Formulating Cast Urethane To Produce Wire Guide Rollers
The manufacturing of wafers for the microelectronics and
photovoltaic industries involves a multitude of critical processes. One of the
first of these is the precision cutting of ingots or crystals, typically
composed of hard and brittle silicon, into uniformly sized wafers that are
suitable for the processing of silicon chips.
In a production environment, this cutting is typically
performed by multi-wire saws composed of diamond-coated wires precisely and
evenly spaced in a belt-like configuration.
The cutting of the ingot or crystal of silicon material is performed in
one pass, resulting in dozens or even hundreds of wafers.
To achieve the ideal spacing between wires, as well as
maintain the ideal wire tension, precisely manufactured rollers are used. This includes smaller “tensioner" rollers
along with a larger roller with machined notches that keep the diamond-coated
wire in place.
Because the accuracy of the wafer cutting process is
dependent on the durability and consistency of the rollers, those that lack
material strength or precise geometry/dimensions can fail prematurely. This can lead to costly unplanned downtime,
wasted materials, and potential delays in subsequent operations.
Fortunately, high performance rollers are available that
utilize unique, more durable, formulations of urethane and can be machined to
meet the high tolerance requirements with consistency from part to part to
ensure the ideal multi-wire cuts.
The majority of wire-cutting rollers in use today are
composed of urethane (short for polyurethane). Shorter rollers, which typically
cut up to 30 wafers simultaneously, are made of high-performance, cast and
machined urethane. The longer rollers, which often cut up to 300 wafers, are
often coated with urethane sleeves having similar properties.
“Urethane is the preferred material for both long and short
wire cutting rollers," says Greg Cappaert, Technical Sales Engineer at
Argonics, one of the U.S.’s largest producers of wear-resistant urethane
products. “Cast urethanes are tough, abrasion-resistant elastic materials that
combine many of the performance advantages of high-tech plastics, metals and
ceramics along with the resiliency and flexibility of rubber parts."
Because urethane’s properties can be adjusted over wide
ranges of hardness, resilience, spring rate and dynamic properties, its
versatility offers custom solutions to difficult design challenges.
“Urethane, even in relatively small quantity production, can
be adapted and tailored to prioritize the physical properties that design
engineers need," says Cappaert.
Urethane’s properties are vital to high-volume production
operations that utilize multi-wire saws to slice wafers as thin as 80-100
microns. Any deviation in wafer
thickness or profile will bring a cutting operation to a halt and, unless spare
rollers are on hand, create serious production delays.
Cappaert adds that urethane rollers used on wafer
wire-cutting equipment are essentially “consumable" items. The shorter rollers,
also known as tensioners or pulleys, are replaced several times a year. Longer rollers that have urethane linings
must be recoated as much as 40-50 times annually.
Cappaert says that much of the turnover of rollers results
from rollers falling out of spec prematurely due to shortcomings in urethane
formulation, casting or machining.
Because urethane is an engineered material, many problems
can be overcome by adjusting formulation to meet the specific requirements of
an application, as well as rigorous testing and stringent quality control of
individual parts. He also notes that polyurethane rollers can be engineered to
deliver extreme accuracy in both wet and dry operations, such as machined
tolerances to +/- 0.004" (+/- 0.10mm).
Cappaert characterizes the production of cast urethane
rollers as complex and highly sensitive. For that reason, Argonics casts,
machines and inspects each individually. When larger urethane-coated rollers
are approaching wear limits, these must be stripped and re-coated using the
same types of stringent procedures.
Although specific customer formulations and design
information are considered proprietary, Cappaert says these methodologies can
lead to improvements in wafer warp, consistency of cut, and thickness variation
while also increasing roller longevity.
Another difficulty that many suppliers and users of
precision wire cutting equipment can relate to is supply chain issues.
Depending on inventories on hand, untimely deliveries of urethane rollers –
possibly due to premature failures – can shut down wafer cutting, resulting in
costly delays down the rest of the production line. Cappaert recommends working
with a manufacturer, like Argonics, that is willing to help solve these types
of costly delays.