Managing The Heat

A low-cost method for heat management using a conductive, high temperature polymer that can eliminate the need for conventional soldering.

Abstract

Technology in the 21st century has made seemingly miraculous leaps in the design of miniature components that are manufactured to exacting specifications. Unbelievably powerful processors are being made available in smaller and smaller packages as the world of computer driven lilliputian devices grows larger and larger. These small devices are tightly packed with incredibly tiny components engineered to infinitesimal tolerances. As each of these components come alive, they generate heat that multiplies and builds to levels which can be detrimental to the life of the motherboard.

Introduction

Unbelievably powerful processors are being designed to fit in to smaller and smaller packages. It seems that nearly every component is being designed on a scale that was inconceivable just a decade ago . These small devices are tightly packed with incredibly tiny components that generate heat and endanger the life of the motherboard. The subject of this paper is meeting and controlling the thermal challenges that arise in applications where electronics are placed in environments that subject them to high temperature.

The Product

Thermal control of portable and handheld electronic systems is an issue that includes high power electronics as well as pagers, cell phones, laptop computers and supercomputers and satellite-based electronics. Designing attachments for heat sinks used with assorted electronic systems that operate at higher temperature as well as swing through a range of varied climate conditions presented a challenge. Initially we set out to find a suitable low-cost method to manage the heat and protect the PCB. It soon became clear that the weakest link in the system was the conductive material used to integrate components and heat sinks with the board. Finding a way to protect high-performance computing equipment from heat-generated breakdown, meant bypassing traditional solder techniques. We knew from past experience that solder-less bumping was a better and less expensive system for making electrical-mechanical connections and substituting a polymer for solder could increase the operating temperature of the board itself.

Using the nickel based AC-78; a multipurpose conductive and electroplate-able polymer, solder-less bumping connections could be made by uniting unlimited numbers of surfaces together during the polymer cure. The bonded surface proved reliable and able to operate at 260 degree C without failing. The process is fast and efficient and unlike lead-tin solder, the copolymer attachment has the advantage of being integrated into the substrate and is strong enough to pass cross hatch pull tests. Further investigation showed that the product could be used in applications that require multilayering, screen printing and also in through hole design techniques.

Characteristics of the Polymer

Resistively:

Resistivity is a function of both curing conditions (cure temperature and cure time) and the substrate used. Under ideal conditions, the volume resistivity can be as low as 0.65 ohms/square/mil.

The enclosed charts show decreasing resistivity over cure time with curing at different temperatures (125°C, 150°C, 175°C and 200°C) for four substrates (glass microscope slides, 96% alumina, glass fiber epoxy board, anodized aluminum metal sheet).  However the decrease in resistivity over curing time is small. With higher cure temperatures, the plateau of constant resistivity is reached more quickly.  Increased cure temperatures result in a lower level for the final plateau of constant resistivity.

Different substrates demonstrate different resistivity readings.  For lower cure temperatures the resistivity differences are significant; whereas for higher cure temperatures, such as 200°C, different substrates show similar resistivity.

Resistivity Tests 

125°C               30           60        90     120     150      180 (min.)
Glass slide        1.51      1.48    1.4      1.39   1.37    1.29
Alumina          1.698   1.63    1.55    1.5      1.495  1.23
Glass/epoxy    1.4        1.33    1.27    1.25   1.23    1.15
Anodized Al     1.3        1.26    1.21    1.18   1.15     1.15

150°C                30       60       120     180 (min.)
Glass slide        1.34    1.33     1.3     1.25  
Alumina           1.25    1.22    1.17   1.17
Glass/epoxy     1.25    1.24    1.24   1.14
Anodized Al     0.97    0.96    0.94  0.93   

175°C                30       60        90        120       150 (min.)   
Glass slide        0.84    0.82    0.83    0.83    0.82
Alumina          0.87    0.81     0.82    0.82    0.79
Glass/epoxy    1.07     1.06    1.05     1.03    1

Anodized Al     1.1        1.1       1.09     1.03   1.02

200°C               30       60        90 (min.)
Glass slide        0.8      0.76     0.76
Alumina          0.77    0.73     0.73
Glass/epoxy    0.89    0.71     0.69
Anodized Al     0.75    0.75     0.73

222°C-225°C    15         30 (min.)
Glass slide         0.936   0.8
Alumina            0.77     0.76
Glass/epoxy      0.76     0.75  
Anodized Al       0.74     0.74

Corrosion Resistance:

Applied on a steel substrate with a thickness of 1-2 mils, the polymer coating passed a fog test (ASTM B117) for 1,000 hours in a salt spray chamber (5% salt solution, 95% humidity at 75°F) with no sign of rusting.

Adhesion:

The polymer coating has shown very good adhesion with actual wire and glass substrates breaking in the adhesion tests. 

Test parameters: The solder strips from the solder-ability tests (approx. 3 to 4 mm in width) were used.  Tinned copper wire was butt soldered using a hand held soldering iron.  No additional flux was needed to get a good solder joint.  The pull tests were then conducted with a Chattilion tester at 3 inches/minute.  All failures were from the conductor occurred where the conductor was seen on the bottom of the solder joint attached to the copper wire. The exception was some of the glass slides where glass broke at >20 lbs and the conductor remained attached to the wire.

EMI/RFI Immunity:

Though it is made of a ferrous metallic component nickel, the polymer coating acts as an effective shield to electric and magnetic fields. The attenuation is comparable to that of conductive nickel paint. If applied at 3 mils thickness and measured between the frequency of 3 MHz to 1 GHz the attenuation is approximately 40 dB (measured in a dual chamber test setup).

Applying The Coating

1. Substrate Preparation:

Cleanliness of the substrate is of extreme importance for the successful application of a conformal coating.  Surfaces must be free of moisture, dirt, wax, grease and all other contaminating materials. Use solvent to thoroughly clean the surface of the material.  Contamination under the coating will cause problems that may lead to assembly failures.  For some substrates, additional surface preparation steps, such as sandblasting might be necessary to ensure quality adhesion.

2. Mixing: 
Mix thoroughly before applying.  The formula contains nickel particles (.25 microns to .75 microns) in suspension in a solvent. It must be mixed thoroughly before applying.

3. Applying: 
The formula can be applied by spraying, brushing or dipping on selected components that will be electrically and mechanically attached.

4. Curing:
Coated components need to be cured together. The curing temperature and cure time depend on the desired resistivity and the substrate used. Carefully place the coated components into a preheated oven for curing. The following table outlines the preferred curing method:

100°C-120°C for five minutes (to remove moisture without generating bubbles), then at  220°C for 10 minutes and finally at 260°C for 5 minutes (ideal cure cycle). After cooling to room temperature, the coated material is conductive and electroplatable.

Test results indicate that an alternative infrared curing method can be used. Infrared curing is recommended for low-temperature substrates and may also be used when the curing time needs to be significantly reduced (<5 minutes).

 Conclusion

Recognizing that there is a critical need for thermal management of high performance electronic equipment in applications ranging from military electronics to telecommunications, a new low-cost method to manage the heat and protect the PCB was developed. The process relies on bumping; a better and less expensive system for making electrical-mechanical connections. Substituting a polymer for solder in the bumping or bonding procedure can increase the operating temperature of the board itself and allow the electronics to operate under harsh conditions that include external thermal loads. Connections can be made by bonding several surfaces together during the polymer cure. The bonded surface is able to operate at 260 degree C without failing and unlike lead-tin solder, the copolymer attachment has the advantage of being integrated into the substrate.