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Placing components on both sides of the board

Posted on November 16, 2017.

One of the questions that we've been getting lately is how do you deal with the boards, populated with the components on both layers? Do you have to glue them, so that they don't fall off? How do you rework them if you need afterwards? How do you dispense solder paste on the second layer, so that the first layer doesn't get smeared? All of those are very good questions.

Actually, this feature wasn't supported by Cirqoid until recently. There were two obstacles there:

  • software support - this required some hacking with the Gerber files to get both layers dispensed
  • hardware support, i.e. having a possibility to lift the board while dispensing and placing the second layer, so that the first layer doesn't touch Cirqoid's bed

Software support was introduced in cirQWizard a while ago. As for the hardware, all new Cirqoids include a pack of spacers for double-sided dispensing (the same pack is available for all previous owners of Cirqoid in our store at a symbolic price - Double sided dispensing/placement spacer).

So, to show how this all works and what the procedure is, we've thrown together a demo project. The gadget is a USB temperature logger - an MCU, FT232 to communicate over USB and a connector for an external 1-wire thermometer. Not a lot of stuff, and the board measures 27x14mm. It is designed to be soldered directly to the USB male connector - sort of like a USB flash drive.

That's the layout of the board:

USB thermometer board layout

As usually, all the relevant files are available for the download. Here's the link to schematics, layout, exported Gerber files and firmware sources - USB thermometer.zip

Milling and drilling

We started as usually: milled both top and bottom layers (we also did a rub-out this time), drilled a few holes and milled the contour. Here's the board after these operations:

 

Vias

On this board we opted to use copper rivets for most vias as they had to be under IC to keep the layout small. We also added just a bit of solder between the rivets and traces, just to be on the safe side:

Placing top layer

Now, we've got to the solder paste dispensing. We put the board on the spacers:

Even though, on the first layer it doesn't matter much, it's just easier to follow the same process and have the same Z offsets for both operations (by the way, we've seen some our customers milling the boards on the spacers - this doesn't seem to be a particularly good idea, the board is poorly supported when it's on spacers). We now dispensed the paste for the top layer and proceeded to place top layer components. 

Here's the board with populated top layer:

Now the board goes into reflow oven for a usual reflow cycle. This is what we've got out of the oven:

One solder bridge between FT232 pins, some tiny solder balls around the pins - nothing unusual. We'll take care of after the second layer is done.

Placing bottom layer

After letting the board to cool down, we placed it back on the spacers, dispensed the paste on the bottom layer and placed the components. The board is now ready for its second reflow cycle:

During reflow, top layer components are going to be upside down. Unless the components are unreasonably heavy, surface tension will hold them in place while solder is liquid. After the second reflow cycle, we've got the board with the components soldered on both layers:

Final touches

Nothing has changed on the top layer, which was upside down in the oven. We can now get the board out of the panel, solder in connectors and do a little touch up of a few pads here and there. After all that and a bath in ultra-sonic cleaner, this is what how the final board looks:

Test run

We threw together a little testing stand to see how our new gadget works:

In case you are wondering what happens when you heat a component with hot air gun, here's the chart (for the record, the hot air gun was set to 150º):

Summary

So, to summarize, the procedure for placing the components on both sides:

  • Mill and drill as usual
  • Dispense paste on the top layer
  • Place components on the top layer
  • Reflow the board
  • Dispense paste on the bottom layer
  • Place components on the bottom layer
  • Reflow the board

It doesn't matter whether you start populating the top or the bottom layer - you can dispense and populate bottom layer first if it's less tricky than the top.

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