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  • Author Author: Jan Cumps
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Hercules LaunchPad and GaN FETs - Side Note B: Look at the PCB

Jan Cumps

I'm designing a BoosterPack to evaluate GaN devices with the help of a microcontroller.

I've received PCBs from Seeed. Let's have a look ...

image

 

This is my first Seeed order (a kind element14 community member gave me a rebate coupon that I happily used).

Board specifications:

 

  • PCB Dimension - 10cm Max*10cm Max
  • Layer - 4
  • PCB Thickness - 1.6mm
  • PCB Qty. - 5
  • PCB Color - Green
  • Surface Finish - ENIG
  • Copper Weight - 1oz.
  • Panelized PCBs - 1
  • Expedited Option - NO

 

The price without shipment for this board is 65.12€. Shipping was 7.37€. The coupon got me 9.15€ off ($ 10). Totals to 63.97€

image

This design requires 2oZ copper layers. My budget can't carry the cost of such a board - so I will not be able to draw full current out of my design.

This is a compromise, one of the many I had to make. I tried to avoid compromises in the design, but did make them when ordering.

Getting a board that has enough copper on it will not require a design change. It's just ticking a different option when ordering and coughing up the monies.

I'd rather do that after I validated that I didn't make mistakes in the PCB layout.

image

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Just like previous designs I did with OSHPark, the Seeed boards look very good at first inspection.

This design is 4 layers, so I have no mechanism to look at the two inner layers. The top and bottom layer look very nice, so that's a good sign.

 

In the images below, the artist renditions on the left side are generated by the Seeed portal.

The photos on the right side are the actual thing. Click on them to get a detailed view.

 

imageimage
imageimage

 

 

Populate Order

This is the first time I'll populate a two-sided board. I'll use hot air for the smd components.This is how I plan to approach it:

  1. solder the GaN IC and the inductor. These two have the pins under the device. I'll have to massively bombard them with heat to transfer the energy underneath where the solder paste is.
    I'll use the pre-heater that shabaz gave  me to get the whole board to a close-to-reflow temperature.
  2. then all the smd components on the bottom side. If they fll of later, that's not a big deal, because the are only few.
    I'll also use the pre-heater but on lower temperature, because its heat will be radiating on the inductor and GaN chip.
  3. Then I'll do the remaining top smd components.
    The pre-heater again on a temperature low enough to not desolder the bottom components.
  4. the trough-hole components by hand

 

It is my first time. I'd be happy to take your advice on the preferred order to solder the components.

The tools I have at hand:

  • hot air
  • soldering iron
  • pre-heater

 

 

edit: I've attached the waveform of one High input pulse as hi01.wfm.

If you have a Rigol DS1052E you can upload that to your scope and examine the ringing with cursors.

image

 

 

Related Blogs
Hercules LaunchPad and GaN FETs - Part 1: Control Big Power with a Flimsy Mouse Scroll Wheel
Hercules LaunchPad and GaN FETs - Part 2: Make a BoosterPack
Hercules LaunchPad and GaN FETs - Part 3a: BoosterPack Layout - Reference Design
Hercules LaunchPad and GaN FETs - Part 3b: BoosterPack Layout - my version
Hercules LaunchPad and GaN FETs - Side Note A: BoosterPack Layout - Custom KiCad Parts
Hercules LaunchPad and GaN FETs - Side Note B: Look at the PCB
Rotary Encoders - Part 1: Electronics
Checking Out GaN Half-Bridge Power Stage: Texas Instruments LMG5200 - Part 1: Preview
Rotary Encoders - Part 4: Capturing Input on a Texas Instruments Hercules LaunchPad with eQEP
Vintage Turntable repair: Can I fix a Perpetuum Ebner from 1958 - part 4 - Hercules LaunchPad Enhanced PWM try-out
Attachments:
hi01.zip
Parents

  • The board is built up, not tested yet.

    Reflow worked ok, except for 0402 and some 0603s.

    C22(0402) toombstoned, I had to rework C15, C21 and LED D2 too.

    I'm happy that the (somewhat) complex footprint of quadrature encoder - with one oval and two round mounting holes fitted perfectly.

     

    The two screw terminals however, P2 and P3, are a complete goof-up. The holes are too small, the distance between them is wrong and the total size of the footprints doesn't match the component.

    I have to check how that happened, because I made this footprint myself from the data sheet. (wrong part ordered or delivered, wrong drawing selected in the sheet, inch/cm switch, I don't know yet)

     

    image

     

    image

Comment

  • The board is built up, not tested yet.

    Reflow worked ok, except for 0402 and some 0603s.

    C22(0402) toombstoned, I had to rework C15, C21 and LED D2 too.

    I'm happy that the (somewhat) complex footprint of quadrature encoder - with one oval and two round mounting holes fitted perfectly.

     

    The two screw terminals however, P2 and P3, are a complete goof-up. The holes are too small, the distance between them is wrong and the total size of the footprints doesn't match the component.

    I have to check how that happened, because I made this footprint myself from the data sheet. (wrong part ordered or delivered, wrong drawing selected in the sheet, inch/cm switch, I don't know yet)

     

    image

     

    image

Children
  • in reply to Jan Cumps

    Hi Jan,

     

    That's looking good! The terminals could be resolvable, because they come in a few different sizes so maybe the wrong part was ordered. 3.81mm, 3.5mm are common so it could be that the PCB pads are for 3.5mm pitch but the physical part is 3.81mm for example. But anyway it is minor, since you can attach wires directly of course.

    My usual mistakes are similar to yours with the capacitor - in my case due to copying-and-pasting a capacitor in the CAD schematic and then changing its value mistakenly assuming it is the size I wanted, but it sometimes isn't..

     

    EDIT:

    ..and sometimes for a prototype when I don't have the right parts, build vertically..

    image

  • in reply to Jan Cumps

    Two steps forward one step back at the moment.

    When I apply bias voltage to the GaN device, power supply goes trough its knees. Even if this is supposed to be a 3mA maximum. The led indicator in parallel is taking more current normally than the whole bias part.

    The LMG5200 is getting real hot too.

    I first used a DMM to check if there are shorts (I did that just after mounting the GaN and inductor too). No.

     

    So I have now desoldered the GaN device and will check all of the signals going there. I may have damaged the device.

    Hope not. Because I only have once IC left and they aren't cheap.

     

    image

     

    On the other side, this is a chance to go and probe the circuit without fear of damaging the GaN device...

  • in reply to Jan Cumps

    All 'soft' signals are ok.

    pulse generation works, bias stable, quadrature encoder is fine.

     

     

    image

     

    Now over to the power part - just a few caps and an inductor.

    I don't have an L-meter, DC resistance similar to an unused identical coil.

  • in reply to Jan Cumps

    High-res probing photo:

    image

     

    especially for DAB, I've taken care to probe with signal integrity in mind.

    The absence of patch wires -  and using ground springs in stead of probe crocodile clips - does the job.

    I've made all test points on my board in a way that allows ground clip probing.

     

    image

     

    Focus on ringing. It's acceptable, I think.

    Rising edge of channel 1:

    image

    Falling edge of channel 1:

    image