Engineering, Design and Usage Precautions Relating to High Vacuum Fasteners

Introduction

The use of high vacuum and related semiconductor manufacturing equipment presents many detailed and complicated engineering problems. This section will discuss, in laymans terms, some of the challenges associated with using both vented and non-vented fasteners in vacuum environments.

Vented vs. Non-Vented

We supply screws which are vented using a coaxial hole down the middle of the screw. Of the various methods used to vent screws for vacuum use, this one best preserves the fastener integrity. All vented screws have had some material removed and will have a corresponding reduction in strength vs. the same size non-vented screw. Contact our technical department for additional information.

Precision Clean vs. the Effect of Lubrication

All Benson Associates' parts are shipped precision cleaned and Class 100 packaged to assist in maximizing your system's performance. However, precision clean parts may present an application problem. With no surface contamination, a stainless steel screw may gall, or seize inside a stainless steel tapped hole or stainless steel nut. This makes it difficult or impossible to remove the screw without destroying it. To reduce the threat of galling, use of a vacuum stable lubricant such as Molybdenum Disulfide (MoS2), Tungsten (WS2), graphite, or silver plating is recommended.

Coated and Plated vs. Plain Stainless Steel

Some coatings may generate particulate contamination and may present a surface with low adhesion to some sputtered or deposited films and coatings. Further, a coated or plated screw has a lower friction coefficient than an uncoated one, resulting in a higher tension (or compression) value for a given torque. The word of caution here is that the tensile strength of a screw can be exceeded without exceeding the recommended maximum torque. This could distort or even destroy the fastener.

Using a Screw Twice

When tightening down a screw, you cause small stress fractures to occur. Taking the screw out can cause additional stress fractures. This will weaken the screw and with repeated use, the screw will eventually fail. Additionally, many vacuum processes leave behind materials which build up on the fastener, then flake off, causing particulate contamination and, subsequently, reduced yield. The cost of replacing each screw after one use is often much lower than the risk of downtime and the expense of removing a broken screw. As always, please feel free to contact our technical department for additional information.