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Humidity question

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Our shop doesn't have a humidity controlled environment. We ... - Oct 13, 2008 by Mariss  

#56903

Humidity question | 13 October, 2008

Our shop doesn't have a humidity controlled environment. We reflow our SMT board production at the end of the day. That means some boards can soak in 8 hours worth of humidity before reflow with the obvious negative consequences.

What has come to mind is building a sealed holding enclosure that keeps the SMT mounted boards at 40 - 45C while they are waiting for reflow to minimize moisture absorption. Basically an insulated cabinet with doors and a thermostat controlled heater.

Is this idea sensible? Are there any drawbacks?

Mariss

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#56904

Humidity question | 13 October, 2008

Why don't you just reflow them right after you populate? Then you'll quit losing parts because someone bumped a board after the paste lost its tack.

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#56905

Humidity question | 13 October, 2008

Main reason is the oven is 38kW and reflows a day's worth of production in 90 minutes. If it ran all day this place would be a sauna. That and the cost of electricity.

Mariss

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#56908

Humidity question | 14 October, 2008

You must be a director or somewher ein the management chain.

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#56914

Humidity question | 14 October, 2008

How much cost do you absorb in touchup and replacing missing parts, and how does it compare to the cost of operating a reflow oven that runs whenever you have product available to process?

I don't necessarily have an answer for you but I'm having a hard time grasping exactly what you do there and why.

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#56916

Humidity question | 14 October, 2008

How much flux will be lost if you do that? And I would think that the paste would be too dry by the end of the shift.

And don't forget that power rating of the oven doesn't mean that it is using that much power the entire time it's on.

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#56926

Humidity question | 14 October, 2008

Why not use a cabinet like those used for limiting the exposure of misture sensitive components to humidity?

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#56927

Humidity question | 14 October, 2008

The mounted panels (8" by 8") are stacked in carriers, 0.5" between panels, 20 panels per carrier. There are no missing parts or other damage due to carelessness. Anyone bumping a panel is summarily executed by the company firing squad.

As someone else surmised, I'm in management, also engineering, purchasing and floor-sweeping, crawling my way up the corporate ladder in an 8-employee conglomerate I own.

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#56931

Humidity question | 15 October, 2008

So, are you putting the loaded carriers into a batch oven, or unloading the carriers onto an inline oven conveyor, or something else?

Also, have you talked to your paste supplier to see what they might suggest? They might have a paste with an extended stencil life that would perform better for you.

I'm thinking that if you're using a batch oven, you might need to think about adding nitrogen to that process. Then you just load it as product is assembled and flush the oven with N2 every time you close the door.

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#56933

Humidity question | 15 October, 2008

You have a lot more problems than what you are asking about. If you are running a shop that is un-air conditioned and high humidity you will have nothing but trouble in ALL aspects of your operation. I know this is harsh but I am really surprised in this market you would have any customers at all running such an operation.

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#56934

Humidity question | 15 October, 2008

From your question you must be having rework issues due to the extended wait before reflow. What is the cost of the rework compared to the cost of just running the oven? Maybe you should look for a better used reflow oven that won't turn your shop into a sauna or maybe you just need to add external exhaust to your current oven.

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#56937

Humidity question | 15 October, 2008

Anyone ever tell you that your efforts to help were unwelcome?

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#56948

Humidity question | 15 October, 2008

Who does the ambiguous "you" refer to?

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#56949

Humidity question | 15 October, 2008

No, there have been very few rework issues. We use 0603 passives, a few SOICs and a couple of 48-pin QFN packages. The boards are 4-layer, densely populated and 6-mil traces. We also thru-hole reflow 9 TO-251 packages on the bottom and a 3.5mm pitch terminal block on top.

We use AIM no-clean lead-free paste with a manual stenciler.

We have 3 Juki KE-750 machines and a Manncorp CR-5000F 10-zone (5 up, 5 down) zone oven. The equipment runs only our product, about 60,000 units a year; we don't job-shop. The thru-hole reflow solder joints are near perfect as are the SMT joints, even on the 0.5mm QFN packages. Every panel is inspected under nice Nikon SZ-660 microscopes, every panel has about 2,000 SMT parts on it. Rework is 5 to 10 parts per panel, rising to 10 to 20 when humidity is high. 98% of boards pass first functional test, 96% pass when humidity (>50%) is high.

With high humidity, there is also minor amount of spatter (10 to 20 'balls' per board). It is this that I would like to eliminate.

I'm just a small guy running a young company here in Tustin, CA. Our product is very profitable and we build a lot of it. One day soon we will be able to afford a nice environmentally controlled facility but until then I have to deal with what we have and control the process using what we are given.

I'd be happy to post microscope photos of what we turn out on a good day versus a bad day. There won't be a lot of differences. We work very hard to optimize what we have.:-)

Mariss

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#56952

Humidity question | 16 October, 2008

How about changing the rules, and running the reflow twice a day _if_ humidity is above a certain threshold ? The oven should not use 38 kW continuously.

Of course, a continuous flow process - assembly -> reflow -> immediate test - will save you a lot of grief when something goes wrong.

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#56957

Humidity question | 16 October, 2008

We don't associate solder balls with no-clean flux and humidity. For solder balls with no-clean flux, we'd rather have you thinking about: * Paste selection * Aperature size * Paste print process * Reflow recipe

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#56959

Humidity question | 16 October, 2008

Sorry...I was depending on the formatting to indicate that my comments were in response to qdp's comments.

Whether he/she likes it or not, your process is probably well entrenched and you were asking for help with it. I didn't think his opinion as to the breadth/strength of your customer base was particularly pertinent to the discussion.

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#56963

Humidity question | 16 October, 2008

Not sure why the modiaters of this Forum have removed my posting when theres lots of folks here advertising there services. Mariss the offer stil hands if you red my previous pots of offering my consultancy services,

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#56964

Humidity question | 16 October, 2008

Oh I thought you were referring to WML's post where he asked Mariss out on a date. LOL

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#56966

Humidity question | 16 October, 2008

We not sure why the moderator would one of your posts either, but maybe it goes something like => If someone asks for sources, anyone can post their recommended supplier[s].

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#56967

Humidity question | 16 October, 2008

"We don't associate solder balls with no-clean flux and humidity." I'll let the former speak for itself, but on the latter, not even spatters?

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#57035

Humidity question | 21 October, 2008

Mariss, I would run a power analyser on your oven to see the real power consumption during warm-up and during production. Then you would be better able to decide how much money to invest in a humidity controled cabinet big enough to contain your daily production. You might not need that. Maybe you could start the oven earlier, say middle of the afternoon and start reflowing. That way you reduce the waiting time of your populated PCBs. I don't know the layout of your shop, but maybe you can isolate your oven in a separate room, so that the heat of the oven is contained. An insulated curtain comes to mind. If you have doors or windows in the oven room, so much the better.

On the far-out solution proposal, install a water cooling jacket arount the oven to capture the heat and use it to warm water for the lavatories..;-)

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