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.
- SMD parts
- R3, R36 🔥
- Q1, Q2, Q5, Q6 🔥
- Q1 to Q2 jumper (optional) 🔥
- Q5 to Q6 jumper (optional) 🔥
- Resistors, diodes, thermistor, ceramic capacitors
- RN1
- U7, U6, U4 (optionally, sockets for each)
- D2, D4, D5, D6 (LEDs)
- TO-92 parts
- J12 (12 pin connector)
- J13 (Terminal screw block)
- 12V fan connector
- C7, C1 (electrolytic capactitors)
- Terminal Lugs 🔥
- F1 connectors 🔥
- U2, Q3 (TO-220)
- Heat sink (optional)
The Bill-Of-Materials for Digi-Key is located at this address https://www.digikey.com/en/mylists/list/4BDBKJ7A9E. There are two optional parts in the BOM (heatsink and push-pins) with quantity zero.
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
Optional parts:
- IC sockets Example Amazon listing
- Heatsink (in BOM)
- Large thermal pad for heatsink 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.
The big pad should be pre-soldered and cleaned with alcohol or other flux remover. The solder will wick through the thermal vias during pre-soldering, this is normal. After pre-soldering, apply flux to the pad and solder the two legs. Once the legs are soldered, begin applying heat to the pad and tab.
The Power Board has a copper plane on the top and bottom, and the mosfet pads are connected through to the back with thermal vias. It will take some time for the board to heat up sufficiently for solder to flow. Once the existing solder starts to flow, apply fresh solder until adequate fillets are present all around the tab.
🔥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.
Some of the resistors on the power board are high precision (0.1%)
- 49.9 kOhm: R1,R6,R8
- 10 kOhm: R2,R4,R9,R11
- 259 Ohm: R5,R10
These are mainly used as high precision voltage divider for accurate sensing of the temperature, voltage, and current.
The remaining resistors on the power board are pull-ups, pull-downs, and current limiting resistors. These are 5% precision resistors.
- 1 kOhm: R15, R17, R24, R27
- 10 kOhm: R16, R21, R25, R28, R29, R30
- D10
- D1
There are two diodes on the power board. They are different part numbers, so be careful not to mix them up. D10 is a general recitfier diode, and D1 is a flyback diode for the fan.
Install TH1. Try to ensure the part is flush with the board so it can accurately read the temperature of the PCB.
Install the 0.1µF ceramic capators: C2, C12, C13
Install RN1. This resistor network is used by the status LEDs.
There are three ICs on the power board. Two of them (U6 and U7) have the same footprint, but are different parts. IC sockets can optionally be installed at this point.
There are four status LEDs on this board. The three green LEDs are for +12V, +5V, and RPi. The yellow LED is for Battery. The long leg of the resistor goes on the right where the small "+" symbol is printed.
There are four TO-92 parts. Three of them (Q4, Q7, Q8) are the same transistor and one is a precision voltage regulator (U1). The three transistors are used to drive power mosfets which are used for 12V switching. The voltage regulator is used to provide a precise voltage to U4 in order to get accurate current sensing.
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.
Install C7 and C1.
🔥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 800-850F 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.
There are two TO-220 parts which look the same, but are different parts. U2 is a voltage regular and Q3 is a power mosfet. Take care to not mix them up, also take care to install them in the correct orientation.
To accommodate high current installations, the Power Board has mounting holes for a 50mm square heatsink. 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.