Sunday, May 8, 2011

Populating the backplane...

I finally set aside the time to add the components to the backplane (it's better if one is in the right mood when there are so many solder points).  The 5 din 41612 connectors provide power/gnd to the attached cards as well as a bus of 21 differential pairs and 20 single ended lines.  There is no termination on the backplane itself.  One the top is a microcontroller (Microchip pic 18F64K22) that connects to some of the single ended bus lines (for i2c and spi and msten), a single unique line to each connector (for spi select and i2c configuration remapping) and a series of indicator leds for status information and a low voltage serial interface for debugging.  I haven't used this particular one before but it should be ideal for this board.

The status connector can be left unpopulated (or the led array removed) to provide access to the status signal lines for testing purposes.  I here I am using larger current limiting resistors than I could because I don't need them to be very bright and I don't want to draw much current (I want aorund 1 mA).  They could be changed to lower values if you want. 

There is a little variation with the offset 40 mil holes but easily within spec and not a problem at all.   Plating through the holes is very uniform.

Looking at the bus lines (6 mil) (yes every single line to every single point) the alignment uniformity is excellent - the copper layer is the most critical... slight alignment shifts on the solder masks are no problem at all as long as the copper is right.  (Ignore the dust here - I was handling the boards a lot testing the electrical connections).

We all have our checklists and sequences for putting things together - (collect and sort the components first, start with the smallest components first, check the fit, clean the boards... an so on).  Because I have keyed connectors here I decided to mark the first pins on all the din connectors first (it would not be fun to find out 1 was in the wrong orientation later).

I have connectors from several companies and they should all interconnect - (I still like to try them all anyway).   On this board I put holes for the retention clips (they make it much easier to keep the connector in place until everything is soldered and add an additional bit of support).   Since I didn't have enough on hand I also used a version that doesn't have the clips.  Interestingly, while the upper part of the connectors are equally compatible with the male connectors, the bottoms are setup a bit differently.  The ones without the retaining clips were slightly easier to solder (I don't know why though).

The order I put it together was, clean twice with IPA on a lint free cloth -> after the smd components were finished -> I then did the din connectors -> followed by the sockets for the controller, power and status indicators -> them the single row connectors -> then cleaned the flux off the board with warm water and a soft toothbrush -> then flick almost dry -> check that the components are still all attached (yay they were) -> clean with IPA and a different soft toothbrush -> flick dry -> IPA on a lint free cloth to make certain residual flux is gone ->add electrolytic caps -> add a couple of batteries to check the 3v3 ldo (that's the happy green led here) -> add the controller chip and indicators -> done.

If all the boards come out this well I will be very happy... (two more boards to come before I can really start signal testing - the controller code will be posted when I get it finished).  The design for this one can be found here.