SMD bronze board assembly

As exercise #2 let's assemble the AVR32 board called bronze. See attached files below for detailed information on the bronze board.

For background information and an overview, see smd.


Parts are kept in the ring folder in transparent envelopes. Each envelope is labeled with the bronze board part designator, e.g. R5, D1, etc. Please put the parts back immediately after taking them out. And please please please make sure that parts go back inside the labeled bag for each part!!!!!!!


Your setup for assembling SMD prototypes consists of

  1. a soldering iron with large and fine tips, and maybe a heated soldering talon. Always wipe your iron on a damp sponge before using.
  2. a magnifier, very useful for inspecting solder joints from different angles. Your solder joints should be shiny and bright and form a clear wedge from the pad (on the board) to the pin (on the part). We often see beginners only heat the pin and not the pad, resulting in all the solder sticking to the pin and none to the board.
  3. a solder paste dispenser' and 'rosin-core solder wire. Both are useful.
  4. solder wick, which is handy for removing excess solder and solder bridges which short between pins.
  5. liquid solder flux, which is handy for deoxidizing metal better and helps the solder to flow.
  6. tweezers, essential for handling parts.
  7. pins, which are handy for moving parts around and scraping out gunk.
  8. razor knife, which is used for cutting board traces.
Step 1: Getting Started with the 5V rail and the PWR LED

Before starting with this step, watch this video: to see in general how to solder small 2 pin parts.

In this video you the assembly and testing of some components on the bronze board which are simple to solder is shown.

On the bronze board, start with soldering the following

  1. D1 - the power LED. Check the polarity carefully. The blue LED needs the fat gold wire on the right side - the C cathode/minus side.
  2. R5 - the power LED limiting resistor: 1k Ohms.
  3. P3 - the male type A USB connector.
  4. L1 - Ferrite Bead with 600 Ohm @ 100MHz and 150 mOhm DC to block noise in VBUS out of the 5V coming from the USB host.

Your soldering should end up shiny and bright and should clearly flow up from the pad onto the pin of the part.

When you solder the USB connector, bend the pins sticking through the board outwards before you solder the USB connector. To avoid possible stress on the SMD joins, solder these holding pins first, the the SMD pads.

After all these components are soldered, the blue LED should go on once you connect your board to a computer.

The orientation of the LED is shown here:


Check 1

The power LED should light up when you plug in the board to a computer.

Measure the voltage across C2'. It must be within '5V +/- 5%.

Step 2: Soldering the 3.3V rail stuff

Next we'll solder the voltage regulator that makes 3.3V from the USB 5V VBUS and some power supply bypass capacitors.


  1. U2 - the voltage regulator 3.3V.
  2. C1, C2, C3, C4 - power supply bypass caps. Note that two of these are 100nF ceramic chip caps and two are 10uF polarized [ tantalum chip capacitors]. Note that 100nF is the same as 0.1uF if you can't find the part. The positive side of the tantalum capacitor is marked with a bar. The image below shows the orientation of the tantalum capacitors:


Check 3

Measure the 3.3V rail voltage over C3 or C4. The voltage must be within 3.3V +/- 5%.

Step 3: solder the difficult parts

Before soldering the next components, watch this video:

Important: Before attaching the AVR32, you need to cut a trace on the PCB for one of the pins that is incorrectly connected. Cut the trace from pad 36 to the via just inside the frame, as shown below by the short blue line inside the black circle. Use a razor knife, and make the cut towards the inside of the frame, from northwest to southeast in the image. This direction will help avoid cutting the trace above and to the left. You don't need to cut very deeply, just enough that you don't see the metal anymore. Watch out for cutting other traces! Gently does it.


Do you see the problem on the schematic? Hint: look at the VDDIO connections on pins 36 and 48 on the schematic:

When soldering the AVR32, in case you create bridges between the microcontroller pins: * make sure that they don't have to be there: Check with a blank PCB whether the two pins are connected by a trace directly in between. On the bronze board, no pins are shorted by direct traces between the pins. * Remove extra solder to clear the bridge:

Here's what it looks like:


Now it is time to solder:

  1. U1 - our USB microcontroller AT32UC3B1256
  2. X1 - The 12MHz crystal

When handling U1, be careful not to bend any of the pins. If you drop U1 on a hard surface the pins will almost always be bent on the corner and it is very difficult to straighten them. Be careful bending the pins in any case; they are brittle and will break off easily.

When you solder U1, make absolutely certain that the part is lined up properly on all four sides after you solder only one corner pin.' If the alignment is not correct, fix it before soldering any other pins. Once you solder one corner correctly, solder the opposite corner. If everything is lined up properly, the solder one of the sides that has no solder. Now the rest should be easy. 'Do not mash the pins, a gentle touch of the soldering iron is all that is needed. If you bend the pin while soldering it is very hard to get it back on top of the pad. You can use either a very thin line of solder paste or apply a bit of thin wire solder to the iron tip and get it to flow onto the pins and pads. A bit of liquid flux applied to the line of pads helps in this case quite a bit.

Step 4: Other parts and don't forget the zero ohm resistor on the back side

If you are confident that U1 and Q1 have been soldered correctly, you can solder the remaining (easy) components. Refer to the schematic for values. All parts are in the folder of parts labeled by part number.

  1. C5 & C6 - the 22pF crystal load capacitors
  2. R3 - 10kOhm TCK pullup resistor
  3. R1 & R2 - 39Ohm USB data lines series resistors
  4. RESET & DFU - the two micro switches
  5. Any other capacitors or resistors except the LDR (= Light Dependent Resistor, the thing with the curly pattern on its face, R7) and the RGB LED (D2, = Red/Green/Blue Light Emitting Diode). Note that C7, C8, and C12 are arranged as shown below


There is one component on the back side of the board, L2, which should just be a shorting jumper. Solder a zero ohm resistor (a jumper). This connection isolates the AVR analog supply voltage. If you forget it nothing will work!

Your board should now look like this (except you will not have the LDR or RGB LED):


Check 3a: Visual inspection

Look at your board under the high power microscope. Look especially between the pins. You should not have leftover solder balls! If there are too many they will cause a short - maybe not today but later on. In any case, your solder joints should be shiny and clean and clearly weld the pins to the pads with nice wedges.

Check 3b: Electrical test

First, plug your board into the current-limited supply with the USB cable on it. If your board draws too much current the voltage will drop. A working board draws <40mA @5V. If your board draws too much power you must find the short(s) and fix it/them.

Next, after plugging in the board to a computer, check all rail voltages again:

Measurement Site' || 'Nominal Value
C1 or C2 5.0V +/- 5%
C3 or C4 3.3V +/- 5%
C7 or C8 1.8V +/- 5%

If any of the voltages are out of range disconnect the board immediately and start visual inspection, and have someone else look at the board.

Check 4a

If you have an oscilloscope, let's check the oscillator: * Attach a wire to some GND pin. * Connect the scope ground clamp to the wire * Check the signal on C5 or C6, on the signal side, not the ground side. * You should see a (more or less sinusoidal) 12MHz signal with about 1Vpp amplitude.

Check 4b

Connect your board to a Windows computer and in the Device Manager you should see the AVR32 USB DFU device show up (if you installed the driver earlier).

Or, connect your board to a Linux computer, then run the command lsusb in a shell.

Besides other devices you should see: bus_device}

Check 5b

To get further information on what Linux found out about the device, run sudo lsusb -d 03eb:2ff6 -v

You should get this: <nowiki> Bus ??? Device ???: ID 03eb:2ff6 Atmel Corp. Device Descriptor:

bLength                18                                                                                                                          
bDescriptorType         1                                                                                                                          
bcdUSB               2.00                                                                                                                          
bDeviceClass            0 (Defined at Interface level)                                                                                             
bDeviceSubClass         0                                                                                                                          
bDeviceProtocol         0                                                                                                                          
bMaxPacketSize0        64                                                                                                                          
idVendor           0x03eb Atmel Corp.                                                                                                              
idProduct          0x2ff6                                                                                                                          
bcdDevice           10.00                                                                                                                          
iManufacturer           1 ATMEL                                                                                                                    
iProduct                2 AT32UC3B DFU                                                                                                             
iSerial                 3 1.0.2                                                                                                                    
bNumConfigurations      1                                                                                                                          
Configuration Descriptor:
  bLength                 9
  bDescriptorType         2
  wTotalLength           27
  bNumInterfaces          1
  bConfigurationValue     1
  iConfiguration          0 
  bmAttributes         0xc0
    Self Powered
  MaxPower              100mA
  Interface Descriptor:
    bLength                 9
    bDescriptorType         4
    bInterfaceNumber        0
    bAlternateSetting       0
    bNumEndpoints           0
    bInterfaceClass       254 Application Specific Interface
    bInterfaceSubClass      1 Device Firmware Update
    bInterfaceProtocol      2 
    iInterface              0 
    Device Firmware Upgrade Interface Descriptor:
      bLength                             9
      bDescriptorType                    33
      bmAttributes                       15
        Will Detach
        Manifestation Tolerant
        Upload Supported
        Download Supported
      wDetachTimeout                      0 milliseconds
      wTransferSize                   65535 bytes
      bcdDFUVersion                   1.01

Device Status: 0x0001

Self Powered


Step 6: Pin headers

Now solder the 2×16 pin round pin gold plated headers into the bottom of the board to allow you to plug the bronze board into a solderless breadboard, as shown below. Solder the thin pins from the bottom into the board so that the thick tough part sticks out. You will need to cut apart some pieces. Make sure the pins are nicely vertical and properly aligned so that you can plug the bronze board into the solderless breadboard later.


Step 7: LDR and RGB LED

Finally, you can solder the LDR and RGB LED if you want to run this test. The RGB LED has one ground cathode pin and 3 positive anode pins, one for each color. It is oriented as shown below:


You made it till down here? Congratulations! Your board passed all tests that do not require a test-firmware to be flashed…