Maria Montero

How to Design a Custom PCB Holder: Mechanical Considerations for a …

When working with high-precision sensors, it is crucial to consider the housing. In this article, we discuss the mechanical design considerations at play in a custom board design for an inclinometer subsystem.

There are many considerations for mechanical design when designing a system with a high level of precision. Recently, I designed an inclinometer-based custom PCB with such precision that I need to design a heavy and stable case to protect the integrity of the data it collects.

As I mentioned in my summary of the inclinometer subsystem, the general project should allow a resolution of 0.001 °, which means that the sensor will detect it if the plate moves even 1 µm. This means that you will have to digitally recalibrate the board each time it snaps into its mount. To make things easier, I decided to fix my card to a holder for less need for constant digital recalibration.

The picture above shows the assembled board that is kept in the aluminum tester during the preliminary test.

See the links below for more information on the Precision Inclinometer Project Series. This article specifically refers to the PCB layout article.

Manual plate adjustment with differential drive screws

Since I’m Mark “Hard-way” Hughes, I decided to add a manual adjustment system. At one end of the bracket, I chose a differential drive screw mechanism. On the other hand, I added two additional polished M3 acorn nuts on the opposite end of the table mount. These two non-adjustable feet bolt to the bottom of the dash and become a pivot point opposite the differential screw. Also, this arrangement allows a certain amount of tuning to test the project.

An exploded rendering of the PCB holder, alignment pins, PCB, and PCB clamps

Differential screw mechanisms have two slightly different pitch threads that rotate simultaneously about a central axis. They are arranged so that as one screw advances, the other is withdrawn. The result is a compound screw whose effective pitch is the difference of the pitch of the two original screws.

This video shows a generic differential screw while this video shows the Thorlabs differential screw used in this project.

A render of the differential screw. The thread size has been exaggerated in this illustration. Image by Mark Hughes

This differential screw mechanism can be adjusted in multiple ways. Rotating the thick part (shown in blue above) provides a rough fit. Rotating the fine part (shown in red above) provides fine adjustment. Rotation of the center part (shown in green above) simultaneously affects both roughness and fineness, providing a microfine (differential) fit.

If you have access to the company’s credit card, your friendly local machinist will be able to make custom differential screw mechanisms. Due to budget concerns, I opted for a 25 µm / rev differential set screw manufactured by Thorlabs.

A complete revolution of the mechanism changes the height of the end of the PCB holder by 25 µm, but the screw can be turned by smaller amounts.

In the case of this differential screw, 1 µm corresponds to a rotation of approximately 15 °. While this is not an ideal level of fit, it is at least within the realm of plausibility. And, as a backup, I have the ability to calibrate the device digitally.

PCB Mechanical Design: Heat and Mechanical Stress Mitigation

We incorporate some mechanical design considerations into our PCB design, itself.

Six 3mm notches are placed 1 inch on center along the long edges of the board that will engage the indexing pins on the aluminum bracket. The PCB is held in place by two 3 “long aluminum fasteners that will be held loosely against the PCB with M3x0.5 or # 4-40 hardware.