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double side reflow soldering

Chinaren

#19818

double side reflow soldering | 7 May, 2002

Hello, As you know for the double side assembly PCBs, we normally first place the bottom side, then place top side after reflow soldering of bottom side. are there any other considerations for the sequence of reflow soldering ( bottoms side first, top side later) besides the component falling down into the oven?

Thanks

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Hussman69

#19825

double side reflow soldering | 8 May, 2002

One thing most over look is the time between the last reflow and the second print. You don't want to print on a hot board.

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Steve T.

#19845

double side reflow soldering | 9 May, 2002

The lighter the component, the more likely it is to stick to the solder on the bottom reflow side. All boards I have worked with, the descrete components (R's and C's) are placed first (bottom side), reflowed, then the active components (IC's) are placed on the top side and reflowed.

I've never had a problem with components dropping off.

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#19893

double side reflow soldering | 13 May, 2002

The other important issue relates to moisture sensitive devices (MSD). If you have MSDs on the first side, you need to track the remaining floor life of the components on the partially assembled boards between the first and second reflow. Contrary to popular belief, the first reflow does not reset the floor life of the MSDs because it does not remove any significant amount of moisture.

Re : J-STD-033, section 8.4.4 "If more than one reflow pass is used, care must be taken to ensure that no moisture sensitive components, mounted or unmounted, have exceeded their floor life prior to the final pass."

Francois Monette Cogiscan Inc.

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ianchan

#19924

double side reflow soldering | 14 May, 2002

Hi mates,

Yes, the lighter mass is a valid reasoning. in addition to the "lighter (mass) than thou" point of view,

1) the first reason for mounting active Parts (eg. IC, QFP, LGA...) is to minimize the temperature exposure of the Parts to a single reflow cycle. (if we did the active IC first and the discrete RES,CAPs second process, we would be exposure the IC to 2 times a reflow). although intensive study by the R&D mfg of components claim active ICs can go for "so-and-so" number of cycles of reflow "so-and-so" deg-C, most of us get that warm and fuzzy feeling knowing that active ICs ain't getting more thermal exposure than is "necessary" ie. one reflow cycle at best. for more info one can ref to JSTD-020A (IPC source).

2) MSD control is the second reason why we do active IC last (2nd cycle) so we can control from the time we rip-open the mositure barrier bags and start exposure the IC to the environment.

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#19928

double side reflow soldering | 15 May, 2002

Francois: How much moisture does a typical reflow cycle remove from popular SMT packages?

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#19933

double side reflow soldering | 15 May, 2002

Hi Dave,

The short answer is "not any significant amount" of moisture is removed during reflow. The temperature excursion is much too short.

If you look at the physics of moisture diffusion inside a package you will see that only the surface moisture can be removed in a fairly short timeframe at high temperature. A standard reflow cycle does not remove any of the moisture absorbed deeper inside the package, which is actually what we are concerned about.

This is why the standard specifies that it takes up to 48 hours of continuous exposure at 125C to remove enough moisture to reset the floor life clock.

If you need more than this simple answer I could always try to find experimental data to demonstrate the above.

Regards, Francois

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ianchan

#19939

double side reflow soldering | 15 May, 2002

Hi mates,

any production floor controls that are efficient and effective? to control MSD.

am talking about practical shopfloor "good mfg practices".

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#19946

double side reflow soldering | 16 May, 2002

My company offers a simple and cost effective control system for MSDs on the factory floor. It is a turnkey solution to comply with J-STD-033.

If you want more information feel free to contact me offline.

Francois Monette Cogiscan Inc. Tel : 450-534-2644 Fax : 450-534-0092 fmonette@cogiscan.com www.cogiscan.com

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C.K.

#19950

double side reflow soldering | 16 May, 2002

Sounds like this 'Hussmann ' character is taking credit for my findings again. You'd "fit right in" where I work.

Heh he hehe hehe.

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Hussman69

#19951

double side reflow soldering | 16 May, 2002

I fit you in where you work!

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JoAnn

#19981

double side reflow soldering | 20 May, 2002

Greetings! The SMTA will be offering a course on this topic at SMTA International in Septmber. If you want a complete overview of this course please check out http://www.smta.org/smtai. Hope we might get to see you in September.

JoAnn

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Reflow Oven