use time" onto the FIFO label (label attached to the Can).

3) For the WS paste we use, its a special formulae we commissioned for our own use, and supposedly has a 8hrs exposure(usage time) to ambient tempeatures. we set 6hrs to give buffer for machine downtime arising from dilly-dally and some setup debug timing for the Technicians. Your supplier will have their own set of "usage time" specs.

Heyz...no need to for the mush. all of us previously started somewhere at the bottom, and then worked our way up. life is not only how hard you work, its also about all the opportunities that other people shared with us. remember those people. Cheers!

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ianchan

#20516

Reflow PBGA | 26 June, 2002

Hi mate,

there is another thread ongoing about such an BGA vs oven length issue, suggest posting your question there.

As for that someone's opinion, he/she is entitled to it. Then again how did that someone determine this level of "insufficient"?

It could be my stubborn streak ingrained into my genetic makeup, I believe its gonna be "harder" to get the same profile on a 5zone compared to a 12 zone Oven, then again I'd gain great professional satisfaction in making possible what was "insufficient" and difficult for that diapproving other person.

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Dreamsniper

#20528

Reflow PBGA | 28 June, 2002

Hi Steve,

We have a Heller 1088 (4 Zone Oven) and we are using Indium NC92 with 2% silver. My Oven setting is:

Zone 1 Top & Btm 125 Zone 2 Top & Btm 145 Zone 3 Top & Btm 188 Zone 4 Top & Btm 265 Belt Speed 34 cm/min

I have run a profile using 6 thermocouple point on my pcb and I did not put any point on my PBGA but I locate the 2 points on the 2 components beside the PBGA. My thermal profile peak temp (using Kic) ranges from 215'C - 228'C. The board has 1 256PBGA and 8 fine pitch components. I checked the PBGA's solder joint and noticed a second drop had occured. We compared the solder joint of my PBGA with one that is processed in a Vapor Phase oven...there's not much difference between them except that from the naked eye the Vapor Phase process looks better...but my PBGA 2nd drop occured...we have produced more than 1000 boards since last year but none of the bds. returned with a PBGA solder joint problem...I know that we need a longer oven...8 zones at least...we are planning to buy 1 this year as we go for more BGA quantity on our new products...I hope that this will help you all...

regards, Dreamsniper

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Dreamsniper

#20529

Reflow PBGA | 28 June, 2002

Forgot to include the following:

Dwell time is from 55 secs to 68 secs. Indium specs for the paste is maximum peak @ 228'C and 30 to 90 secs dwell time above 183'C. PCB is a 6 layer board with Dimension of 205 x 166 mm but is slightly dense with 11 SMT Relays, 1 PLCC44, SMB's,0603's, 0805's, Tant Caps, 4 Dpaks, SMT Bridge Diodes, SOIC's etc. PCB design is quite good as the PBGA is place at the center of the PCB. It is also a double sided PCB using Glue @ the bottomside and passes the Wave Soldering Process after manual insertion of PTH Components.

regards,

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Dason C

#20531

Reflow PBGA | 28 June, 2002

You need to check your pot temperature and the board surface temperature. You know the melting point of the 63/37 at 183C. If your top side surface temperature is above this point and second reflow may happen. Also, the board will warp and may cause the open at the BGA especially at the center of the board.

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#20532

Reflow PBGA | 28 June, 2002

I would be concerned about condensation with re-refrigerating (or re-re-refrigerating, even) paste that's been exposed to room air.

I suppose the answer would be to getting a paste that allows for long term room temp storage. I've heard of new chemistries designed for that purpose, although I haven't investigated them.

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RDR

#20533

Reflow PBGA | 28 June, 2002

Is that 5 top and 5 bottom zones? We use a 4 zone and a 3/4 zone oven for BGAs and have success. I don't believe that the amount of zones is the issue but what type of profile you can get out of them. I will admit that the large ovens are much easier to profile than our little "tabletoppers" but if the oven has good convection it should work. When you profile a board do you see a lot of temperature difference across the board at different locations? If not you will probably be fine. I also work at a C.M. and I should be telling you to send your BGA work to us since "you aren't capable with your oven" but I won't

Russ

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Steve L

#20571

Reflow PBGA | 2 July, 2002

You guys are really helpful. Thanks again for all your suggestions.

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#20684

Reflow PBGA | 15 July, 2002

If your just getting into BGA assembly or rework, I strongly suggest taking this class. It covers profiling and considerationds for heat sensitive BGA'sin depth.

http://www.pcb-repair.com/bgarework.htm

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#21791

Reflow PBGA | 1 October, 2002

WE USE A HELLER 1500 EXL OVEN (5 ZONE OVEN) AND HAVE BEEN DOING PBGAs FROM 292 TO 780 BALLS WITH .O32" PITCH (.8 mm) AND METAL STIFFENERS ON TOP WITH NO PROBLEMS. I AM GETTING 210 - 215 DEGREES C AT THE BGA BALLS WITH 50 - 65 SECONDS ABOVE LIQUIDUS DEPENDING ON BOARD AND BGA SIZE. OVEN SETTINGS ARE IN C: 150,170,180,190 AND 250 AT 54 CM/MIN. WE HAVE FOUND OUT THAT BGAs ARE EASIER THAN .015" PITCH FINE PITCH PARTS. HOPE THIS HELPS.

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