Failure with Solder Points and Mechanical Pressure

The good thing with failure is: it is an opportunity to learn :-).

So here is a case: For a STEM roadshow (see “MINTomat: World’s Most Complicated Bubble Gum Automata?“), we have produced in a rush an autonomous robot with a shiny printed 3D cover:

LED effect in blue

Prototyp with LED Effects

The board on the top has 4 I²C ToF (Time-of-Flight) sensors to measure the distance through a hole on each side of the sensor board on the top:

Sensor Head

Sensor Head

The 3D printed part is attached with screws to the PCB:

Screws

Screws

The robot worked fine for several weeks, until it failed. Failure is indicated with a red LED error code: all LEDs were permanently showing in red:

Failure Indication LEDs

Failure Indication LEDs

The robot firmware uses USB CDC for a command line interface. Hooking it up on the host showed that the I²C communication to the sensor fails:

Failure Report

Failure Report

So there is something wrong with the sensors? The cover is attached with screws to the sensor PCB, removed the screws to inspect the board:

3D Printed Cover with Board

3D Printed Cover with Board

Because things were on a very tight schedule, we had to produce the prototype board internally. And because of a layout error, we had to fix two traces. A close inspection of the PCB showed where the problem was:

Pressed Solder Point

Pressed Solder Point

The board was working fine initially, but over time the mechanical pressure of the 3D printed part pulled down the solder of that fix, making a shortcut with the ground plane :-(.

With this, the temporary fix was easy: fixing the shortcut and putting in a mechanical spacer so there is no pressure applied on the board surface in these areas.

Summary

We better would have spent a day more to get good PCBs and not to rely on internally produced ones. Temporary solutions won’t last long (that’s why they are temporary). Don’t underestimate mechanics and mechanical pressure on solder: solder will flow and can create all kind of effects (not talking about high acceleration or vibration forces here!). And always think about the unexpected, and yes we have learned yet another thing which comes with experience :-).

Happy Learning 🙂

Advertisements

4 thoughts on “Failure with Solder Points and Mechanical Pressure

  1. Yes, solder can deform under mechanical stress. I performed failure analysis for many years, and have seen numerous cases of solder (and other solids) failing due to such conditions. Oddly enough, although solder itself is a soft alloy, it forms a few very brittle intermetallic compounds with copper. Quite a large number of solder-related defects are actually brittle fracture failures.

    Liked by 1 person

  2. Pingback: Mintomat: An Overcomplicated Gumball Machine | Hackaday

What do you think?

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google+ photo

You are commenting using your Google+ account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

w

Connecting to %s

This site uses Akismet to reduce spam. Learn how your comment data is processed.