Flat VIAs for double sided PCBs

Who does not know the problem, if working with PCB design a single sided PCB reaches its limits.
Double sided PCBs offer some more freedom in routing. But also when working on high frequency designs a ground plane is a must. Doing double sided PCBs is a bit harder from the alignment point of view, but also doing all the vias is a pain.

One way to produce vias is to put some wires through the holes and solder them on both sides. Some annoying problems doing this is that the wire is soldered on one side and needs to be soldered on the other side. By soldering on the second side the wire falls off since the cooper heats up the first side and both sides are melt. Another problem is that one usually gets a higher bump which makes it very complicated to have vias under ICs and other components.

So it would be good to be able to produce flat vias which fit under components, but also hold in the hole by them self.

What I did is to achieve this goal is to use silver wire of the same size as the hole one wants to get the via through.

I have 1mm holes and bought 1 mm silver wire. This wire is basically a cooper wire coated with some silver.

But I’m not soldering it to hold I press it into the hole from both sides that it sticks inside the hole. And gets nearly as flat as the whole PCB. This is probably already enough to get contact with the cooper but to be sure I solder it additionally with the board.

I will explain the technique in a bit more detail with the USB to FPGA converter board I designed for the FTDI UM232H USB 2.0 Developement board.

Starting with the drilled PCBs:

FPGA_Back_USB_Interface FPGA_Front_USB_Interface

The next step is do cut from the silver wire small pieces. Approximately 2mm length should do the job when using a 1.5 mm thick PCB.

FPGA_USB_Interface_VIA0

The next step is to put the cut pieces into the holes and push them through to the other side.

FPGA_USB_Interface_VIA1 FPGA_USB_Interface_VIA2

Now comes the part which makes the vias flat. The silver wire is pressed from both sides into the hole. This can be easily done by tongs or by a bench vice.
The result are very flat vias which in principal already have contact with the top and bottom layer of the PCB cooper.

FPGA_USB_Interface_VIA3

After having all vias placed on the PCB and pressed. Top and bottom side:

FPGA_USB_Interface_VIA4 FPGA_USB_Interface_VIA5

To be sure that the vias have contact I solder them as well.

FPGA_USB_Interface_VIA6 FPGA_USB_Interface_VIA7

Looking at the vias and the soldered board one can see that they are much lower compared to all the other components.
Using better solder wire or some flux one can get even lower in height.

FPGA_USB_Interface_VIA8 FPGA_USB_Interface_VIA9

And finally how the whole PCB looks like after being equipped with ICs and the FTDI UM232H USB 2.0 Developement board.

FPGA_USB_Interface_Done

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