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I'm in the process of assembling an embedded (Raspberry Pi-based) machine that has a lot of sensors wired together via I2C. I find myself adding headers for the I2C lines to all the little tiny breakout boards on which the sensors are mounted, and I have an I2C "bus board" that I've assembled using a prototyping board to simplify connecting all the sensors in parallel, but I'm wondering if there's any advantage to soldering the connecting wires instead of using 0.1" headers and jumper wires. The boards are not very rigorous timing-wise (the bus is set to a low bandwidth.)

For the record, what I have now -- everything assembled via headers -- works just fine. I'm concerned about reliability, noise, capacitance, etc.

What's best practices when it comes to assembling systems like this?

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  • \$\begingroup\$ One more thing to note: this system is going to be in an uncontrolled environment, meaning it won't get wet but it will see extremes of humidity and temperature (anywhere from -10 to 60C for the latter.) \$\endgroup\$ – joemadeus Mar 18 '18 at 20:44
  • \$\begingroup\$ There is no answer to "best practices." Every situation presents unique considerations. Sometimes, there are categories of operation that have a lot of common requirements and someone will spend time providing some useful guidelines. But there is no such thing as "best practices." You know your situation better than anyone here. I have no clue, for example, what your box will be made of and how it "breathes." With temperature variations like that and high humidity, how do YOU plan to keep moisture from condensing, for example?? No info even on that. No best practice suggestions possible. \$\endgroup\$ – jonk Mar 18 '18 at 20:57
  • \$\begingroup\$ You need a NEMA 4X plastic or stainless steel container, or condensation will destroy it (your electronics) within a year. Your connections to the enclosure need to be water-tight. \$\endgroup\$ – Sparky256 Mar 18 '18 at 21:08
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    \$\begingroup\$ Direct solder connections between a wire and a PCB are OK but MUST be prevented from moving due to handling or vibration or they will fail. Crimp connections are more reliable if there is vibration and/or motion. Most of the connections in an automobile are crimp connections. The danger with crimps is that if they are not done properly they will fail. If I was making something for myself, I would not solder any wires directly to the PCB. Zero. \$\endgroup\$ – mkeith Mar 19 '18 at 3:18
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    \$\begingroup\$ It is true like another person said down below that non-locking connectors can vibrate loose (if there is vibration). So that is something to keep in mind. \$\endgroup\$ – mkeith Mar 19 '18 at 3:21
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Advantages of solder:

  1. Lower part usage
  2. Mechanical strength
  3. Part cost

Disadvantages:

  1. Effort to replace if needed
  2. Effort to assemble

There isn't much to worry about for capacitance and noise for a header + connectors vs soldering. It really boils down to assembly cost/time/effort. Keep in mind commercial products use both in similar environments. The most critical thing here is if your use case involves a lot of vibration. Solder would beat a non locking terminal strip.

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  • \$\begingroup\$ There won't be much vibration but if there is any condensation, despite good venting and such, I'm concerned that the headers might corrode? I suppose another advantage of solder is, I can protect those solder connections with a conformal coating. \$\endgroup\$ – joemadeus Mar 18 '18 at 21:21
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In addition to what @Passerby said, I can think of few other considerations.

1. Space limitations: Fairly obviously, headers and pin connectors require more physical space to accommodate them without pinching or stressing the wires.

2. Fail-safe needs for wire snags/ tension: Depending on where and how the sensor will be mounted, and how the wires will be secured if at all, the relatively insecure connection of headers vs solder could actually be an advantage. Better to have a connector unplug than to damage a PCB. Of course good wire management and proper strain relief is the best option when possible regardless of termination type.

3. Corrosion risks: Solder is inherently less susceptible to corrosion than pin connectors. Granted, unless you've got a fully potted or well coated board, by the time corrosion becomes an issue with header pins you'll probably also start having to worry about any small pin pitch and sensitive components on the board. Gold plated headers and pins can help here too, albeit more expensive.

4. Redesign and re-purposing flexibility: I do a lot of prototyping work. Sometimes I go with headers simply because I know there are good odds I'll have to change the wire lengths or scrap the project.

No clear overall winner. For me, the decision usually ultimately comes down to how I have to mount it and how likely I am to unplug/replace it. Really just a matter of balancing project needs and life cycle expectations with personal preferences.

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Nothing wrong with solder, until you have to unplug it.

Except soldered wires fatigue where the solder filled strands meet the strands. Support with hotmelt glue so the bending cannot occur where the solder has wicked up the strands.

If you are making pcbs, I find it very easy to have oblong pads 0.050" apart, and solder ribbon down directly without splitting the conductors apart.

For I2C we use Micro-match connectors (see i2c connectors), which are

  • low height
  • mass-terminate ribbon cable,
  • can crimp in a vice -no special tool needed
  • can make daisy chain cables with multiple males on one ribbon for bussing.
  • drawback: zigzag footprint bad for Veroboard
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