Saturday, December 29, 2012

Breakout PCBs from OSH Park for the MCP9808

About a month ago I put in an order for some PCBs from OSHPark.  They were nothing special, just a break out board for the MCP9808 temperature sensor from Microchip, the order was more about testing out the service and getting some practice designing PCBs.  Well, they finally turned up a couple of days ago and I couldn't be happier.  So I though I'd document the design and final product.

The board is pretty basic.  It's only 0.75 x 0.50 inches with a couple of ESD protection diodes, current limiting resistors and a capacitor to filter the power supply.

MCP9808-Breakout_ProjectFiles.zip
MCP9808_Breakout_Schematic.pdf
MCP9808-Breakout_Gerbers.zip

Printed Circuit Board
PCB Top Layer
Printed Circuit Board
PCB Bottom Layer

I'm a bit of a cheapskate, so I designed the board in Kicad.  I'm familiar with Altium designer so Kicad took a bit of getting used to, but it wasn't too bad.

PCB Layout
Final PCB Design in Kicad

There's a built in viewer in Kicad, but I prefer to use gerbv to get an idea of the final result.

PCB Layout
gerbv Render of the Top Layer

PCB Layout
gerbv Render of the Bottom Layer

The MCP9808 temperature sensor comes in two different packages, an 8 lead DFN, and an 8 lead MSOP.  Ideally in my design I want to use the 8 lead DFN but I'm not sure if I'll be able to solder it successfully, so in the event that I can't, I've put footprints for both packages on the PCB in parallel.  That way if I can't solder the DFN, I can still test the design on the board with the MSOP package.

Printed Circuit Board
Duplicate Footprints

When designing the board, I was worried about a couple of things related to alignment.  One of them was the alignment of the drill holes with the copper layers.  I followed the tolerances specified and was happy with the result.  The picture below shows a slight misalignment of the of the hole, but it's within spec.

Printed Circuit Board
Drill Hole Alignment

The other thing I was worried about was the solder mask between the pins of the DFN package.  There's only 0.2 mm between the pads, and in case I want to reflow the board using solder paste, having some solder mask in there is going to come in handy to prevent bridges.

You can kind of see it the photo below, but there does seem to be a pretty good amount of solder mask between the pads. Further inspection with a jewellers loupe confirms this.

Printed Circuit Board
Solder mask between DFN pads

I've also had a little bit of practice placing the parts on the board.  Not easy, but doable.

Printed Circuit Board
Parts in position

Printed Circuit Board
Parts in position
So overall I'm pretty impressed, but there are a couple of problems with the board.  The silkscreen wasn't exactly as I'd expected, but that was my fault for not leaving enough clearance around it.  There's also the standard ever so slight misalignment problems between the copper, solder mask, drills, and silkscreen, but nothing outside of the norm and what you expect when you design a board.  In the future I need to pay attention to this a little more and alter my design accordingly.

The other major issue is delivery time.  Stupidly I went for the cheap postage forgetting that it would take about 5 weeks to get to Australia, next time I'm going to fork out the extra couple of buck for the faster option.  Why on Earth does it take so long to get through Australian customs?  It's excruciating.

Overall I'm thrilled with the price and quality of the boards, the service was great and I'm definitely going to be using OSH Park again.  I've designed boards by committee before and the results were pretty average, this is the first time I've had a go myself and I really enjoyed the experience.  I'm still amazed at the process though, send off some design files and a few weeks later the PCBs turn up in the mail.  Barely had to leave the house.  I think my logo on the board below sums up my feelings pretty well.

Printed Circuit Board
Neville logo

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