Why magnetic permeability is worth a second look in semiconductor manufacturing

Stainless steel has a reputation for being non-magnetic, and for austenitic grades in the annealed condition that is a broadly fair assumption, with relative magnetic permeability sitting very low. The complication is that the material rarely stays in its annealed condition once it is manufactured into a product or component, especially a fastener.

Manufacturing a bolt means cold working it, through processes such as drawing, cold-forming and thread rolling. This plastic deformation of the steel can result in part of the austenitic microstructure changing into strain-induced martensite. Also known as deformation-induced martensite, this phase is ferromagnetic.

In the vast majority of applications, that shift is academic. A bolt's magnetic permeability is unlikely to be a consideration when it is holding a pump bracket together. However, there are some industry sectors where magnetic permeability in fasteners becomes more of a concern, and semiconductor manufacturing is typically one of them.

Semiconductor fabrication runs on equipment that doesn't like magnetic surprises, such as electron-beam lithography and inspection tools, metrology and measurement systems, and a host of sensitive instrumentation. Components or assemblies carrying a magnetic signature can introduce unintentional errors that were not accounted for in the initial design. In critical areas, the hardware around this equipment (fasteners included) needs to be carefully specified with magnetic permeability in mind, rather than left to chance.

This is the gap that BUMAX® 88 and BUMAX® 109 are designed to close. Both grades benefit from very low magnetic permeability compared to standard A4-80 fasteners. BUMAX® 88 has an approximate relative permeability (µr) at 20°C of 1.006, and BUMAX® 109 is slightly higher at 1.007, despite being a considerably stronger fastener. This is achieved through a combination of factors. Firstly, the material specification for BUMAX® 88 and BUMAX® 109 is tightly defined and less susceptible to the formation of deformation martensite during cold forming. Secondly, many aspects of the cold-forming process are closely controlled, including reduction ratios, process temperatures and tool design.

Interference with magnetic fields, sensors and electronic equipment is therefore minimal where BUMAX fasteners are concerned. This sits alongside the other considerations these industries tend to demand: low particle emissions, low outgassing, cleanroom packaging and low-cobalt content (in high-radiation environments). Magnetic permeability is the one that is most often misunderstood, because of the counterintuitive notion that a "non-magnetic" material could be magnetic at all.

As geometries keep shrinking, tolerances tighten and performance expectations rise, there is less and less room for variables that can't be accounted for. A fastener that holds a steady, low magnetic permeability from raw material through to the finished joint simply takes one of those variables off the table.