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SMT electronics assembly manufacturing forum.


Intrusive Reflow

iman

#22442

Intrusive Reflow | 20 November, 2002

Hello,

need help on intrusive reflow process, thanks in advance to the experts and general info sharing :)

need some help to obtain answers, to following questions:

1) PCB is FR4, 1.6mm thick, 4 layers, what stencil thickness (stencil design guidelines?) is optimal to get sufficient solder in the thru-hole, without generating excess solder lump on the opposite bottom side of PCB? the customer specified a concave solder in the thru-hole, and that conventional convex solder is not permitted.

2) for 63/37 paste, is type 3 or type 4 optimal for intrusive solder processing? what reflow considerations need to be looked at? was verbal informed the most frail components on this model PCB can withstand up to 250 deg-C.

3) is there any need to artificially block the bottom side of the PCB, before paste screen printing onto the top side? If so, what methods avaliable to ensure coplaner volume distribution of the "blocking material"? We were thinking of screen printing a layer of water-peelable mask on the bottom side PCB, followed by top side paste screen printing with subsequent component mounting and Hot Air Reflow. Any comments or alarms ringing on this line of thinking?

4) Generally anything else that is overlooked in this thread? any direct process experiences anyone can pls share?

Thanks!

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Guy

#22469

Intrusive Reflow | 21 November, 2002

The information you are seeking is found in IPC-7525. Section 3.3 describes an equation for calulating the solder paste volume required to produce a desireable fillet. I simplified it a bit.

V=1/s[Tb(Ah-Ap) + Ft + Vp] - Vh

V is the volue of paste required s is a shrink factor base on the percentage flux in the paste. Ah and Ap are the cross sectional areas of the hole and the pin. Ft is the total fillet volume Vp is the solder volume on left on the board pad (after reflow) Vh is the solder paste filling the hole during the printing process.

Most of these will have to be established empirically. You can get a good starting point by doing a little arithmetic. Before you purchase a stencil dispense some paste, reflow and measure results.

Good luck, have fun.

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Bill Barthel

#22477

Intrusive Reflow | 21 November, 2002

There have been a number of papers and articles on the subject as well as tutorials and workshops sponsored by SMTA, etc. I can send you a paper I presented in 1995 that may cover most of your questions. There are also many other resources that might help you: Printing equipment and stencil suppliers Consultants (I can give you contacts if you like) Trade magazine archives Technical symposia (SMTAI, APEX, NEPCON)

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iman

#22529

Intrusive Reflow | 27 November, 2002

Hi and thanks for the replies.

does anyone have soldering acceptance specifications, for intrusive reflow soldering?

any reference (eg. IPC) Standards to follow?

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iman

#22530

Intrusive Reflow | 27 November, 2002

Thanks! all the help would be most appreciated! how do you think it best to share your paper with me?

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Guy

#22531

Intrusive Reflow | 27 November, 2002

I am not aware of any IPC specifications but Bob Willis in the UK has some real nice stuff. http://www.bobwillis.co.uk/

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iman

#22542

Intrusive Reflow | 28 November, 2002

ran an internet search on "intrusive reflow", out popped various links on the subject. Bob Willis was rather prominent on the topic's links.

anyway seems intrusive reflow soldering specs are the same as conventional manual insertion (radial, axial) components, IF the parts supplied and used are conventional radial & axial lead parts. In such event, all lead protrusion and solder coverage/fillets apply.

in our case, the leads do not protrude out from the PCB bottom "solder" side. hence my inquiry into any established specs?

Once again, thanks to all the generous folks who posted replies to this thread, even though it may not be exactly considered new technical inquiry.

Thanks!

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#22554

Intrusive Reflow | 2 December, 2002

Most modern spec do not distinguish between process methods. So, there is no allowance "if you are using bla bla bla method, you should see ... . But if you are using bla bla de bla method, ignore that and look for ... ."

Most specifications require lead protrusion for PTH parts. A-610 is pretty much the industry standard. As a minimum, you should see the lead trying to push through the solder.

From one perspective, it's pretty coo that you are putting-down so much paste that you get no lead protrustion. We usually can't even fill the hole. But then again we haven't relayed-out the boards and the holes are about the same diameter as a no. 2 Ticonderoga.

Finally, while Mr Willis has done much to popularize the term "intrusive reflow", other purvayors of process methods have coined terms like: * Pin & paste * Paste in hole

So, consider these terms also as you mine the fine SMTnet Archives.

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