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Closed loop TIM processing: printers offer better process control and cost savings over dispensers.

While the pace of thermal interface materials development has generally kept up with the thermal transfer demands of smaller, higher-powered devices, it is debatable as to whether the dispensing mechanisms often used to deposit these materials have maintained--or are capable of maintaining--such a development cycle. As I've often pointed out in this space, because dispensing is a serial process, the only real way to increase throughput is to add more dispensers.

However, it's not just speed limitations that need to be considered with traditional dispensing, but material quantity use as well--especially in the case of expensive TIMs. The inherent throughput advantages and deposition control of screen printers alleviate these issues. To fully understand the screen-printing TIM process benefits, we first need to discuss traditional deposition methods versus that of mass imaging.

TIMs are used to facilitate heat transfer within a package at the die level (TIM1) or from the exterior of a package to a heatsink (TIM2). Historically, dispensers deposit these materials. For efficient heat management, complete material coverage is essential and, with dispensing, this requires the deposition of a fair amount of material--on the order of 48 mg for a die of 10 x 10 mm in size--to ensure complete coverage. The material is then pressed by either the processor lid or heatsink so that all the gaps are filled, which means unused material is pushed out from the sides of the device, producing material waste. When you consider that these materials can cost on average anywhere between $1 to $2 per gram, keeping more of the TIMs on the device and out of the waste bin will result in considerable savings. Second, material uniformity may be problematic because of the reliance on the pressure of another device, which may have its own imperfections, to evenly spread the material. Lack of TIM material flatness and uniformity can result in problems such as insufficient thermal conductivity or inferior coplanarity.

Through use of a printing process to deposit TIMs, material volumes can be controlled; thickness is precise with little to no waste; material uniformity is dramatically improved, and the formation of voids is substantially reduced because the process does not rely on another device (i.e., a processor lid or heatsink) to spread the material. In a project our company worked on, we were able to put down 30 mg of thermal interface material (TIM2) with a standard deviation of 1 mg; the average height of the material was 250 [micro]m with a standard deviation of 11 [micro]m (Figure 1). In comparison, with a dispensing operation, 55 mg of material would have to be deposited and, again, thickness and uniformity control is extremely limited and material waste is relatively high. With screen printing, material use is optimized because of the precision of the print process. Also, an enclosed print head prevents the material from exposure to environmental conditions, and only the volume of material required is deposited, which results in additional material cost savings.

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Now, let's talk about speed. The print process is inherently faster than the dispensing process because of its ability to image multiple substrates with unlimited material volumes on equipment capable of sub 5 sec. cycle times. Using specialized tooling--or virtual panel tooling--to transport the device carriers into the printer, multiple packages can be printed in a single stroke (Figure 2). In the case of TIMs, our company has successfully processed up to 10 substrates in approximately 25 sec., or 2.5 sec. per substrate, as compared to dispensing, which averages about 5 sec. per substrate.

Finally, using a post-print inspection tool to verify material presence and the absence of voids closes the process loop without slowing cycle time and dramatically improving end-of-line yield. By using an inspection technology that operates at the line beat rate, there is no slowdown in production time or UPH (units per hour) and packaging specialists can enjoy print verification peace of mind and superior yields.

Clive Ashmore is global applied process engineering manager at DEK (dek.com); cashmore@dek.com. His column appears bimonthly.

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Title Annotation:Screen Printing
Author:Ashmore, Clive
Publication:Circuits Assembly
Date:Jan 1, 2007
Words:682
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