PCB design for QFN reflow soldering

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:

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.

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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.


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

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.)


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.

Hello KiCAD Community,
I designed my own symbol, downloaded the PSOC5LP schematic and footprint from the web.

I made connections with eeschema and updated the PCB from schematic.

  1. the wires in PCBNEW shouldn’t they appear automatically or should I draw them too?
  2. I want to make my system compact so I was thinking to put the sensor on the controller.
    so how can I draw pins on he other side and connnect them to my Pads ?
  3. Other solution; put the controller above the sensor. So how can I draw pins that are from the same side (chose size, height, etc.)

" but I don’t use vias in VCC as I route it at top" I didn’t understand what to use other than vias ( sorry I’m a total newbie in PCB design and in KICAD

Routing is its’ own skill. There should be ‘air wires’ to guide you. You might have them turned off. Note four dot picture with white lines in the left tool bar.


Hello again,
I found that my problem is that the pin that I connected with wires are ‘not connected pins’ as I run the electrical rules check
What did I do wrong ??

In KiCad there is a weird naming *&^%$#@! that confuses left with right.

If you design a schematic symbol, you have to choose “right” if you want to add a pin on the left side of the symbol.

The quickest way to fix it is to open the 832m1 again in the symbol editor, then draw a box around three pins, move them, then press y to mirror them in the Y-axis and place them at the right location.

Also, You can add the yellow background if you select the rectangle, edit it’s properties and set Fill Style to Fill with body background color.

@paulvdh Told you what is wrong and one (valid) way of correcting your mistake. I want to show you how you can spot this problem for yourself in the future.

Look at the unconnected pins on U1 vs the unconnected pins on U2. For example, look at U1-4 and U2-J8.3. Note the small, red circles. They indicate pin connection points that aren’t connected to anything. On your U1 the connection circle is against the graphical rectangle for U1. But on U2 the connection point is away from the graphical rectangle. This is an indication that you accidentally put the pin on your symbol backwards. Once you turn the pins around then the pin name text ("+EXC" for U1-4) will now be inside the symbol.

Now that you see that the pins on U1 are backwards you should also notice the green squares on the ends of the wires. This is an indication that the green wire isn’t connected to anything at that point. Once you fix the pins on U1 and the circles are on the outside then the wires and pins will be connected and both the red circle and the green square should vanish. At that point the wire to pin connection should look more like U2-J8.6.

To help hide some of the pin unconnected ERC errors for pins that you intend not to connect to anything, you can use the “no connect flag” to tell ERC that “yes, I intend for that pin to be disconnected”. That way the accidentally unconnected pins won’t be hidden in a sea of intentionally unconnected pins. See how I have the no connect flag on pin 7 of this IC:
2021-10-20 15_10_59-Window

NOTE I’m using KiCad version 5.1.10. If you are on the 5.99 nightlies the toolbar might look a little different, but the functionality of the no connection flag should be the same. (I haven’t heard that it changed…)

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