PCB assembly/microsoldering options

[misaacson01]

One of the more challenging aspects of building the VR headset is assembling components onto the custom PCB. Assembling circuit boards can often be very straightforward with basic soldering, but with the miniscule surface-mount connectors used by the displays of this headset, the PCB-assembly step becomes non-trivial. For those who don’t currently have the expertise and/or equipment for PCB assembly nearby (and I think this will be the case for many) then I just wanted to list here some basic solutions to consider. If you’re reading this, then I would guess the cheap DIY option (option 3) is what you’ll prefer so feel free to skip to that, but I at least wanted to mention some very professional options first:

Option 1: Use a PCB assembly service

Just like the bare PCB is ordered through a PCB printing service, an assembled PCB can be ordered through an assembly service. In fact, many PCB printing services have options to bundle in assembly, which can be the most convenient way to go. However, this option can require that the printer/assembler already has all the components in stock, and I have yet to find a service that stocks the 13-pin FPC connector our headset display uses (if you find one, let me know!). So, instead of bundling printing and assembly into one semi-automated service, you can directly contact assembly services with a description of the PCB you have and the components to be placed, and they can give you a quote per board. They might require you to purchase/ship the boards and components to them, after which they would have a one-time setup fee (which can be expensive) followed by a relatively low price for each assembled board. This process makes the most sense for large batches where the setup fee is a small fraction of the total cost, so if you just want to assemble one or a handful of boards that don’t have many components (like this VR headset), one of the manual-assembly options below may be better. But if you’re OK with the upfront costs and longer lead times and really want those perfectly assembled boards, then using an assembly service is ideal.

Option 2: Assemble yourself using professional microsoldering equipment

Manual PCB assembly is a relatively common practice but can take serious skill to master and often uses more expensive tools than typical soldering stations. Microsoldering small surface-mount components may rely on hot air baths, pencils, hot plates, and/or ovens to flow solder paste and create the clean electrical connections, and a microscope or magnifier to work with the smallest components. One such method is described in this nice video from zephytronics. To use this method and equipment to solder the FPC connectors of the VR headset, the process would go something like this:

• Apply a thin bead of solder paste along the rows of connector pads on the PCB (2 rows per FPC connector, 2 connectors per board).
• Position the connector onto the pads, pressing the pins into the solder paste beads. The edge of the connector should line up with the silkscreen outline on the PCB, with the connector's black flap pointed toward the cutout of the PCB where the display's ribbon cable feeds through.
• Mount the PCB above an air bath and turn it on to safely heat the PCB and components up to a working temperature.
• Direct the air pencil at the FPC connector pins one at a time until all the solder has melted and flowed, and all pins and pads are connected.

The obvious downside to this method is that the initial cost of the equipment (such as a hot air pencil) is pretty high. If you’re planning on doing a lot more PCB prototyping then it can be a good investment, but if assembling PCBs is not something you’ll be doing often then I would point you to the next option for significantly cheaper manual PCB assembly.

Option 3: cheap DIY microsoldering

The concepts behind this option are very similar to those in option 2, but are simply accomplished using much cheaper (and only slightly less ideal) equipment. There are many different approaches for this, but the method I’ll recommend will require (in addition to a basic soldering setup, including a soldering iron and tip cleaner) just a few inexpensive tools.

Tools:

• Tweezers
• Solder paste
• A heat gun (capable of reaching temperatures for reflow/rework)
• A fine-point tip for your soldering iron

Instructions:

• Apply a thin bead or layer of solder paste along the rows of connector pads (2 rows per FPC connector, 2 connectors per board)
  • Often times, solder paste can be difficult to adhere to the board in a thin layer, especially if the paste is a little dry. In this case, apply a clump of paste somewhere on or near the solder pads, and gently blow some warm air from the heat gun onto the paste until it becomes more liquid and easier to work with. Then, you can use the tweezers to manipulate the paste into a small layer along the rows of pins. Excess paste can be wiped away with tweezers holding a small piece of cloth.
• Position the connector onto the pads, pressing the pins into the solder paste beads. The edge of the connector should line up with the silkscreen outline on the PCB, with the connector's black flap pointed toward the cutout of the PCB where the display's ribbon cable feeds through.
• Start gently blowing hot air onto the PCB and components, beginning with the heat gun held farther away (e.g. 8+ inches) from the board.
• After about 15 seconds of slowly heating the board and components, gradually bring the heat gun closer and closer to the components, just until you can see the solder start to melt (gaining that metallic sheen of liquid metal) and create the electrical connection between pads and pins. Don’t hold the air gun any closer to the components than necessary to flow the solder -- if the connector gets too hot, the black plastic flap will begin to warp. If the connector has shifted at all during this process, using the tweezers to gently slide it back into position, maintaining the flow of hot air to keep the solder liquid.
• Once all the solder has flowed, turn off the heat gun and leave the PCB to cool untouched for a few minutes. Bumping the PCB at this stage could shift the components off their pads, as the solder will stay liquid until the PCB has cooled off.
• After the PCB has cooled, visually inspect the connectors and pads for loose solder microbeads, and solder bridges between pins. Loose solder can be wiped away with the fine-point tip of the hot soldering iron. Solder bridges (where adjacent connector pins are connected by solder) can occur when a little too much solder paste was applied to those pads, but these are easy to fix: simply place the fine tip of the hot soldering iron on the center of the bridge until it melts, then quickly pull the tip away from the connector. Repeat this process, cleaning off the soldering tip each time, until enough excess solder has been pulled away and the bridge disconnects.

Whichever option you choose, after the PCB has been populated with components (both surface-mount connectors and through-hole headers), continue with the rest of the assembly and installation instructions to verify that everything is in working order. Or if you want to more quickly check that your board is assembled correctly, use a fine-point multimeter probe to verify the connections match the pinout information described here.

Personally, I have preferred option 3 for the many VR headsets and monocular displays I’ve assembled, but I would love to find a nicer, easier option for people who don’t want to mess with microsoldering on their own. So, if you have any suggestions for easier PCB assembly, either through a service I don’t know about or an easier DIY method, feel free to add to the discussion below!