The big one

I built the top side of my bityExpress-B board last night.  I had to stack some PCBs under my stencil frames to account for the components that I loaded previously on the back side of the board.  The paste worked well everywhere except U4 (the on-board programmer's CPLD).  Since I can bypass the programmer I went ahead and loaded the components and baked it.

All of the solder joints looked good except... you guessed it... U4.  It had at least 3 shorts.  I cleared those and tested.  So far the power supplies all work and I can program the CPLD and the FTDI FT245R.  I stopped when the on-board programmer failed to recognize the FPGA.  I'm betting that is related to solder issues on U4.  I'll try a bit longer to resolve U4's issues and if I fail I'll just bypass it and use an external programmer to check the FPGA.

Ready for paste

Paste looks good (except for U4)

Bakin'

Another day, another stencil

I decided to try my hand at two-sided reflow before I run the BGA on bityExpress.  The board that I designed is a simple LED board that I can use to verify if all of the pins on the bityExpress expansion connector are connected to the FPGA correctly.

When I ordered the stencil I found OSH Stencils (no connection to OSH Park).  They make polyimide (Kapton) stencils like OHARARP, but they will build smaller stencils down to a minimum order of $5.  They also sell adjustable stencil frames for $5.  Based on what I see I'm sold.  Now I don't have to try to combine stencil orders to make them affordable.

Stencil and frame from OSH Stencil

Paste applied

Parts placed

Ready to reflow

Reflow complete

Backside build

I ran the backside of bityExpress-B using my toaster oven on Monday night.  I was planning on running the backside for all three PCBs and pick the best on to run again with the topside components, but I changed my mind when I saw how well the backside ran the first card.



Solder paste applied:

Parts placed:

After reflow:

Now I'm just waiting on my Altera Cyclone V FPGA to arrive before I build the topside.

Simple might be better

I was trying out a few additional manual profiles with my toaster tonight and surprisingly this one looks best:

• Set the oven to its maximum setting and turn it on
• After temperature has been above 220C for 30 seconds turn the oven off
• 60 seconds after turning the oven off open the door

This profile isn't perfect, but it is really close:

• Peak temperature: 227C
• Time preheating (between Ts1 and Ts2): 135 seconds
• spec:  60-120 seconds
• Time above 219C (Ta): 96 seconds
• spec: 60-120 seconds
• Time from ambient to peak temperature: 8 minutes and 14 seconds
• spec: 8 minutes

Looks like I have a profile.  My new paste should be arrive in a day or two.  Solder paste is supposed to be shipped overnight so that it arrives before the included cold pack warms too much.  I saved a few bucks counting on the polar vortex to keep my paste nice and cool.  Hopefully I won't regret that.

Toaster

I couldn't resist getting a toaster oven today to see how hard it would be to profile.  I found a Black and Decker TO1675W at a local retailer.  The first order of business was to drop in a few thermocouple probes and see just how hot this oven can get:

With the oven set to 11 it reached 250C in around 12 minutes.  This is 15C beyond the typical maximum safe temperature for lead-free parts and certainly well beyond the liquidous temperature (219C) of most lead-free solders:

I then attempted a manually controlled profile.  I plan to automate my profile, but for now I'll just close the control loop myself to build a few boards.  I placed a bityExpress rev A card in the oven with a thermocouple placed near the center of the PCB:

My initial profile attempt was the following:

• Set the oven to 180C and turn it on
• After the temperature has been above 150C for 60 seconds, turn the oven up to its maximum temperature
• After the temperature has been above 220C for 30 seconds, turn the oven off

Comparing this to Altera's SMT Board Assembly Process Recommendations (AN-353-4.0):

• Time preheating (between Ts1 and Ts2): 143 seconds
• spec:  60-120 seconds
• Time above 219C (Ta): 102 seconds
• spec: 60-120 seconds
• Time from ambient to peak temperature: just over 8 minutes
• spec: 8 minutes

Why did I ever trust the paste?

After looking again (and getting some advice), I've decided that the real problem was that the paste was too old.  When solder paste ages it looses flux due to its volatility.  When I heated the PCB last night it smoked much less than when I ran the bityBGA boards (a clue I missed).  Today's inspection settles it:

Oh, thermal mass

I used my rev B stencil to apply some paste to an extra rev A PCB to test my ability to solder bityExpress.  Unfortunately I had poor results.   I couldn't get the paste hot enough to flow evenly.

I'm not sure if the issue is with the larger thermal mass of this card or if I need to adjust the IR lens a bit more.  I was discouraged enough to look into toasters.