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REFLOW PROFILE NEED HELP

Views: 2290

james

#36276

REFLOW PROFILE NEED HELP | 24 August, 2005

I need some help here. I just to see what suggestions anyone else has. Our Man. Eng. has 3 profiles on our 9 zone Heller Reflow oven. These are the profiles:

1. 120 140 150 150 180 200 215 250 220 29in per minute 2. 120 130 145 150 150 185 205 220 245 28 in per minute 3. 120 130 145 150 150 185 205 245 225 27 in per minute

I want to say our oven has its own cooldown zone, so I dont think we need to add it in our profile. We have had alot of solder issues lately and I am trying to resolve them. But first I need other opinions to do that. Reflow profile no. 1 is used for a board with heavy BGAs and the customer is complaining that they are not working. If anyone could give me suggestions I would really appreciate it.

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PWH

#36277

REFLOW PROFILE NEED HELP | 24 August, 2005

What kind of problems/defects/process indicators are you having? It is likely that you will have to bite the bullet and get a thermal profiling mole to prove that your selected profiles are subjecting parts to the correct tempertures per part specs. and paste recommendations. It is difficult (especially with BGA's) to assign numbers to the zones of your ovens and know that the parts are reflowing adequately. In my opinion, the best way to profile a BGA board is to build a scrap board in full, drill holes through the PCB from the reverse side of the BGA as close to BGA lands as you can. Holes should be close diameter to thermal-couples. Install the t-couples in the holes and profile the board until you closely match part/paste specs. Another critical step in BGA placement is close inspection of paste deposition on PCB pads as well as the obvious - part placement. Have your PCB x-rayed to prove part placement is good w/no bridges. If it hasn't happened already, your customer will get fired up enough and will ask for profiles and x-rays related to the specific BGA in question. You cannot tell them the oven zone temps. and please them if they know anything about SMT ovens. Oven zone temps. are only an arbitrary # that is a starting point for what is really happening in the oven based on part density, PCB thickness, etc. You also need to find info related to BGA temp. ramping and how that affects solder joints. (IPC 7095). Sounds big but... with the proper tools (mole/defect info) it is a piece of cake! Hope this helps and Good Luck - PWH.

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aj

#36278

REFLOW PROFILE NEED HELP | 24 August, 2005

James,

I understand that you would like a quick fix answer but profiles take time and analysis to get right.

Firstly, your paste supplier should provide an example of the correct profile for the paste you are using.

Secondly, what makes you think its the profile that is causing the boards to fail.

Do you have a profile recorder i.e Datapaq or slimkic. If you have ,profile your oven and see what results you get.

Best take a step back and look at your overall process.

aj...

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james

#36280

REFLOW PROFILE NEED HELP | 24 August, 2005

We do have a slimkic and I will be profiling a board asap. Yes I am looking for a quick answer and I know profiling takes some time. It is just hard to get the M. E. to understand what I want. We just never had a cooldown as part of our profile. Thanks for the suggestions.

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#36282

REFLOW PROFILE NEED HELP | 24 August, 2005

Why is your big heavy BGA profile your fastest one? I'm guessing because it's hotter therefor faster? To achieve some kind of "balance" with the others. But you want those boards to get more heat, so you don't blance higher temps with faster belt, you just have higher temps. Or same temps but slower belt speed, or a combo of higher temps and slower belt speed.

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james

#36285

REFLOW PROFILE NEED HELP | 24 August, 2005

That is what I was looking for. The profiles did not make much sense to me. We are getting alot of cold solder joints or bumpy solder joints.

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#36288

REFLOW PROFILE NEED HELP | 24 August, 2005

I think where the fuzzy thinking starts is by calling the recipe's; profiles. Or maybe I"m being too fussy. Idealy you want every solder joint to have the same thermal "profile" (not recipe). That is the temperature over time is best if identical for every joint on every board. You can't do that of course because every different component (and PCB) has different thermal characteristics. This means the recipe gets changed. (and I internaly cringe when someone says use a new profile) Changing the recipe does change the profile but they are two separate entities.

The second source of fuzzines is Temperature vs Heat. You want the same temperature exposure for every joint. But different comps have different thermal characteristics. This means to get the same temperature exposure you need different heat exposures.

WIth big heavy BGA's they can soak up a lot of heat. Therefor you need a slower belt speed to let them absorb the heat and get up to temperature. The best way to slimkic profile one is to drill a small hole in the board form the other side into a center ball. I"ve only ever been able to have the boards and BGA's to do this a couple times though. Otherwise try and get a thermalcouple tip in as far as you can.

Some genius designer thought it would be a good idea to build in heat sinks on some BGA's.

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james

#36289

REFLOW PROFILE NEED HELP | 24 August, 2005

Well that Genius did it on this BGA. It does have a heatsink and absorbs alot of heat. It just stinks because the BGA cost over 1000 and I really can not scrap a board. But that first profile does not look correct for this board with the BGA esp. the belt speed.

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dougs

#36292

REFLOW PROFILE NEED HELP | 24 August, 2005

if you don't scrap a board you'll struggle to get this right, i've seen it in other places i've worked where the management wouldn't let me scrap boards to test the profile and also wouldn't let us charge customers NRE for the cost of a board, needless to say there were many failed units due to poor soldering, some of which look fine under the scope, you need to insist that you get to run a properly set up recipe or you may end up with scrap in the 10000s

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#36294

REFLOW PROFILE NEED HELP | 24 August, 2005

We've been down this road before, and it doesn't have to be as expensive as it would seem. First, ask your board supplier for a solder sample of the board. They are likely to have a scrap board that you can use for your profiling. As for the BGA(s), ask the manufacturer of the part if they have dummy, or nonfunctioning, versions the BGA(s). Otherwise try one of the dummy component suppliers (Practical, Topline). Attach the dummy BGA(s) to the solder sample board using your best guess profile, and then follow the other's advice on thermalcouple attachment. Even without the rest of the components on the board, your recipe will probably be close enough. Good luck.

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james

#36309

REFLOW PROFILE NEED HELP | 25 August, 2005

I ran a profile on a dummy board with no parts and the peak temp is 220. The 220 is just the temperature of the board of course. With that BGA on it, I am pretty sure it sucks up alot of the heat. Also I am looking at the profile and notice that it is staying about 220 for about 50-60 seconds. This seems too long? Maybe I am wrong. I could see being above 200 for that period of time but not the 220. Could this be what is damaging the parts?

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Rob

#36311

REFLOW PROFILE NEED HELP | 25 August, 2005

From reading back through the post, it looks like you still need to establish whether the parts are actually being damaged, or just not soldered properly.

If you're worried about heat damage get the manufacturers recommended reflow profile and check how long the device can take at 200 or 220 degrees, and also if the PBT is being exceeded.

This is a science, you need to be scientific about how you gather your data, inteprit it & derive conclusions, else you'll be running from quick fix to quick fix without ever sorting out the real problem.

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Slaine

#36398

REFLOW PROFILE NEED HELP | 30 August, 2005

whats the actual failure mechanisum your seeing which balls on the BGAs? my bet is its the ones on the corners, this is mainly due the the large heatsink on the top of the BGA when it starts to get hot it acts as a bi-metalic strip and lifts the corners especially on larger BGAs ie 30mm square. can also work the other way and give unsoldered joints in the center. If this is the case you either need to heat it up slower or faster if possible.

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james

#36498

REFLOW PROFILE NEED HELP | 2 September, 2005

Thanks to all. I have it straightened out. We were running too hot and too fast and too long of a reflow time. I am running with Kic2000 and it works great. Thanks for all your input.

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