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When accuracy counts, count on the encoder

03 June 2010

A developer of bespoke machinery capable of working to micron and sub-micron accuracies uses sealed compact incremental linear encoders that not only provide the required level of positioning accuracy and repeatability but also a brand name that gives its customers a ‘renowned reference standard’

The prototyping and production of silicon wafer and related substrates is a challenging industry, but its demands have played directly into the hands of one particular company – Loadpoint, whose equipment is widely used for the cutting, dicing, grinding, polishing, slitting and drilling of components used in the semiconductor, printer head, optical, ultra-sound scanning and medical diagnostic sectors.

Based in Cricklade near Swindon, Loadpoint has progressively built its business over 40 years to provide a micro-machining sub-contract operation as well as a machine, process development and prototyping facility that is reckoned to be one of the most advanced facilities of its type in the UK. The company has a turnover of some £3 million and employs 25 people. High on its agenda is the continuous development of bespoke machinery capable of working in micron and sub-micron applications.

The MicroAce 66 150x150x10mm capacity micro dicing machine is a recent example and a demonstration of how Loadpoint goes about meeting these challenges. This machine has to obtain – and, crucially, maintain - an axis positioning resolution of 0.1 micron, as well as achieving ultra-precise control over the programmed cutting process, which can involve dicing wheels as large as 75mm diameter and as thin as 15 microns.

Loadpoint’s choice of Heidenhain’s LF481C sealed compact incremental linear encoders not only provided the required level of positioning accuracy and repeatability for this machine but also, as Loadpoint chief engineer Clive Bond points out, a brand that gives the customer a “renowned reference standard”.

Loadpoint has enjoyed a long relationship with Heidenhain, having utilised and capitalised on the latter’s linear scales for 35 years while its competitors continued to rely on open loop stepping motors. This enabled the company to achieve higher orders of precision and repeatability, and it inevitably led to a particular focus on special applications.

Customer challenge
With almost 90% of Loadpoint’s equipment being exported, Mr Bond reflects how one European customer challenged the company’s ability to build a machine that would be able to provide the precise levels of pitch accuracy required for slitting silicon wafers. As he recalls, his company’s response was to contact Heidenhain directly for its Heidenhain VM182 comparator system, which ultimately put the customer at ease.

The VM 182 incorporates a steel scale with two co-ordinate phase gratings and a scanning head connected to the scale by an auxiliary carriage. The scale is clamped to the table and the scanning head to the machine spindle. “This qualified, without any doubt and according to ISO 230-2, both linear and non-linear error curves as well as any reverse error in each of the machine axes,” says Mr Bond. “So successful was the trial that we now take this reference to customer sites when we check and recalibrate machines.”

Loadpoint’s development laboratory has seen various special machines and processes created for machining silicon, silicon carbide, glass, quartz, gallium arsenide, germanium, lithium niobate, lithium tantalate, sapphire, lead zirconate and titanate (PZT), as well as equipment for ‘processing’ components such as wafers, substrates, plates, laminates, rods, blocks, tubes and assemblies.

With such a wide range of different and challenging applications, the company has become largely self-sufficient as a developer of bespoke systems - it even has its own Nano control system featuring high-level diagnostics and remote communication. Loadpoint also develops proprietary software and some in-house programs are able to monitor process quality by including the company’s own Kerf Check for measuring the width of cut, size and area of chipping while also identifying and correcting alignment offset errors. The software will also monitor and display spindle current, blade wear, chuck vacuum and the co-ordinates for each of the required machine axes.

The MicroAce 66 has X and Y axis travels of 160x164mm and a Z axis that is able to accept components of up to 10mm thick when using the maximum size slitting wheel of 76.2mm. Each axis is fitted with a Heidenhain LF481C linear scale for position feedback, which also includes Heidenhain’s mounting spar. This means that, in the event of an encoder change, there is no requirement for drawn-out clocking procedures to establish datum positions.

The linear encoder is fully protected from contaminants, and an aluminium housing and elastic sealing lips protect the scale, scanning carriage and guideway. Indeed, the fact that the LF481C is a sealed encoder helped Heidenhain secure this business against a supplier of exposed scales.

The 1.2kW air bearing spindle has a top speed of 80,000rpm (optional is 2.4kW/60,000rpm) and this works in conjunction with a Theta axis bearing rotary table with vacuum chuck. The table has a high resolution direct drive torque motor with an axis resolution of 0.0004 degrees. A four-direction coolant delivery system has outlets for main blade jet, side wash bars, wafer fan jets and flood jets.

Also incorporated in the full screen display vision and alignment system is monocular video alignment that incorporates pattern recognition with a Z axis auto-focus to set up the alignment of the image. Through-the-lens LED illumination and external high intensity LED fibre ring illumination are also included.

The LF481C is one of a range of encoders available from Heidenhain (GB). Sealed units are available with full-size scale housings offering high resistance to vibration at measuring lengths of up to 30m, as well as slimline scale housing for limited installation space that are capable of measuring lengths of up to 2,040mm.

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