• Assembling densely populated PCBs with 0201 components

    05/30/2021 at 22:07 0 comments

    I recently assembled a tiny 7.5 x 9.5 mm PCB (actually two) with 0201 passives, LEDs, a BGA and a nasty LGA. Components on both sides. It was difficult, and I was not sure if could do it, but it went well with the right tricks and some experimentation.

    TL;DR: the result:

    So how did I get there?

    Tools and helpers

    A lot of little tools and helpers are needed, and I'll try to include everything here. First of all - for lack of a dedicated bench - I assemble my boards in the kitchen. That means that I have to keep my kitchen tidy to cut down the time it takes to convert between food and PCB output. Same for my assembly equipment:

    • stereo microscope. I use 10x magnification and that's enough for 0201
    • a heavy vise
    • those pointy dentist things (I use them to apply paste, flux gel, and poke parts)
    • tweezers
    • a squeegee to spread paste/flux gel
    • a small glass plate for a blob of solder paste and flux gel
    • a small aluminium plate
    • soldering iron
    • hot air station
    • a soft brush to apply flux gel and to clean the PCB
    • solder paste, low temp SnBi and high temp SnAg
    • tacky flux gel and a flux pen
    • alcohol (isopropyl is recommended but I use ethanol) for cleaning the PCB and a flat container that you can rinse the PCB in
    • some paper towels to clean tools and the PCB

    PCB Preparation

    Not that complex. I clean the PCB with ethanol and prepare a cheat sheet with part location, value and orientation for both sides.

    Bench Preparation

    As in the picture above, I put all my tools in approximately the place where I need them. I put a blob of solder paste (each type) and some tacky flux gel on the glass plate. The flux gel is spread with a squeegee to give me a thin, even layer of flux. The idea is that I can pick a part with tweezers, place it on the tacky flux layer, and as a result have a part with flux on the bottom, but not everywhere else.

    Adhesion is always higher between tweezers and part than it is between part and PCB. That's a law.

    BGAs and other high temp stuff first

    I solder high temperature components first. Since BGAs already come with the right amount of solder in the right place (good!), we can't pick the type of solder in this case. It's most probably high temp SnAg solder, and if it's something different, you'd probably know that.

    The BGA is dipped in the tacky flux layer and placed on the PCB. The solder mask helps figuring out if it's placed correctly: I place a heavy part on top to get some pressure between the solder balls and the PCB. Then I can feel it "snap" into place in the solder mask openings. the extra weight stays on during soldering. The solder's surface tension is pretty high, so the extra weight won't squash the joints.

    I solder BGAs on a hot plate, which is conveniently included in pretty much every kitchen worth its name:

    That's an STM32WLE5, with a 4x4 mm inductor on top. Here you can also see what the aluminium plate is used for. It spreads the heat and doesn't allow radiated heat from the heater reach the PCB. That's a thing because these glass ceramic plates transmit infrared radiation, and the aluminium plate will block that. Turn it to max and wait until the solder reflows. And extra blob of high temp paste somewhere on the PCB helps to see when reflow temperature is reached Then switch the plate off, wait 10 to 20 seconds and slide the whole thing (alu + PCB) off the hot plate.

    Other typical high-temp parts are connectors. I do these with SnAg for its higher mechanical strength. This PCB has a u.Fl connector on the bottom that I solder with an iron and normal SnAg wire in the vise. One pad first, align the part, then solder the rest.

    Parts with small pads will require something other than wire. In that case, I pre-tin the pads with paste and an iron, apply some flux again, and then reflow the pre-tinned pads with hot air to level them. Touch up until all pads for a given part have solder pillows of roughly the same height. That's right, no stencil!

    Clean the PCB with alcohol to get rid of excess paste...

    Read more »

  • Test ESC stuff

    12/30/2019 at 12:05 0 comments

    I'm building a BLDC ESC based on

    • EFM8BB2 running BLHeli_S
    • NCP81253 Gate Driver
    • SIZ322DT NFET pair
    • and a DMN2990UDJ fual NFET for level shifting between 3V3 (MCU) and the 5 V gate driver logic

    Link to current schematic (possible error in MCU to level shifter signals: pwm and com signals might be swapped so "Ap" should actually be "Ac", same for B and C signals):

    https://s3.amazonaws.com/static.projects.hackaday.com/9684061580420948473.pdf

    power FETs: https://www.vishay.com/docs/79370/siz322dt.pdf

    gate driver: https://www.onsemi.com/pub/Collateral/NCP81253-D.PDF

    NFETs for level shifting: https://www.diodes.com/assets/Datasheets/DMN2990UDJ.pdf

    MCU datasheet: https://www.silabs.com/documents/public/data-sheets/efm8bb2-datasheet.pdf

    MCU reference manual: https://www.silabs.com/documents/public/reference-manuals/efm8bb2-rm.pdf

    I assembled a fresh board without the MCU:

    Measurements I've done so far to make sure the board is healthy (without powering it up):

    • level shifter DMN2990UDJ body diodes show normal forward voltages and block in reverse
    • power FET SIZ322DT body diodes show normal forward voltages and block in reverse

    Older content:

    As long as the MCU isn't programmed, it should be possible to inject the level shifter input from an external source.

    Schematic of one half bridge driver:

    The level shifter is to the left (Q5, R12, R13), gate driver in the middle (U4, C7, C8) and motor driver FETs to the right with some feedback resistors (labeled "easy access" because I'm not sure about the values).

    VBat can be between 2.5 V and 12.6 V (maybe 16.8 V) and I'm currently using a 2S LiPo charged to 7.5 V.

    Gate Driver:

    The gate driver will switch on the high side FET if PWM is high. It will switch on the low side FET when PWM is low, and it will disable both if PWM is somewhere in the middle.

    Level shifter logic:

        Cc Cp PWM PFET NFET
    1)  0  1  2V5 off  off
    2)  1  1  0V  off  on
    3)  0  0  5V  on   off
    4)  1  0  0V  off  on

     This makes the whole driver stage look like some sort of an imaginary PFET-NFET pair (line 4 in above table is an exception to that).

    The problem

    When I apply 0 V to Cc and 0 V to Cp, the high side FET isn't on. I can only measure some 3.8 V, but it should be VBat. It was pointed out in the hack chat that the gate driver might require PWM with a duty cycle less than 95% to work correctly, so I also tried with 24 kHz PWM and 90% duty cycle. Since Cc logic is inverted, I actually injected 10% duty cycle on Cc.

    The above was measured with a multimeter.

    Things look different with a scope

    Input to level shifter and its output to the gate driver; Ch1: PWM to gate driver (5V logic), Ch2: input to level shifter (3V3 logic, signal Cp):

    That the PWM signal (Ch1) is going to 2V5 instead of 0V is caused by the voltage divider (R12 and R13). The gate driver handles a 2V5 input by disabling both output FETs. Input Cc was low all the time.

    The output of the gate driver can't really be accessed so I took a look at the output (testpad to the right in the schematic). Ch2 is still the 3V3 input, Ch1 is the testpad output:

    Indeed the output goes to VBat so the high side FET is switching on. It doesn't go down to 0V when off because the low side FET is never on.

    Next steps:

    • Connect Cc to Cp to have complementary output with the low side FET
    • Add a light load to the output. Since I only have 1/4 W resistors, a couple of 680 ... 1K Ohm resistors in parallel should be fine.

    Again thanks to Bharbour for helping.

    That helped!

    Driving both FETs and adding a load of 6 x 1 kOhm (lack of power resistors...) helped. The following screenshot was made with a 3S battery at 11.4 V, and that's what I'm seeing at the output. Ch1 is the input to both Cp and Cc, and Ch2 is the output at the motor pad:

    Time to figure out how to program the on-board MCU.

  • Light box

    01/06/2018 at 21:43 0 comments

    Created a foldable light box out of

    • cardboard
    • painter's tape
    • some white cloth
    • double sided tape
    • white paper
    • four round 10x2 magnets
    • four M5 nuts

    Tools:

    • Scissors
    • knife
    • hot glue gun
    • pen

    The first two picture show the general layout. The flaps you see in the first picture to the left are just painter's tape with magnets to hold the thing together when it's put together for use. You can also see - in the third rectangle from the left - the two nuts that the magnets hold on to.

    Plugged together:

    First light: