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Rectangular uBGAs with 1 raised edge after reflow

#17103

Rectangular uBGAs with 1 raised edge after reflow | 6 August, 2001

Folks,

I need some help or advice with a potential soldering problem with a set of uBGAs. We are seeing two rectangular uBGAs (Intel package type GT28F320C3), which are located within 150 mils of each other, exhibit a slope away from each other after reflow.

The uBGAs are located in the lower right-hand plane while looking at the PCB upright. The lower edge will be the one closest to you while passing through reflow oven. The reflow oven is forced air convection type with 7 zones. The profile is >|120 |140 |160 |180 |180 |190 |260 | with a belt speed of about 28 IPM.

The uBGAs are placed parallel to each other along the smallest side. The small edges closest to each other are raised slightly and form a "hill" when looking from the horizon of the PCB. The same small sides run parallel to the edges or tabs of the PCB used to hold it moves through reflow oven.

The uBGAs are rectangular, 0.75mm pitch, weigh 83mg, have 47 balls, and the dimensions are 10.850 x 7.286mm .

Could this be a thermal issue? I'm thinking it's related to air flow. Let me know if you can help

Thanks

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CPI

#17119

Rectangular uBGAs with 1 raised edge after reflow | 7 August, 2001

Are there other components on the board? If so, how does solder quality look on them? Are they all reflowing? Are the sections of the uBGA�s sitting on a ground plane? Sounds like un-even heating. You should attach a couple thermal couples and see how the area is heating. attach one at the end that is low and one where it is high. Just looking at your temps it doesn�t look like the board would be above liquidus very long you only have one zone where the board would actually exceed 183. Remember the temps you set your oven to does not indicate what the board temp will reach. Run a profile and see what your ramp rates, soak time and time above liq are. Good place to start. Your paste supplier should have a recommended profile.

Good Luck

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#17135

Rectangular uBGAs with 1 raised edge after reflow | 7 August, 2001

Good problem description.

Your profile seems reasonable. Where did you take it? Thermocouples on: * Reflow oven internals * Pads were the solder balls are not collapsing * Another location

Intel publishes a BIG pdf on CSP flash memory for gear heads at: http://developer.intel.com/design/flcomp/packdata/csp/298161.htm

It seems easy to guess to shoot => slowing-down the conveyor to allow the solder to heat these balls more. But that may not be the answer, because if the flux is exhausted, more heat will not help. Then if the flux is exhausted, the answer is to crank the belt. Two different answers to the same question, hmmm.

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jbieber

#17256

Rectangular uBGAs with 1 raised edge after reflow | 17 August, 2001

Gregory, I think that the profile is too radical, i.e. zone 6 to zone 7 temperatures are too far apart. I would guess that the solderjoints have not all reached 183C in zone 6 which is why the uBGA's do not have a symmetrical ball collapse. You may want to increase zone 5 by 5 degrees and zone 6 by 5 or 10 degrees. Keep in mind that the set point for zone 7 (260 degrees) may be too high and may yield excessive board temps. Follow the solderpaste specs. and the BGA case temp. specs., typically 215C and 220C max. respectfully. An aggressive profile is not desirable for BGA's. You may need to slow down the belt speed and lower temps to yield a lower temp slope of around 1 degrees C/second. This will also prevent the BGA's from "potato-chipping" where the package becomes warped causing opens or shorts at the corners. Keep in mind that the paste may dry out if the profile is not within the mfgrs. specs. We have found that some pastes can withstand pushing their envelopes, thus widening their spec. windows a good solderjoint quality throughout all areas of the board. Another factor that just came to mind is "Has the oven been on for a long time?" Even though the oven temps might have been reached, the drive chain, etc. may not be up to temp. Convection ovens have cold spots as well as the substrates (pcbs)being reflowed due to heavy ground planes, etc. Our oven's fixed rail at the edge of the oven is a few degrees cooler than the variable one in the middle. Try rotating the board before reflowing it. Another factor, PCB thickness needs to be considered. A normal pcb (.062" thick) will heat up faster than a thick pcb (e.g. .120"). That's all folks! Happy Baking! Jay

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#17274

Rectangular uBGAs with 1 raised edge after reflow | 19 August, 2001

Hi Greg,

Have you resolved your problem yet? Daves reply to slow the belt may be your best bet. It appears you are not heating the BGA's uniformily. The temp in zone 6 seems to be a bit odd, are you trying to prolong the soak? If so, try to increase this temp and lower zone 7 a bit. it's possible you are drying out the paste. you may also speed up the belt a bit and raise the temps in the 5 & 6 zones.

Greg

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#17483

Rectangular uBGAs with 1 raised edge after reflow | 5 September, 2001

Well, it appears after running both a bare and populated PWB we were seeing flux exhaustion on the uBGA. The problem apparently resulted by a recent maintenance effort where the air blowers in the reflow oven were replaced. The blower replacement must have improved the thermal transfer of heat and since the profile was not adjusted to compensate, it lead to higher peak temperatures, ramp rates, and time above liquidus.

To find the problem we used a ECD Super MOLE Gold to test the surface temp of the PWB under and around the uBGA. The paste type was Indium NC-SMQ92. The Indium spec calls for a 0.5 to 1 degree C/sec thermal ramp. Between zones 6 and seven we observed ramp rates close to 1.2 degree C/sec ramp. Also time above liquidus averaged 120 seconds above liquidus which exceeded Indium spec by 30 seconds.

Our next run of these CCAs is planned this month so I wasn't able to verify the newly adjusted profile. After we run them I'll post an update with the results for fellow smtNet'rs.

Cheers!

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#17486

Rectangular uBGAs with 1 raised edge after reflow | 5 September, 2001

tks

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#17737

Rectangular uBGAs with 1 raised edge after reflow | 28 September, 2001

Folks,

We completed our run of these boards yesterday and indeed the problem was resolved by modifying the reflow profile. Thanks to all who responded to this posting.

Cheers!

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#17740

Rectangular uBGAs with 1 raised edge after reflow | 29 September, 2001

Gregory,

Thank you for posting the results of running your boards yesterday (glad to hear everything worked out well). I am sure other SMTneters are glad to see the sharing of their information helped.

Perhaps others who have used information provided by SMTneters could post results obtained? Maybe this will encourage even more people within the industry to participate? And, therefore, make this forum even better then it already is for everyone....(if that is possible).

Thanks again for posting the results!

Roland

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#17742

Rectangular uBGAs with 1 raised edge after reflow | 29 September, 2001

What did you change in your profile?

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reflow oven profiler

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