Author: Alan Hawse

Summary

If you have to use a hot air solder tool, you are probably having a bad day.  But when you need a hot air solder rework tool, you are very, very glad to have one.  I am sure that there are people who can get a QFN off a printed circuit board without one, but I am not one of them.  It is even more fun to put a QFN back on a board without one.  But, once again, I am not one of those people.

Hot Air Solder

I have a Metcal HCT-900-11 Hand Held Convection Rework tool aka Hot Air Solder tool.  This machine can blow out hot air … really, really hot air, up to about 950 degrees Fahrenheit.  I’m pretty sure you could cut your arm off with it.  The tool has a bunch of nozzles of different shapes and sizes.  Using one of these things is an art form because you need to get the temperature right, the airflow right, the distance from the board right or you will find yourself blowing components off the board and making things worse.

Pinball machine PCB Solder Problem

In the post where I discussed the first test of the Pinball board I told you that the power supply was not working.  It turns out that the cause of the problem was a big blob of solder under the accelerometer.  To fix that problem I had to remove the tiny little 3x3mm QFN accelerometer using the hot air solder tool.  Once I did, there was a big mess of solder under the package which I cleaned up with a braided copper wick and a soldering iron.  I suspect that I put too much solder onto the pads with the metal stencil when I built the board.

Here is what the board looked like after I cleaned it up:

And here is a picture of the damn tiny chip.  The problem with all of these chips is they are meant to be used in cell phones, so they are as small and thin as possible.  And to make matters worse, the pads are on 0.5mm pitch or smaller.

Hot Air Solder Rework

When I first started fighting with these QFN problems, I talked to the guys in the Cypress Development Kit team in India.  The manager of the group recorded this video of Jesudasan Moses, who is a wizard with hot air solder tools and a soldering irons.  This video was a huge help and I am grateful to them for recording it.

To fix my problem I followed the same process.  Here is the video:

After three attempts, I got the chip  mostly on.  As you can see in the picture below, I was missing a connection to one of the pins.  The other problem was that I melted the screen with the hot air.  Suck.

After I touched up that pin with a soldering iron I still couldn’t talk to it with the PSoC, and I didn’t have easy access to the I2C bus to figure out what was going on.  In the future I need to make sure that there is a place to plug into the I2C bus to debug network traffic.  I will add this to my PCB design guidelines.

Back to the soldering iron to fix it.  I started by putting some flux on the pins of the QFN package.  Then, I draged a very fine tipped iron with a little bit of solder on it across the pins.  After a couple of rounds of this, I got the chip working.  I still need to cleanup the flux, but here is a picture of one side:

And here is a picture of the other side.  You can see that I mangled the capacitor to the right during the rework process.

I think that a significant source of problems was not having the entire underside of the QFN with Solder Mask.  That would have helped repel the solder and kept the blob from forming.  I think that I will add that to the checklist as well.

Unfortunately when I was looking at the layout trying to figure out what was going on, I realized that the accelerometer interrupt pin was not connected to the PSoC.  This happened because I had the pins connected by “name” in the schematic.  This is something else that I will add to the checklist.

Using a hot air solder tool is definitely an art form.  One that I will need to practice quite a bit before I get to anywhere near Jesusdan level of skill.

You can find all of the source code and files at the IOTEXPERT site on github.

Summary

I spent a lot of time the last few days building and testing the Pinball board.  This morning when I plugged in my oscilloscope to test the PSoC motor driver, I saw this:

Here is a screenshot directly from the oscilloscope.  When I saw this, I wondered what was going on.  Why does the driver pull-up very quickly, but have some kind of RC delay on the pull-down?  This was particularly troubling as the PWM was set to 50 Hz (see on the picture where the (1) Freq is showing the measurement)

Things got much worse when I increased the frequency to 500 Hz.  In fact, the motor driver doesn’t pull-down at all.

PSoC Motor Driver

So where do I start debugging?  First I looked at the PSoC Creator schematic.  When you look at the schematic you can see that I took the PWM output and steered it to either M0L or M0R (or M1L or M1R).  Then I used a control register to flip it.

The M0R/M0L signals are used to control the switches on the H-Bridge.  Here is the actual schematic from the PCB:

I chose the Toshiba TB6612FNG motor driver chip for this design.  It has two H-Bridges built in and is the same one that we used on the PSoC 211 Robot.  You can see from the schematics above that I save 1 pin by attaching the PWM input to “H(igh)” and that I pulse either In1 or In2.

When you look at this table, or the schematic, you’ll see my problem.  I connected one of the inputs to “L”  That means that the NMOS switch that controls the pull-down is always off.  When the PWM is low, there is no pull-down.  That explains the funny RC pull-down, which is just some leakage on that node.

To fix this, I make this change to my PSoC Creator schematic.  In this configuration I get the following:

CR=00 the output = 00 (stop)

CR=02 the output = 1PWM = CCW

CR=03 the output = PWM1 = CW

Now when I run this thing, here is what I get (at my desired 20KHz):

And finally a video showing the PSoC Motor Driver working:

Summary of my PCB Solder Reflow Process

Getting the purple envelope from OSH Park is always exciting.  The big question is,  “After all those hours studying the Eagle PCB editor, did I make the same error three times in a row, or did I finally get it right?”  In this post, I will take you through all of the steps that I follow in in my PCB reflow solder assembly process.  I use a Qinsi QS-5100 PCB Solder Reflow Oven which I talked about in detail in a previous post.  One of the interesting things about this blog website is that it has driven me to write down some of my procedures and move away from the “just wing it” method.  I believe this will pay long term dividends for me.  So, Thank You!

My board assembly process is as follows:

1. Take solder paste out of the refrigerator and let it warm up (probably at least 2 hours)
2. Test fit the through hole and major surface mount components
3. Print out the parts list to use as a checklist
4. Setup all of the parts on the desk in some order (using checklist)
5. Tape the board holders to the desk
6. Make sure the solder stencil registers correctly
7. Screen on the solder paste (video)
8. Inspect the solder paste with a microscope
9. Place all of the parts
10. Reflow the PCB in the Qinsi QS-5100 oven
11. Visually inspect the solder and clean up any solder balls
12. Solder in the programmer connector and program the board
13. Cleanup the disaster area

Take the Solder Paste out of the Refrigerator

You need to have solder paste at room temperature as cold solder paste does not spread very well.  I recommend that you take the solder paste out of the refrigerator at least two hours before you use it.  I use Kester EP256 solder which comes in a syringe or a tub.  In this case, I purchased it from OSH Stencils when I bought the original stencil.  In two of the videos I watched on the internet, the person put extra solder back into the tub but obviously that cannot be done with a syringe.  My experience has been that older solder paste that has been exposed to air will not reflow properly.  That will make you crazy, and is the reason why I have not gone down the road of saving extra solder.  I cannot emphasize enough to keep your solder cool and fresh.

Test Fit Components onto the OSH Park /Eagle PCB

When I get a new board, the first thing that I do is make sure that the major parts align to their footprint.  You don’t want to get down the road with paste on your PCB and find that you screwed up the size or placement of a component.  Here is a picture of my lab assistant test fitting the PSoC.

Parts Checklist

While I am designing the PCB in Eagle I always create a BOM spreadsheet with the names of the parts, the part number, the label on PCB silkscreen, etc.  I use this spreadsheet to ensure that I have all of the parts available to put on the board.  It sucks to get a PCB with solder paste on it, then be missing a part that you need.  Moreover, you need to move reasonably quickly once you have paste on the board (or the paste will harden) so making sure you have everything at hand is critical.

Setup all of the parts on the desk in some order

For some strange reason, it always feels stressful placing the parts onto the PCB.  In order to make the process smoother, I arrange the parts on my desk in some order.  In this case, all of the parts are by type i.e the resistors are all together.

Tape the PCB Brackets onto the Table

Once I have things setup, I use angled brackets (which came from oshstencils)  to hold the board.  I use blue painters tape from Lowes to hold it all together.  The other thing that I do during this step is remove the tabs from the PCB manufacturing process and file off the rough edges.  The file is just a finger nail file I bootlegged from my wife (don’t tell).

Make sure the Stencil Registers Correctly

The next step is to insure that the stencil registers properly with the board.  You need to verify that the stencil is aligned in both the x-y and angle directions.  It is easy to be “close” but twisted which will give you bad results.  For this build I am using a metal stencil from OSH Stencils.

Screen on the PCB Solder Paste

Once every is lined up, you screen on the solder page.  Here is a video of me doing it for this PCB.

When I was originally trying to figure out how to make this whole process work I found this video from SparkFun which was very useful.

Inspect the PCB Solder Paste with Microscope

Once you have the solder paste screened onto the board, you should look at it carefully.  I typically use my microscope because my vision is not that great.  However, this blowup picture of the board works pretty well.  The only real problem in this picture is the blob of solder on U2 (the Accelerometer) which will cause problems in the reflow.  It is very tough to not get too much solder on the 0.5mm pitch pads.

Place all of the Parts onto the PCB

I was originally planning on making a video of placing the parts onto the board.  But, I need to look in the microscope while I am placing the parts on the board and I didn’t have a good way to record video.  I use a pair of Hakko 7-sa non-static tweezers  which I bought from Amazon.  I have a bunch of different pairs of tweezers and these are by far the best.  This is a picture after all of the parts are placed:

Place in Qinsi QS-5100 PCB Solder Reflow Oven

Now that you have the parts placed onto the PCB, it is time to place it in the PCB solder reflow oven.  I have found that it works best to have my PCB sitting on another board in the middle of the tray (a tip that I got from Ian Lesnet of Dangerous Prototypes).

Visually Inspect the Solder and Clean up any Solder Balls

I almost always end up with 1 or 2 solder balls stuck between the pins of one of the chips.  When you are running the reflow, solder that isn’t attached to anything will ball up because of the surface tension.  Murphy’s law says that the solder balls will not end up anyplace good, and will for sure short some pins together.  You should look carefully for these balls or bridges and clean them up with your soldering iron.  This is a picture of a solder ball from another project.  You can see it between the right two pins on U6 (the chip labeled LTAJT).  The easy way to fix the ball is to squirt a little bit of flux on it, then touch it with your soldering iron.  That will melt the ball onto the pins next to it.

Solder the Programmer Connector into the PCB

The (almost) last step is to solder on the header for the Miniprog-3.  Then let it rip with a blank program program.

Cleanup the Disaster Area

Once all of that is done, my office is a complete and total disaster area.  Yes, I use gloves when I work with solder and you should as well.

In the next post I will take you through the steps I followed to test the Pinball PCB.

You can find all of the source code and files at the IOTEXPERT site on github.

Goals for fixing the PCB Layout using Eagle

In the last Pinball post I talked about the 1.0 version of the Eagle PCB not working.  You might notice that the version number on this post is 1.2.  It seems strange to skip a version number.  Well, it turns out that I didn’t skip a version number in fact,  I did the most ridiculous thing possible, I made exactly the same error twice in a row on the PCB layout. That is, I flipped a chip footprint.  Enough about that.

In this post I am going to talk about the new printed circuit board.  As I updated the PCB design for this version of the board I had the following objectives

1. Fix the flipped footprint
2. Put a 1″ LCD screen on the board
3. Move the whole system to 3.3v and simplify the power routing
4. Provide a jumper selectable 5v/3.3v source for the motors
5. Fix the USB footprint to a USB plug that can be purchased
6. Make the silkscreen more readable (and fix the errors)
7. Fix the ground plane grid on the CapSense Buttons

Here is the OSH park rendering of the new PCB layout.

Fix the flipped footprint

I would have bet money that I fixed the Eagle library to have the correct foot print when I did PCB 1.1.  However, after I renumbered the pins in the package editor, I did not unhook and then rehook them in the library editor.  The result was a flipped footprint, again.  What is particularly frustrating is that I didn’t recheck it on the PCB.  I now have added that to my PCB checklist.

Put a 1″ LCD screen on the board

On one of our WICED WiFi development kits, the IoT team put a really cool 1″ OLED LCD screen.

After I looked around a little bit, I found that the little screen was very available for $3-ish on eBay.  My original plan was to use a bigger screen with an external connector.  But these aren’t as cool and they cost $8.  See what I am saying?

In the upper right hand part of the PCB, I extended it a little bit and put on the footprint.  The only hitch is that it appears sometimes the pins are “VCC/GND” and sometimes the pins are “GND/VCC”.  In order to handle this, I setup four 0-Ohm resistors to allow me to switch the pins.

Move the whole system to 3.3v and simplify the power routing

There was no reason to run the system on 5V other than giving a little bit of extra power to the motors.  So, to simplify things I moved the PSoC to 3.3V (which it will happily do).  That allowed me to remove the level translator that I was using for the accelerometer.  By doing this, I could move all of the 5V power to the far left of the PCB, and not route it all over the place.  Here is a screen shot of the PCB layout.  You can see that the power comes in from the USB connector, then goes straight to three places

1. The 3.3v Regulator
2. The Mini-Prog-3 Connector
3. The Motor Power Selector

Provide a programmable 5v source for the motors

I setup the motor power to be either 3.3v or 5.0v based on the selection of a jumper.  The jumper is a 3-position header on 100 mil spacing.

Fix the USB footprint to a USB plug that can be purchased

In the first version of the Pinball PCB layout I used a USB footprint that I found on the internet.  But, it turns out that footprint was not available.  In fact, I had to search and search for those parts.  I finally found a scrap shop in Poland that had a few of them available.  That was a bad idea.  Here is the new footprint:

Make the silkscreen more readable (and fix the errors)

My eyes really don’t like any silkscreen that is less than 30mils tall.   There are some people who can see a smaller font, but I am just not one of them.  I really prefer 40mil tall letters to occur in my PCB layout.  I have updated the checklist with a chart of what is legible and illegible.  In addition I had a few errors like the one below (there aren’t two V50 pins next to each other, one is supposed to be ground.)

Fix the ground plane grid on the CapSense Buttons

To get the best CapSense performance you should not use a solid ground plane in your PCB layout (like the one below).  You should use a cross hatched ground (like the picture at the bottom).  You can read about this in the Cypress application note or in my design guidelines.

You can find all of the source code and files at the IOTEXPERT site on github.