Summary: cheap ‘n cheerful, good enough for double-sided experiments, but not good enough for 4-layer impedance-controlled PCBs.
The laser toner technique and very-low-cost PCB manufacturers are good enough for the low-speed digital signals found in Arduino-class circuits. They are not suitable for large boards, or for containing medium speed digital signals capable of bit-rates up to 1Gb/ or 2Gb/s. A requirement for 50Ω or 100Ω differential impedance lines on >=4 layer PCBs implies the PCB cross-section “stack” must be tightly specified in terms the prepreg’s thickness and .
This post outlines experiences for small experimental boards.
Laser Toner Transfer
I’ve used the traditional laser toner mechanism that’s described in many many blogs, but only for:
- through hole components on single and double-sided boards, with acceptable results
- gash test boards when experimenting with SMD components
This is acceptable for producing Arduino-class circuits with analogue and low-speed digital logic.
The results for SMD components are surprisingly good, and would just about be acceptable in an emergency.
I’ve needed to make some quick-and-dirty RF 50Ω attenuators and 50Ω / 75Ω matching pads for use up to ~2GHz. These circuits consist of two edge-mounted SMA connectors and 1, 2 or 3 SMD resistors mounted on double-sided FR4 board in a “co-planar waverguide with groundplane” configuration. At approximately 15mm*10mm, these boards are small physically and electrically.
The track (singular) needs to be the appropriate width for the impedance. That can be determined with any of the many online calculators; I used Chemandy Electronics’ calculator.
The track can be made by using a hand-held Dremel to “mill out” the gap between the track and ground.
For small and/or experimental double-sided boards, I’ve used DirtyPCBs principally for cost, but also to have a benchmark against which I can evaluate a “real” PCB manufacturer. The principal features were:
- 10 off 5cm*5cm double-sided is $14 (i.e £8-9) including sea shipping
- order placed to PCBs received: 21 days
which is remarkably good value.
Comments on the DirtyPCBs board:
- the silkscreen is sufficiently legible but is not a continuous line; this is unsurprising since I pushed their process to the limit (0.15mm width)
- the solder mask can be offset by 0.2mm (board too small for scale problems to be evident); whether this is a problem depends on the PCB’s feature size. Solder mask on top of a pad can be easily removed by scraping with a jeweller’s screwdriver
- one board had an unacceptable 1mm diameter blob of unetched copper, bridging two connections. If it had occurred on the inner layer of a 4 layer board, it would have been for high-speed logic, and would have been invisible and uncorrectable
- another board had several spurious 1mm diameter circular holes in the solder resist. In the wrong places, this would probably allow solder bridges
There is no way that board should have passed a “100% etest”. I believe the etest only checks for continuity on the basis that plated thru holes are the most likely defects – but it does mean a completely unetched board would pass! So, black mark for quality against DirtyPCBs, but was I unlucky or are other manufacturers actually better?
However, even with those problems, I have used DirtyPCBs again and will continue to use them in the future for two-layer boards.
Another blog about one person’s opinion of Seeed vs ITead Studio vs OSH Park might be relevant.