[March 25, 2006]
Thermal Control with Carbon-Composite Materials
|
Feature Article |
| |||||||||||||||
|
|
| |||||||||||||||
|
Thermal Control with Carbon-Composite Materials |
| |||||||||||||||
|
|
| |||||||||||||||
|
|
| |||||||||||||||
|
By Carol Burch and Kris Vasoya, ThermalWorks, Anaheim, Calif. [thermalworks.com]
The growing complexity of semiconductors is creating significant thermal considerations in their packaging, as well as their interconnection on PWBs. Moreover, the European RoHS Pb-free mandates, effective on July 1, will add a new and partially unknown factor to the equation. High chip-connection counts and small connection pads cause difficulty in attaching and adhering to a radically expanding substrate or board, thus connections have become paramount in the performance of critical electronic designs. Constraining the core expansion of a substrate or PWB allows the designer to tailor the surface coefficient of thermal expansion (CTE) of the structure down to the silicon die expansion rates and attach even large-scale die directly to the PWB or substrate. Reducing stress at the solder joint or solder-bump level is the thermal challenge that is moving into the spotlight because it may limit performance. CTE Benefits From car chassis and aircraft, to designing for dimensional stability in space, carbon composites are emerging as a superior technology because of their strength and stiffness-to-weight ratios, as well as their CTE benefits. The same principles that apply to aircraft wings and military protection gear will help the semiconductor industry shrink PWB designs and control the CTE of organic substrates. Carbon fiber composite is an affordable material in comparison to diamond and nanostructure materials and can be utilized to control substrate expansion, weight and rigidity. While thermal transfer benefits in a substrate may not be maximized due to heat saturation in a small area, the advantage carbon brings to substrate designers—compared to polyimide and glass—is its amazing strength and rigidity to hold organic materials together better, even after Tg levels have been reached and exceeded. Higher Reliability Controlling warpage and surface expansion during periods of extreme temperatures enable higher reliability in current designs; more importantly, this will permit designs to expand beyond their current performance boundaries. The surface CTE of an organic PWB can also be tailored to the 6-8ppm/°C expansion rate that closely matches the expansion rates of ceramic ball grid arrays. Placing a BGA reliably, without solder-joint stress due to expansion mismatch between ceramic and glass-based products—such as FR4—is of major concern in the PCBA industry today and is the reason X-ray machines have become a necessity on every production floor. Carbon-composite laminate offers a low expansion rate, generally 1.0 to 3.0 ppm/°C, depending on the type of carbon material used, and dominates or “controls” the expansion rate of the other organic materials in the PWB or substrate (see Figure 1).
Taming Carbon’s Dielectric Constant The dielectric constant of carbon, 13.36 Dk, has been considered too high for an electrical signal layer; therefore, carbon and PWB have never combined well for electronic designs. Innovation, such as high-end mechanical drilling and laser drilling with an added step, allows carbon to be imbedded into a substrate or PWB structure without harming the signal integrity of the structure. Utilizing carbon-composite is possible even in complex structures made with special materials, in sequential laminations, and in blind-and-buried vias—thus allowing complex ASIC and large-scale microprocessor die to be directly die attached. Current studies of flip chips and direct chip attach without underfill attached to the carbon-composite structures are being conducted with promising results (see Figure 2). One study conducted by Boeing and Auburn University proves that a sandwiched construction of carbon/FR4 substrate is not only advantageous for thermal expansion purposes to match flip chip, but passes all space and harsh-environment testing for flammability, offgassing and structural strength.1
Test Procedures Specific strain-gauge techniques2 were used to measure the expansion of a board constrained with carbon composite laminate ST500 and FR4 layers. This technique requires that a strain be applied to both the test sample and a reference material with a known low expansion CTE, such as titanium silicate. Strain gauges utilized in this work were Measurements Group WK-00-125MG-350 with a gauge factor of 2.01. The gauges were mounted using M-Bond 43B adhesive. Thermocouples were also placed on both the test samples and the reference material. The instrumented samples were placed in a thermal chamber and the bridge output was nulled at 0°C.
Measuring Strains and Temperatures The strains and temperatures were measured and recorded from -50°C to +150°C in 10°C increments. Temperatures were allowed to stabilize for 45 minutes at each set point prior to taking the strain and temperature measurements. The data acquisition system consisted of a 350Ω, ¼ bridge strain-conditioning modules and thermocouple input modules. A data acquisition board was utilized to perform the A/D conversion and interface to the PC running specialized software for experiment control. Results A third-order polynomial curve fit was applied to the strain and temperature data output from each gauge. The temperature-dependent strain curves were subtracted and added to the known reference expansion curve. The total curve was then differentiated with respect to temperature to calculate the temperature dependent CTE of the sample. Temperature dependent plots of the strain and CTE in each direction for both carbon composite/FR4 board and an FR4 sample were taken. The in-plane CTE for a typical FR4 board material is 16-20ppm/°C This study demonstrates that a significantly lower CTE can be achieved by the use of carbon composite/FR4 hybrid PWB (~2.5ppm/°C); thus reducing the in-plane CTE by approximately 80 percent (see Figure 3).
Localized CTE In a rapidly changing design world blending “seasoned” technologies with the newest innovations onto one structure, the first assumed obstacle would be attaching materials with different expansion rates onto one substrate or PWB. If the expansion of a board is tailored down to 9ppm, it will have an opposite and adverse affect on the plastic packages which expanded at a similar rate to the original FR4 board (17-19ppm/°C). There is an effective method to isolate a section of the PWB expansion to match that of silicon for DCA or ceramic packages, while leaving plastic packages in the non-isolated area, which will remain at a surface CTE of 14-17ppm. This method, “localized CTE,” enables designers to section off a high pincount, flip-chip- or BGA chipset, while continuing to use plastic packaging technologies in the same design (see Figure 4). Additional Thermal Properties Traditional dielectric materials used in substrate build-up and PWBs show thermal transfer rates of 0.3-5 W/mK. Raw carbon fibers, however, possess thermal transfer rates up to 620 W/mK, higher than copper and aluminum. When manufactured as a composite, this transfer rate does dilute to some degree, but still maintains extremely high thermal conductivity, comparable to a heavy copper layer imbedded into the PWB. The production of a material stack-up with carbon composite laminate is processed much like a PWB with heavy copper or CIC (Copper Invar Copper). The ease of drilling through the carbon composite laminate, however, offers lower production costs than employing other high-thermal transfer materials. Carbon composite laminate is stackable and laser drillable, presenting the designer with thinner profiles that exhibit similar thermal transfer rates to Cu on the X and Y axes (see Figure 5).
Z-Axis Expansion Many new materials developed for surface-core constraint work reasonably well in the X and Y axes and control expansion at 8-12 ppm/°C. Difficulties may emerge, however, with excess heat expansion (reflow) on the Z-axis or through-plane. With RoHS-compliant boards emerging this year, and Pb-free material playing a major role in consumer electronics, oven temperatures are higher; in some instances, FR4 is incompatible with the increased temperatures. The same is true of thermal and core-constraining materials. The Z-Axis “hit” or expansion mismatch in many materials exceeds 3x the Z-axis expansion rate of FR4 or Polyimide. This mismatch causes strain on via barrels and the materials used within them. Carbon-composite laminate expands at a slightly lower, but very similar rate to FR4 and Polyimide in the Z-axis. This will help with the stresses from new, Pb-free manufacturing processes such as HAL (with Sn/Cu-alloys with melting points at 227/°C and process temperatures of 265-270/°Cs. Z-axis expansion is a terrible detriment to the new Pb-free process. Carbon’s Reaction Possibly the most important new discovery of how carbon reacts in a substrate or PWB also involves the new RoHS-compliant Pb-free materials and processes. Processing Pb-free materials exposes Si, packaging materials and FR4 to harmful temperature levels more than once during manufacturing. Thermally conductive carbon composites reduce oven temperatures up to 10 degrees during the reflow process and reduce dwell times in the oven from 5-10 seconds. This is a significant stress reduction on the components and the Si. Conduction Cooling It has been believed that conduction cooling would decrease the surface temperature of a board and cause a rise oven temperatures to melt solder paste. In reality this is not the case. Highly thermally conductive carbon fibers in a weave pattern spread heat throughout the entire board in the X and Y directions much quicker and heat the entire board up more quickly and with greater uniformity. As RoHS advances, and materials become harder to work with at the production level, reducing oven temperatures will become paramount. The price of putting an extra step into a substrate or PWB may be a choice that pales in comparison to the price of trying to do business as usual. Carbon-composite laminate is one answer to production problems involving Pb-free standards. References 1. Material Characterization and Die Stress Measurement of Low Expansion PCB for Extreme Environments, D. Copeland, M. Rahim, M. Islam et al., Department of Mechanical Engineering, Auburn University, Auburn, Ala. 2. Vishay Measurements Group Guide to Strain Measurement Technology, Tech Notes TN-513.
|
