PCB related questions

Hi everyone,

I would like to build a custom PCB for my projet using an Arduino Pro Mini and and BME280 pressure sensor.

So far I could found most answer on Google but there are two finds left.

  1. I want to connect to the Arduino using UBS (and not have to solder some headers). How do I connect the Arduino to the UBS ?
  • VBus goes to 5V
  • GND to ground
  • All EH to ground too
  • But what about D- /D+ and ID ? More specifically where do I connect arduino TX, RX and RESET ?
    Question -USB Connection
  1. The soldering pads of the BME280 chip are underneath the chip which makes it impossible to use "traditionnal" soldering. I therefore need to use solder paste and heat it with a some heatplate right ? (or bake it in an oven ?!)
    BME280 Bosch Sensortec | C92489 - LCSC Electronics

  2. Regarding the BME280 chip, is there any way to make it waterproof and still be fonctionnal ? I suppose that the small hole on the chip is where it measure the pressure and make it waterproof would complety ruin it right ? For context it will not go underwater but me projet might fall in water.

  3. The projet will also contain a LI-PO charger (using a tp4056) and a boost convert (to boost from 4.2 v to 5 volts). I therefore use a 22uF coil. There's not polarity right ?
    SMTDR43-220M 3L COILS | C326226 - LCSC Electronics

Thanks in advance for your time and your help.

Do you mean USB?

Connect for what purpose? To provide power? To upload sketches?

What headers do you not want to solder and why?

You cannot connect those directly to the Pro Mini. For that you need to use a USB to serial converter module. You can find these on eBay, Amazon etc with a variety of different converter chips and USB socket types.

Buy a module containing the BME280 chip soldered on already. These have 0.1" headers similar to the Pro Mini, making them easy to solder to your PCB.

You can buy waterproof temp/humidity probes like the picture below. Inside is a small narrow PCB containing the sensor. However, I have never seen a version of these for bme280. How important is pressure for your project, and why is it important? For temperature and humidity, there are several different sensors available in these waterproof probes such as sht21/31.

Hi PaulRB,

Thanks for your ideas.
I want to put a female micro-UBS in order to upload sketches (and provide power of course). I don't like having headers because I want my custom PCB to be as compac as possible and headers are just to big for my application that why I want to replace them with and USB, which is much smaller. Like this basically:

(it's not a micro USB I know but that's the idea)

In this case, having a CP2102 USB to TTL connector isn't necessary right ?
But how do I connect RT, TX and reset to the arduino ?

Regarding the BME280, I already have the module and it's great. But I want to integrate the BME280 chip directly to my custom PCB. I don't want the entire module but only the chip itself (all other components on the module will be add so that the chip works correctly). The issue is how to solder it on a custom PCB. Probably using some soldering paste and a heat plate right ?

USB and Rx/Tx are two different interfaces. USB is 3V differential signal containing data and clock, the CP2102 or FT232 or ... convert the 3V signals from a PC/RPi/etc. into a 3.3V or 5V Tx signal that the Promini can read on its Rx pin, and accepts the Tx signal from the Promini and converts to the 3V differential clock/data signal that the PC reads.

Yes, the BME280 will need solder paste stenciled onto the board which is then melted in a reflow oven, or with a hot air rework station, or a heat plate perhaps.
All SMD components would typically be placed and refllowed together with an oven or heat plate.

You might find they are in short supply:

BME280 pad footprint, from it's datasheet

Couple of thoughts:

  1. If you design your own board be sure to add the capacitors shown on many µP schematics. The are easily overlooked to those who have never been "bit" by not having enough filtering.

  2. Why add the USB to Serial connector and chip on your board? If you want it small keep the Tx/Rx and have the USB to Serial off the board. Note there are many boards on eBay that perform such a function.

  3. Connectors and water do not mix. If you will be in a moist environment colder directly to the board (smaller and more reliable.)

  4. Why do you need 5V? I believe the BME280 is 3.3V and a Pro mini can run down to 1.8V. You can run right from the Lipo's

  5. The BME280 needs some "conditioning" after soldering.

Hi everyone,

Thanks for your contributions.

It's seem like having a serial to USB is necessary in this case^^ Didn't knew that thanks for sharing.
Is there a "better" chip between the CP2102 / FT232 or others ? Or are they similar ?
Might be silly but can you reflow soldering paste in a conventional oven ? I currently don't have either a heat plate nor an oven for that task.


  1. I will do exactly as the schematic will show (once I've decided with chip to use) ^^. I don't have much experience in PCB design und such so my best bet it to replicate it exactly as shown in the schematic.

  2. It's a good idea, but I really want everything "self-contained". I don't want to have other components to make the PCB work. Just plug the USB and start coding.

  3. Mixing both will indeed be complicated. I will try to avoid water as much as possible but my question was just in case it would happen.

  4. The BME280 runs on 3.3v indeed and I could use an 3.3v pro mini too, but I'm also using 2 servo motors and I need the processing power of the 5v pro mini. I'm actually using a 1s lipo and a boost converter.

  5. What do you mean by "conditioning"? Do I have to do something special after soldering ?

There is a section in the Bosch specification describing their recommendations.

I use an old toaster over for reflow soldering. A cycle is heat up to 150C, hold for 90 seconds to activate solder flux, heat up to 190C for 90 seconds to melt the solder, then let cool.
Check yard sales, look for one with 4 heating elements.

This would work also for smaller boards

LarryD has been posting interesting things he's done with it.

I use the FT232. It's reliable and works every time.

If you use a Atmega32U4, it has the USB interface built in, so a separate chip is not needed. You could prototype with a Leonardo or whatever the 32U4 based boards are called, wire up the BMP280 via I2C, get that working with the battery charger, then work on making up a board containing it all.


There's indeed some recommandations at section 7.8. Basically lead free soldering with max peak heat and min height. The chip might also need some reconditioning.

I've been looking at the MHP30 for a while now and it's seem pretty good and exactly what I need.

Thanks for all your advises

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