Easy Rework with Thermal Compounds

Not long ago I had a conversation with a thermal engineer who was using a phase change pad as the thermal interface material (TIM) in a certain device. Some of the devices failed pre-maturely due to overheating. The engineer felt this problem could be solved by using a higher performance TIM like a thermal compound (or grease) from Dow Corning, but was afraid to change to a grease because he thought that using a grease for field rework would be too challenging for his technicians.

The focus of this month's thermal theme is to show that thermal greases are the clear thermal interface of choice for rework applications.

One need that nearly all customers must consider either during the manufacturing process or during the warranty period of their product is the need to "rework" their device. Sometimes rework is done in the factory after a faulty part is discovered during product testing; however, many times a device malfunction occurs after leaving the factory and must be addressed in the field. Regardless of the situation, it is important to have a reliable rework solution that is simple and provides consistent quality results whether in a controlled factory setting or in the field.

Thermal Interface Material (TIM)

Let's examine the reworkability pros and cons of the four primary categories of TIMs:

Thermal Pads - Of all TIMs, thermal pads are usually the easiest to rework and can in some cases even be reused. Substrates can be pulled apart easily because most pads do not have adhesion. The replacement pads are usually pre-cut to size for easy reassembly. The down side is that many applications require higher thermal performance than pads can offer. For many devices thermal pads are not an option.

Adhesives & Epoxies - These are generally the most difficult materials to rework. Often these materials cannot be removed without damaging the device, or they require strong solvents. Once they are removed, applying a new adhesive many not be an option because many require a heat cure step which is usually impossible in field rework situations.

Phase Change Material (PCM) - PCMs are materials with a matrix usually made up of some kind of wax that is solid at room temperature but softens or even flows at the device operating temperature. PCMs are often precut to size which makes re-application of the TIM simple. The downside is that used PCMs are generally difficult to remove. They often require scraping, solvent treatment, or heating the part to remove all of the previously applied material from the substrate. In addition, during reassembly some PCM pads may require an extra preheating step to partially soften the material in order to achieve a final thin bond line thickness (BLT) and avoid device overheating during the PCM "break-in" or "burn-in" period.

Thermal Compounds / Greases - Thermal greases are uncured semi-flowable materials that are generally screen printed or dispensed onto substrates. Because of this, greases can be easily removed from the substrate by simply wiping with a cloth, tissue or rag. A common myth is that greases are difficult to rework because they are messy and require too much technical expertise to apply the proper amount of material to the substrate. This paper will dispel this myth and show that thermal grease is really the product of choice for device rework.

Why Dow Corning Thermal Greases Are Easy To Rework

First, let's address the question of mess. This is somewhat subjective, because grease is only messy if you get it on you. During rework, the old grease can be wiped away with a tissue and discarded-not very messy. The best way to handle application of the new grease is to dispense from a syringe. When dispensed properly there is no excess grease to worry about. As you will see in the next paragraphs, there is no need for any tools to come in contact with the grease, so there is no clean up required.

Technical Features

There is one key feature that makes Dow Corning thermal greases ideal to use for rework. They are thixotropic, which means they maintain their shape and do not flow unless some stress or sheer is applied. This thixotropic behavior results in two very important benefits:

1) Once the grease is dispensed and then assembled between substrates in a device, it reliably stays in place and can be used in a variety of positions, including vertically mounted applications.

2) Dow Corning thermal compounds have very low yield stress, meaning that they are capable of spreading out to a very thin BLT under very little pressure loading. This thin BLT enables the greases to achieve minimum thermal resistance under low pressures. After the grease is dispensed from a syringe and clamped between substrates, the clamping force exerts the stress or sheer necessary to cause the grease to spread out to a thin bond line to entirely cover the mating surfaces of the device. No precise dispense patterns or tools are required for this.

This second point is very important for rework. It means the grease can be applied using a variety of methods and variable amounts of grease without a difference in thermal performance. For this test TC-5688 was applied to a thermal test vehicle (TTV) using screen print, syringe dispense and spatula application methods. The heat sink was then attached to the assembly under factory recommended pressure load, in this case 45 PSI. This data indicates that the application method had no significant bearing on the thermal resistance since the performance differences in this graph are within the margin of error for the test method. It is also important to note that on the syringe dispense, two different volume amounts were used with the thermal resistance measurement once again being within the error of the test. This means that if you accidentally dispense too much grease it can still spread to a thin bond line and achieve maximum performance.

Thermal greases are an excellent choice for rework.

There is little or no mess and a precise application method is not required for most devices. There is no requirement for fancy dispense patterns and technical expertise to apply thermal grease and obtain great performance results. If you have been sacrificing thermal performance for reworkability, now there is a solution to give you the best of both worlds.

This information has been sourced, reviewed and adapted from materials provided by Dow Corning.

For more information on this source, please visit Dow Corning.

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