PCB design for QFN reflow soldering

Hello!
I am new to soldering and PCBs so excuse me if my questions are basic
My project is reading the data of an accelerometer
with raspberryPI.
I downloaded the layout and footprint for the ADC
and it’s ready.
I would like to design the soldering points for the sensor because the footprint isn’t available online

  1. I didn’t find anything for QFN or reflow soldering (Square), but just points for pin soldering.
  2. Can you give me advice for the layers that I should use?
  3. Is there anything to design other than the connections between the ADC --> sensor and the pins to connect directly to my RaspberryPI?

Don’t hesitate to ask me for more clarification.
Thankyou for your advice and replies.

The datasheet you linked to is for an ADS7038 and it is in a “WQFN” package. It’s 3 by 3 millimeters. That thing is so small that if you accidentally drop it on the floor, you’re unlikely to ever find it again. On top of that it also has 16 connections.
These chips are not very well suited for beginners because of their small size.

What does this mean:

Downloading a layout?
Maybe you are referring to page 71 of the datasheet:
image

The normal procedure here is to make a schematic symbol (or modify an existing one. the ADS7038 is not in KiCad’s default libraries, then link it to the WQFN package, put it all on the PCB and draw the tracks. The example from TI is just that. It’s an example, and it’s not a very interesting example for this simple IC.

WQFN package with 16 pads is a part of KiCad’s default libraries:

I’m not sure whether you should use the version with the 1.6mm exposed center pad, or the 1.75mm. The datasheet says it has a 1.66mm exposed pad.

It needs power and GND. It also needs a reference voltage, and of course the decoupling capacitors.
I do not know what the I/O voltage is for the raspi. You do have to check if it’s compatible with the ADS7038.

1 Like

Thank you for this constructive reply!

In fact, the footprint and schematic were available online for all design softwares (eagle, KiCAD etc.) So I added it to the KiCAD library and it’s all good.

  1. ADC Section 10.1: The raspi has a voltage of 3.3V so greater than 2.35V. So should I short AVDD and DVDD?

  2. Which type of capacitors should I use?

You should not assume that people have time to read datasheet for you. The question should be clear without extra reading.

I use ceramic 100nF 0402 as decoupling capacitors really everywhere. But I can imagine that some fast AD converter can call for using smaller capacitors (like 10nF) assuming they are better at higher frequencies and worse at lower.

Yep

Nice imagination, but in this case the datasheet suggests to use quite different values and your assumptions are far off.

Hello,
Thanks for your replies.
Yes my bad, I couldn’t upload the second picture of the datasheet page because I’m a new user and I can send only one image.

Anyway, I wanted to draw the solder pads for my sensor in order to manufacture the PCB and solder it along with the other components.

  1. Can I work only with PCBNEW? or should I draw also the circuit in eeschema. If that’s the case, how to draw my accelerometer in eeschema?
  2. Which layer in PCBNEW should I use for the solder pads for reflow soldering? and for the wires between the adc and sensor? Is it the copper layer?
  3. For my GND and VCC, should I use Vias? or there’s an alternative way (Female pins, etc.)

Yes.

You should.

I use only my own sybols. I don’t know if there is one in KiCad library you can use. Time spend on searching is typically enough to make own symbol.

Pads and tracks are made of copper so they are on copper layer.

Don’t know exactly what you are asking.
In my designs GND is at bottom and SMD elements at top. So I connect all GND pins by vias but I don’t use vias in VCC as I route it at top.