Printed Circuit Board Assembly & PCB Design SMT Electronics Assembly Manufacturing Forum

Printed Circuit Board Assembly & PCB Design Forum

SMT electronics assembly manufacturing forum.


Bottom side smt and thru hole

Pat Copeland

#15714

Bottom side smt and thru hole | 26 May, 1998

R&D is currently designing a board with smt components on both sides of the board. They are adding a 16 pin IC Gull wing. on the bottom side of the board. I have asked for only caps and resistors to be placed on the bottom side. Is there a recommended direction for the IC and will I have problems through the wave. If I want to use the reflow for the thru hole components what do I ask for at the stencil house.

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Jerry

#15718

Re: Bottom side smt and thru hole | 27 May, 1998

We have no problems but orientation does matter. Orient with longest side of component body perpendicuar to wave. In other words the same preference as with through hole DIPS. Idea is the same as for through hole, that the surface tension will tend to break the solder before it bridges to the adjacent lead. If the leads are parallel to the wave then the leads act as a rake that tends to cause bridging. Not sure what you mean about the stencil house. You just need to size your apetures correctly so that you get proper hole fill. There are some DFM guidelines for this but it depends on the hole diameter and board size.

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Chrys

#15716

Re: Bottom side smt and thru hole | 27 May, 1998

| R&D is currently designing a board with smt components on both sides of the board. They are adding a 16 pin IC Gull wing. on the bottom side of the board. I have asked for only caps and resistors to be placed on the bottom side. Is there a recommended direction for the IC and will I have problems through the wave. If I want to use the reflow for the thru hole components what do I ask for at the stencil house. Pat, Have the design guys orient the part in the direction of travel through the wave, like Jerry said, and have them put extra pads (for a total of 18) on the trailing edge. Solder tends to bridge between the last two leads of SOIC's, so if there's an extra dummy pad on each side, the solder will bridge the last lead to it instead of bidging the last two active leads together. If you want to reflow through-hole components, there are design guidelines for stencil apertures and pad geometries, but it will still take a few experiments on your part. Particularly with your printing process. It depends on the viscosity of your paste, the type of blades you're using, if you're okay with printing extra solder on mask or hesitant because of solderballs, how thick you can go with the stencil (how fine is your finest pitch) etc. Then you must consider what through-hole parts you are stuffing, and how good is your insertion system (manual, odd-form placer, AI equipment, etc). And finally, how full do you wan the barrels - is 50% okay? 75%? 100%? Full topside fillets?. It's highly do-able, but seems like a longer critical path to take when there's just one IC on the bottomside of the board. If I were you, I'd put theive pads behind the SOIC, use about six glue dots under it, and run it over the wave. Good luck, Chrys

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Justin Medernach

#15717

Re: Bottom side smt and thru hole | 27 May, 1998

| | R&D is currently designing a board with smt components on both sides of the board. They are adding a 16 pin IC Gull wing. on the bottom side of the board. I have asked for only caps and resistors to be placed on the bottom side. Is there a recommended direction for the IC and will I have problems through the wave. If I want to use the reflow for the thru hole components what do I ask for at the stencil house. | Pat, | Have the design guys orient the part in the direction of travel through the wave, like Jerry said, and have them put extra pads (for a total of 18) on the trailing edge. Solder tends to bridge between the last two leads of SOIC's, so if there's an extra dummy pad on each side, the solder will bridge the last lead to it instead of bidging the last two active leads together. | If you want to reflow through-hole components, there are design guidelines for stencil apertures and pad geometries, but it will still take a few experiments on your part. Particularly with your printing process. It depends on the viscosity of your paste, the type of blades you're using, if you're okay with printing extra solder on mask or hesitant because of solderballs, how thick you can go with the stencil (how fine is your finest pitch) etc. Then you must consider what through-hole parts you are | stuffing, and how good is your insertion system (manual, odd-form placer, AI equipment, etc). And finally, how full do you wan the barrels - is 50% okay? 75%? 100%? Full topside fillets?. It's highly do-able, but seems like a longer critical path to take when there's just one IC on the bottomside of the board. | If I were you, I'd put theive pads behind the SOIC, use about six glue dots under it, and run it over the wave. | Good luck, | Chrys Pat, I agree with Chrys 100%. Send it over the wave. Intrusive soldering is difficult at best. You have to either hand stuff the parts or have a good and FORGIVING odd form placer. If your board has a dense surface architecture, it's extremely difficult to print. You would probably need to use odd form apertures and thus would be required to do a ridiculous amount of geometry problems. There's also the issue of whether or not to use solder preforms to make up some paste volume. Wave it.... Regards, Justin Medernach

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Bob Willis

#15715

Re: Bottom side smt and thru hole | 7 June, 1998

There are some guidlines on design and through hole reflow on my web site to download. Also on pin in hole reflow the SMTA Office has a document on using the tecnology.

| R&D is currently designing a board with smt components on both sides of the board. They are adding a 16 pin IC Gull wing. on the bottom side of the board. I have asked for only caps and resistors to be placed on the bottom side. Is there a recommended direction for the IC and will I have problems through the wave. If I want to use the reflow for the thru hole components what do I ask for at the stencil house.

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