This is an advanced DIY PCB project. Many components on this board do not have leads for hand soldering, and you will at least need a hot-air rework station to place those. Additionally, this project is almost entirely built on flex PCBs, which have their own set of challenges, particularly getting them to sit flat on a hot plate. Many efforts were made to keep components to just one side of the board, but there are some connectors that just have to be on the rear of the board and some of these connectors are fine-pitch and very challenging to hand solder.
DCMini is intended to be folded up and put in a small case, or worn as a flexible patch with an articulation between the digital and analog side of the boards. If your application doesn't need fold/flex, feel free to order an FR4 board, but otherwise, you'll need to find yourself a capable flex-pcb manufacturer. We have used OSH Park throughout prototyping to great success.
The DCMini PCB and "4head" electrode patch requires a fab that's capable of:
- 2 Layer flex PCBs
- 6mil/4mil Trace Width/Spacing
- 10mil Drill Hits // 5mil Annular Ring
The "4head" electrode patch additionally benefits from a fab that's capable of:
- 4mil/4mil Trace Width/Spacing (If you intend to use the FPC connector)
Note: These boards run right up on -- and in some places, slightly exceed -- OSH Park's minimum flex tolerances. We've had good luck so far, but your mileage may vary. Make sure you visibly inspect boards under a microscope before populating them!!
DIY assembly is easiest using a small hot-plate, lead-free solderpaste and a stencil. We've had great success ordering 4mil stainless steel stencils from OSH Stencils. Since most of the components are on the front of the DCMini PCB, we typically only order the front stencil and hand-solder the remaining components on the rear of the board.
DCMini can be outfitted with various configurations to meet all sorts of requirements/budgets. The most expensive chip on the device is also the central feature of the DCMini -- the Texas Instruments ADS1299. It comes in various channel loadouts: a 4, 6, and 8 channel version. Additionally, the DCMini has the capability to stack (daisy-chain) two of these together. Depending on which ADS1299s you pick, you can achieve different channel counts from 4 channels to 16 channels! Do note though, that if you choose to use the "DAISY" daughterboard, you'll need to populate it with additional components/connectors that drive the price up more.
DigiKey carts for the DCMini's components are available here:
The "4Head" patch has two components:
- 3-claw 4.0mm female "ECG" snap button connectors
- (Optional) board-to-board connector
PCB-mount snap connectors are difficult to source consistently.
The 3-claw 4.0mm female "ECG" snap button part the footprint is designed for was found on Alibaba, but this product listing could change or become unavailable at any time. If you have a more consistent source for PCB-mount snap connectors, consider a contribution with a new footprint/entry for BOM.
Additionally, you will need to use one of the DF40HC-60DS connectors if you cannot use/accurately fabricate the FPC connector. These sockets come in various "mating heights" that can give you additional clearance between the boards. The snap connectors protrude a bit through the back of the flex PCB, and the 3D printed case additionally increases clearance demands so you'll need to pick a DF40HC with sufficient mating height. 2.5mm mating height is likely the minimum. Don't populate this connector if you choose to use the FPC -- plugging in both will probably result in noisier recordings.
Assembly is made much easier using the interactive bill of materials included with the project. We recommend placing fine-pitch components (DF40 connectors, ADS1299) followed by placing the radio module and QFN leadless packages, then the small passives and everything else. We have used the Pixel Pump successfully to accelerate component placement by hand.
Reflow is easiest using the MHP50 and doing the board in quadrants -- you may need to build a support platform to support the rest of the board around the hot plate.
Rework is challenging with a design this dense, and reworking the leadless packages basically requires a hot air rework station. Once rework on the front is completed, we hand solder the components on the rear of the board. Check the testpoints on the rear of the board to ensure no power rails are shorted before powering up the device.