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Assembling one side SMT one side TH

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#64805

Assembling one side SMT one side TH | 10 August, 2011

Hi all,

I would like to assemble boards that have one side all SMD and the other side all TH components. I have no idea how to do it. Any help will be appreciated.

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#64806

Assembling one side SMT one side TH | 10 August, 2011

There are several ways to assemble a board like you describe.

Since you're asking this question, we assume that your production technology is limited. What tools, machinery, and equipment can you access?

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#64807

Assembling one side SMT one side TH | 10 August, 2011

We have an SMT line (Screen Printer, P&P, Reflow) and a Wave machine.

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#64808

Assembling one side SMT one side TH | 10 August, 2011

Maybe I was not clear in my original post.

So we have boards that have SMD components on one side and TH components on the opposite side. We want to assemble the boards using our SMT and Wave lines. Now the problem is that if we solder the SMD first, then it is impossible to solder the TH components with Wave machine because the SMD components will be floated inside the solder pot. If we do the wave first, then it is impossible to do the screen printing.

I was wondering if there is a way to solder both sides using the machines, or we will have to do the TH parts manually?

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#64809

Assembling one side SMT one side TH | 10 August, 2011

Yes, there are ways to machine process boards like this. The devil is in the detail.

In no particular order, your choices are:

Process flow #1: * Print chipbonder on the surface mount side for SMT parts * Mount SMT components * Cure adhesive, if required by the adhesive supplier * Mount PTH components * Wave solder PTH components Be careful, not all boards are designed for wave soldering of SMT parts. Not all SMT parts are intended to be wave soldered.

Process flow #2: * Print solder paste on the surface mount side for SMT parts * Mount SMT components * Reflow solder SMT parts * Print solder paste on the PTH side for PTH parts * Mount PTH components * Reflow solder PTH parts Be careful, not all PTH parts can take the temperature of a reflow oven. Not all PTH holes can be adequately filled by printing paste.

Process flow #3: * Print solder paste on the surface mount side for SMT parts * Mount SMT components * Reflow solder SMT parts * Mount PTH components * Selective solder PTH parts Be careful, not all boards are designed for selective soldering of PTH parts.

Process flow #4: * Print solder paste on the surface mount side for SMT parts * Mount SMT components * Reflow solder SMT parts * Mount PTH components * Wave solder PTH parts using a pallet that protects the SMT parts

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#64810

Assembling one side SMT one side TH | 10 August, 2011

Not an easy question to answer with out seeing the board. There is selective soldering, gluing, maybe a wave soldering pallet. As DaveF said since your asking the question I would assume your shops expertise is limited.

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#64811

Assembling one side SMT one side TH | 11 August, 2011

Thank you very much for your informative and helpful reply. I have a few more questions:)

1- So for the case 1: should I apply the glue to the PCB with the same method as I apply solder paste? (with screen printer and squeegee). If so, I would guess that the stencil should be different from the stencil for solder paste in terms of the location and size of the holes for each component?

2- For case 2: I think it cannot be applied to our case. Our board is all SMD in one side and all TH on the opposite side. So if I do the screen printing on the SMD side and the P&P and reflow, then I cannot do another screen printing, because the pins of the TH components are on the same side of the SMD.

3- I have attached a pic from my board, I appreciate it if you could have a look.

Attachments:

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#64812

Assembling one side SMT one side TH | 11 August, 2011

We used process 1 that dave detailed. We used screen print adhesive on one line and pin transfer on another. The adhesive dots were applied to the body of the component allowing the wave to form the solder joints.

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#64815

Assembling one side SMT one side TH | 11 August, 2011

Franx: In response to your follow-up questions ...

For the case 1: Q1: Should I apply the glue to the PCB with the same method as I apply solder paste? (with screen printer and squeegee). A1: Correct. Glue can be stencil printed, dispensed, or pin transferred.

Q2: If so, I would guess that the stencil should be different from the stencil for solder paste in terms of the location and size of the holes for each component? A2: Correct. Generally, glue is applied at or in a pattern near the centroid of the component.

For case 2: 'Q3': I think it cannot be applied to our case. Our board is all SMD in one side and all TH on the opposite side. So if I do the screen printing on the SMD side and the P&P and reflow, then I cannot do another screen printing, because the pins of the TH components are on the same side of the SMD. A3: We understood all of that from the start. You’re thinking too much. Stop, take a deep breath. Now, just follow the instructions of the Process flow #2. It doesn't matter anyhow, we doubt those PTH electrolytics and display will bear-up under reflow. Likely, those components would have to be hand soldered post in this approach.

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#64823

Assembling one side SMT one side TH | 12 August, 2011

Dave is correct. Also, you should check the TDS's of the caps and the display before processing to see if they can withstand the process temps, look for a recommended solder profile or info for the manufacturability of the device. On another note, we have seen melf type components fall off in handling the product if using glue so make note to your operators if you choose to wave without a pallet.

Normally would use selective wave pallet IF the volume warrants the NRE.

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#64825

Assembling one side SMT one side TH | 13 August, 2011

Process Flow #4 in davef's response is your best bet for this assemblly judging buy your pictures. We purchase our selective wave solder pallets from Ascentec Engineering - they can fabricate a pallet to hold your assemblies, keep the SMT devices protected from the wave, and will include hold down bars to keep the parts straught and flush in the position you desire, you can even choose to put more than one assembly to a pallet. Of course there are other firms that make these, but Ascenetec Engineering (http://www.ascenteceng.com/) is the best IMHO.

Jim

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