PCB Assembly
This page describes the process of assembling the power board and control board
Required tools:
- Temperature controlled soldering station (e.g., Hakko FX888D, Weller WLC100)
- Hot air rework station
- Precision tweezers
- Silicone work mat
- Fume extractor (recommended)
Several assembly steps for the Power Board require very high heat. A silicone mat can be used to protect your work surface. Be careful handling the PCB in between assembly steps as the board itself can get very hot.
Parts and assembly steps that result in the parts getting very hot are annotated with 🔥
The general sequence of assembly for the Power Board is according to height. Shorter parts are assembled first.
The Bill-Of-Materials for Digi-Key is located at this address https://www.digikey.com/en/mylists/list/4BDBKJ7A9E.
Some additional parts will need to be sourced from other vendors.
- Six (6) Terminal Lugs, part number B6A-PCB-RS from LugsDirect
- 2.4" 128x64 OLED Display. Must have SPI interface and SSD1309 chipset. Example Amazon listing
- 12 pin rectangular ribbon connector Example Amazon listing
- 10 pin rectangular ribbon connector Example Amazon listing
Optional parts:
- 50x50mm ATS-CPX series heatsink. E.g., ATS-CPX050050010-121-C2-R0
- Push-pins for heatsink ATS-HK127-R0 (buy two, they come in pairs)
- Large thermal pad for heatsink Example Amazon listing
- IC sockets Example Amazon listing
🔥Hot Parts🔥
As of rev8 of the power board, these two resistors are assembled by the fabricator (JLCPCB). These instructions are here in case of rework or if you have ordered a PCB without the pre-assembled parts.
These resistors are essentially a chunks of copper with specific dimensions to give a reasonably precise small resistance. For best results, these parts should be soldered to the four pads of the footprint as evenly as possible.
- Pre-solder a very small amount of solder to the four pads.
- Clean the pads with IPA or flux remover.
- Visually inspect to see that the the solder mounds are around the same height.
- Add fresh flux to the pads and bottom of part.
- Place the SMD resistor on the pads and apply hot air.
- Wait for the solder to re-flow and the part should drop into place and self-center.
Getting this step perfect is not critical as we can fine tune the resistance in the firmware. Just be sure you haven't created a solder bridge under the part between the two halves of the pad.
🔥Hot Parts🔥
The four DPAK mosfet are best soldered with a high temperature (750-800F) iron with a large tip. Hot air can be useful as well.
These instructions are intended for SMD newbies. If you have experience with surface mount assembly, feel free to follow your own method.
Read this step fully before proceeding.
Pre-heat the pad for several seconds and apply around 15mm of 0.031" solder to the pad. The solder will wick through the thermal vias at this point, but this is expected. The 15mm should be enough solder to fully cover the pad, but not so much that it starts collecting on the back of the board. Let the solder freeze and clean flux residue with IPA.
Apply fresh flux then place the DPAK on the pad. Using gentle pressure, hold the DPAK with tweezers as you start applying heat to the junction of the tab and the pad. A bit of solder on the iron can help with heat transfer. Once the solder starts to flow, slowly move the iron back and forth along the tab. You should soon see all the solder on the pad melt and some bubbles come out the sides. At this point, hold the part in place with the tweezers and let the solder freeze. Make sure the legs are centered on their pads.
Once the mosfet is soldered to the big pad, continue with the legs. These are much easier, just make sure you get a nice fillet and not a blob on the legs.
When your done with this step, the board will be very hot 🔥
🔥Hot Parts🔥
On newer versions of the power board (rev7 and newer), the trace between the mosfets is sufficiently large to support 30A of continuous current. Most installations will not need this jumper.
Two copper jumpers can be installed to improve the current rating of the power board. A gauge is provided on the edge of the power board to help form the jumpers. The jumper holes are 2.6mm which will accommodate 10 AWG wire. If the jumper is installed, apply enough solder to create a fillet between the jumper and the mosfet leg.
Many different values of resistor are used on this PCB. They are also of different tolerance. The 5% tolerance resistors will be tan colored, and the 0.1% tolerance resistors will be blue.
First, assemble the 5% resistors. There are 13 of them.
| value | reference | use |
|---|---|---|
| 10k | R2, R16, R7, R30, R25, R5 | pull-ups, pull-downs, and RC filtering |
| 1k | R27, R24, R15 | current limiting |
| 470 | R29, R28, R21 | pull-downs |
| 100 | R17 | current limiting |
Next, assemble the 0.1% resistors. There are 5 of them.
| value | reference | use |
|---|---|---|
| 49.9k | R6 | voltage divider for TH1 |
| 10k | R11, R4 | voltage divider |
| 24.9k | R8, R1 | voltage divider |
There are 7 100nf (0.1µF) ceramic capacitors on the power board. These are the only ceramic capacitors on this board. They are used as bypass capacitors and as part of RC filters.
Assemble C24, C12, C13, C2, C25, C23, and C21.
Assemble TH1.
Try to ensure the part is flush with the board so it can accurately read the temperature of the PCB. This part is glass, so do not apply too much pressure to the part itself.
The resistor network is used by the status LEDs for current limiting. It is a polarized part, so make sure the dot on the part matches marking on the silk screen.
Assemble RN1.
There are two DIP ICs on the power board.
| value | reference | use |
|---|---|---|
| MCP6004 | U6 | voltage buffer |
| 74HC04 | U7 | logic inverter |
14 pin IC sockets may be used for these ICs, but it is not required.
Assemble U6 (MCP6004). Ensure it's pin 1 indicator (half circle indent) is pointed to the right towards the ribbon cable connector.
Assemble U7 (74HC04). Ensure it's pin 1 indicator (half circle indent) is pointed up towards the lugs.
U2 is a 5V linear regulator. It delivers power to many parts on the Power Board as well as power for the entire the Control Board (through the ribbon cable). This part is installed flush with the PCB for heat dissipation. Bend the legs 90º backwards just below the change in width on the leg. Test fit the part on the PCB and ensure that the hole in the tab lines up with the hole on the PCB.
Once properly aligned, assemble U2. Optionally fasten it to the PCB with a M3x6mm bolt
There are two diodes on the power board. Diodes are polarized parts, so make sure the band on the part (cathode) matches the silk screen.
Assemble D10, it is a larger DO-201 package.
Assemble D1, it is a smaller DO-41 package. This part is only necessary if also installing a DC fan.
There are four status LEDs on this board. The three green LEDs are for +12V, +5V, and RPi. The off-color LED is for Battery.
These are polarized parts, so make sure the long leg of the LED goes in the right hole where the small "+" symbol is printed.
Assemble D2, D4, D5 with green LEDs.
Assemble D6 with an off-color LED.
There are four TO-92 parts. Three of them are an NPN transistor and one is a precision voltage regulator. They all look similar, so be sure to separate the voltage regulator from the transistors.
These parts are all polarized. Make sure the outline of the package matches the outline on the silkscreen.
| value | reference | use |
|---|---|---|
| 2N3904 | Q4, Q7, Q8 | NPN BJT |
| TL431B | U1 | voltage reference |
Assemble Q4, Q7, Q8. Some lead-forming may be required.
Assemble U1. Some lead-forming may be required.
There is one electrolytic capacitor and one tantalum capacitor on the Power Board. They are noticeably different. Both are polarized, but have different markings. The large electrolytic capacitor has a band indicating the negative lead. The tantalum capacitor has a small "+" on the positive leg.
The silkscreen for C7 and C1 indicate the positive lead with a small "+".
Assemble C7 and C1.
Two connectors on the board provide 12V that can be switched off under low power conditions. These are used to power an offboard DROK (or other buck converter) used by the RPi and to power an optional fan for cooling.
Assemble "12V RPi" and "12V Fan".
🔥Hot Parts🔥
The six terminal lugs are designed to be wave soldered, but hand soldering is also possible. These parts require the most heat of any in the assembly, so take care when handling the PCB during this step. Since these lugs are so large, it is necessary to place some supporting material under the PCB in order to balance it and keep it level. Scrap silicone work mat works nicely for this
Remove the bolts from the lugs and place one of them upside down on the work surface. Balance the PCB on top of it using some supporting material as pictured above. Heat the soldering iron to at least 750F and apply some solder to the tip. Begin heating the terminal lug. This will take some time.
Once the solder starts to transfer from the iron tip to the lug surface, begin heating the pad on the PCB. The terminal lug has very high thermal mass and will stay hot for a long time after pre-heating. Once solder begins to flow on the pad, apply around 30mm of solder (roughly three times the length of the pad). Drag the solder around the all edges where the terminal contacts the PCB. There should be enough solder to form a nice fillet along the two long edges of the terminal and to fill the gaps on either end.
Repeat the process for the remaining 5 lugs.
Once the terminals and PCB have cooled enough to handle, clean the pads with IPA or flux cleaner.
Here are some close-ups showing an acceptable solder joint for a terminal lug:
🔥Hot Parts🔥
The F1 fuse footprint accepts two quick connect terminals. Insert the terminals on the top of the PCB according to the silkscreen and tack them into place from the top. This will secure the terminals well enough to fully solder from the bottom. When soldering, start on the pin that wasn't tacked from the top.
The other TO-220 part in the BOM besides the 5V regulator is Q3. It is a p-channel mosfet used for switching power on and off for the Raspberry PI.
It should be installed so that the back of the part is facing right towards the ribbon cable connector. The silkscreen indicates this as well.
Assemble Q3.
A shrouded rectangular ribbon cable connector is used to ensure correct polarity of the ribbon cable that connects to the Control Board. Make sure the key in the shroud matches the silkscreen.
Assemble J15.
This is an optional assembly step. It only applies to installations with high continuous load requirements.
Mounting holes for a 50mm square heatsink are included on the power board. The hole pattern is 43mm by 43mm on center.
This mounting pattern will accommodate any of the 50x50mm pushPIN™ Heat Sink series from Advanced Thermal Solutions, Inc. One such heatsink is included in the BOM (with a quantity of zero). If ordering a heat sink, be sure to also order two units of the push pins. Each unit of push pins includes two pins, so two units gives the requisite four push pins.
Since the back of the PCB has exposed copper, a thermal pad is required to avoid shorting the mosfets with the heat sink. Also, since the heatsink area includes many through-hole pads, the thermal pad must be thick enough to keep the heatsink flat. A pad of at least 1mm thickness is needed. The leads of the through-hole parts in the heatsink area should be trimmed flush with the solder fillet (not flush with the board). Trimming the leads will avoid having their sharp ends exposed which could pierce the thermal pad and contact the heatsink.
Assembly for the control board is relatively simple compared to the power board. Unlike the power board, the parts are not surrounded by large ground planes on both sides. There are interior ground pours on this 4 layer PCB, but they do not wick away heat nearly as much as the exterior ground power on the power board.
In other words, we will use a lower temperature for the soldering iron for this assemble. Something around 650F should be fine.
Also note that some parts are assembled on the back of the board.
Similar to the Power Board, there are many different values of resistor are used on this PCB. They are also of different tolerance. The 5% tolerance resistors will be tan colored, and the 0.1% tolerance resistors will be blue.
First, assemble the 5% resistors. There are 12 of them.
| value | reference | use |
|---|---|---|
| 10k | R14, R33, R9, R13, R32, R12 | pull-ups, pull-downs, and RC filtering |
| 1k | R26, R20, R10 | pull-downs, current limiting |
| 470 | R19 | voltage divider |
| 100 | R18 | current limiting |
| 100k | R31 | pull-down |
Assemble the 12 5% resistors.
Next, assemble the 0.1% resistors. There are 2 of them and they are both 10k. They are used to configure the voltage regulator U5.
Assemble R22, R23.
There is a single axial inductor L1. It is used as part of an LC filter for the AVCC input.
Assemble L1.
There is a single diode on this board, D3. It is used to allow the RST signal to reset the OLED.
Assemble D3.
Do not assemble D8.
There are many ceramic capacitors used on this board which look very similar. There are 8 total. C10 is also ceramic, but it is blue and a bit larger. It will be assembled later.
| value | reference | use |
|---|---|---|
| 100nF | C11, C3, C9, C8, C6 | bypass, filtering |
| 10nF | C18 | filtering |
| 22pF | C4, C5 | crystal |
Assemble the 8 ceramic capacitors.
There is a single 16MHz crystal. It provides a clock signal to the ATMega328p microcontroller.
Assemble Y1. The orientation does not matter.
There is a single 10k resistor network on this board, RN2. It provides pull-ups for various components. It is a polarized part so make sure the dot on the part matches marking on the silk screen.
Assemble RN2.
An IC socket for U3 is highly recommended. If something ever goes wrong with programming the ATMega328p, the board will be bricked unless the IC can be removed.
Assemble U3.
C10 is a larger 1uF ceramic capacitor. It is used as part of a delay circuit for the OLED reset.
Assemble C10.
A tantalum capacitor is used as a bulk capacitor for this board. This helps stabilize the voltage which can vary a bit due to the OLED. This is a polarized part. The capacitor has a small dot indicating the positive leg. The silkscreen also has a small "+" to indicate the positive leg.
Assemble C20.