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BGA non wetting

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#62678

BGA non wetting | 13 September, 2010

Hi all good morning and need advice from expertise here. One of my customer complain that they have constantly issues with one of their product and ask us to do cross section.The report show there was “no wetting to the PCB copper pad” and cause the solder fracture.Could someone advice the root cause and corrective action eventhough we have solder paste inspection,x-ray and 2Dx-ray for inspection.We had re-test and found X-ray still pass under IPC spec.

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#62679

BGA non wetting | 14 September, 2010

Hi Gani, non-wettting issues can surely be resolved. At first, would appreciate if the following info could be shared. Do you supply just one PCBA product to this client or is the complaint towards one of the many products that you deliver? Also, does non-wetting occur at random or is it traceable to a specific area on PCB or in relation to specific components placed?

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#62682

BGA non wetting | 14 September, 2010

Hi Sir, Thanks for reply and base on our system the same part are using for 2 model only and the problem occur on both assembly.By the way as feedback it happen on random pin of this BGA only.

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#62683

BGA non wetting | 14 September, 2010

Hi Gani, Looking at the pictures in the report it looks as if there is a thin coating of solder on the pcb pad....but it looks like the BGA sphere was barely in contact with the pad. This could be caused by insufficient solder paste on this pad...maybe the stencil aperture was blocked ? If there is a low paste volume then the is also a shortage of flux and the pcb pad will oxidise a lot in reflow and you'll not get the BGA ball to solder to it. Have you cross sectioned a joint nearby that you know is good to compare their appearance ? Good Luck

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#62684

BGA non wetting | 14 September, 2010

Hi Sir grahamcooper22, There should not a issue in stencil since i had open the aperature 1:1(one to one).Futhemore this is HASL pcb coating as now our customer had convert to immersion silver.I had do comparison between both and found HASL wetting not so good as immersion.May this one of the root cause?

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#62687

BGA non wetting | 14 September, 2010

Hi Gany, did you check the temperature at reflow profile on this BGA ?

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#62691

BGA non wetting | 14 September, 2010

Hi Gani, HASL is normally very good for solderability providing there is a good consistent coating of HASL on the pcb pad. But it is not unusual to have a very thin coating of HASL on the pad and this is poor for solderability. You then have tin/copper intermetallic on the pad and this is much more difficult to solder to. Immersion silver is better then this tin/copper intermetallic. Your aperture pad ratio is 1:1, but are you absolutely sure you got a good paste deposit on this pad ? What is the aperture diameter and stencil thickness ? Have you compared its cross section to a nearby good BGA joint ? I'd expect a good joint to look like the ones you have of where this BGA sphere joins to the package.

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#62692

BGA non wetting | 14 September, 2010

If the HASL was poor quality and very thin you would find this defect....but you would expect to see it on more pads also. Also, you'd expect to see your solder from your solder paste either dewetted on the pad or reflowed onto the BGA sphere making it look an uneven circle. If there was only a thin coating of solder paste on the pad then you'd also get this defect. Comparing a nearby good BGA joint may indicate if there was a good paste deposit or not.

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#62698

BGA non wetting | 14 September, 2010

Hi Sir, The diameter of the aperature is 17mil and thickness 6mil and since we are using Solder Paste Inspection i believe that should have a good paste deposit on this pad.Let say if no good paste it will more create insufficient solder which X-RAY able detect this failure.

Hi dan_ems, Here i attach the profile.

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#62699

BGA non wetting | 15 September, 2010

Hello, what type of solder paste use, and this component is made for lead process?

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#62700

BGA non wetting | 15 September, 2010

kester 562 clean process and this leaded part.

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#62705

BGA non wetting | 15 September, 2010

First of all decreases to the minimum the reflow time (45-50 seconds) and if does not work, try with another type of slder paste. Good luck !

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#62707

BGA non wetting | 15 September, 2010

hi All, Here i snap shot previous PCB(HASL) and current PCB(Immersion Silver).Base on the photo it could be PCB oxidize cause the non wetting?

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#62710

BGA non wetting | 16 September, 2010

Hi Gani, Immersion Ag finish has lesser shelf life as it can get more easily oxidized on exposure. However, I see concern in your Reflow profile. A few questions: Have you compared your reflow profile to the solder paste manufacturer's recommended profile? Pl. take a re-look. How many thermocouples do you attach? Have you identified the Hot and Cold spot on board? In your profile, we need to consider all critical parameters such as rise slope in pre-heat, dwell/soak time and temp, time above liquidous, peak temp, and slope during cooling.

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#62722

BGA non wetting | 17 September, 2010

Hi Gani, looking at the HASL icture and the poor wetting of the solder over the pads I would imagine that the HASL coating is very thin on the pads and in actual fact isn't 'solder' but is tin /copper intemetallic and this has poor solderability. It is quite widespread to see HASL pcbs with poor coating. PCB manufacturers attempt to give you a level coating on the pcb pads by blowing the excess solder off after the pcb has been coated....if they blow too hard they remove most of the solder and leave a pad covered in tin/copper intermetallic. Good HASL coating is probably the best for solderability and long term storage life but bad HASL is a problem.

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#62724

BGA non wetting | 17 September, 2010

Based on the last few pictures in the report, the root cause is lousy HASL. In fact, you don't have HASL - you've got SnCu intermetallic. Don't mess with your reflow profile, change paste, or monkey with anything else in your process. Have the board supplier fix theirs.

We've been seeing more and more of this lately.

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