Thermally Conductive Silicone with High Position Tack. Zheng Lu, Chris Verosky ASC Conference, Atlanta, GA, April 3-5, 2017

Transcription

1 Thermally Conductive Silicone with High Position Tack Zheng Lu, Chris Verosky ASC Conference, Atlanta, GA, April 3-5, 2017

2 Agenda 1. Background information 2. Thermal conductivity with particle packing 3. High position tack with modifier 4. Conclusions 2

3 Thermal Management with Adhesive Fills voids between heat sink and component Seals and bonds (replaces fasteners) If it is a good thermally conductive material, it transfers heat easily from component to heat sink component Heat sink adhesive 3

4 Challenges for Adhesives Major Factors for Thermal Interface Material (TIM) Selection FACTOR Electrical insulation Ease of assembly Fixture time Thermal performance Reliability Cost Re-workability RATING >10000 V/mm Single Component <10 minutes >2 W/mK Thermal Cycle, Humidity, UL Low Good RTV Silicone is good fit but fast-fixture time and cost are immediate issues 4

5 Commonly Used Thermal Conductive Additives Filler Thermal Conductivity (W/mK) Cost Alumina (tabular) ~ low Alumina (spherical) ~ medium Aluminum nitride ~ Very high Notes Beryllium Oxide ~ 250 Very high Health hazard Boron Nitride ~ Very high Highly anisotropic Silicon Carbide ~ low Low electrical resistance Silicone Nitride ~ high Zinc oxide ~ 20 low Compare to W/mK for unfilled adhesive 5

6 Trend of Thermal Conductivity and Adhesive Viscosity Silicone adhesive system Viscosity increases much more rapidly than thermal conductivity after 82% loading Thermal conductivity continue to improve with the additive loading 6

7 Filler Packing Models Assume that particles are spheres, and thus, most dense packing will result in greatest contact of particles which leads to higher conductivity. Fuller s model to predict the ratio of course/small particles Using lower cost particles Sphere Diameter # 1st D o *1 1 2nd D o * rd D o * th D o * th D o * U(i) = (D i /D max ) q D (i+1) = U(i+1) U(i) U is the ratio of the Di particle. Dmax is the largest particle. q is the factor for packaging Several particle groups are selected Example Data: Group 1: Larger Particle group Group 2: Smaller Particle group 7

8 Thermal Conductivity Packing group one Additive particle packing effects with larger particles Thermal Conductivity, W/mK Course: fine 1.5:1 Particle Combination 1 Course: fine 2.33: Wt.% Large Particle (total = 87%) Course: fine 4:1 Packing of particles has the direct impact on thermal conductivity Larger particle makes more inter particle connections Thermal conductivity continues to increase with the increase of course size particles 8

9 Impact of Additive Particle Ratio Packing group one Packing of particles not only affect the thermal conductivity, but also on viscosity ~30% (fine/total) ratio has the lowest viscosity, indicating best packing 4:1 course:fine 2.3:1 course:fine 1.5:1 course:fine 9

10 Thermal Conductivity Packing group two Additive particle packing effects with smaller particles Course: fine 1.5:1 Packing of particles has the direct impact on thermal conductivity Thermal conductivity reaches a plateau regardless of continuing to increase the course size particles 10

11 Impact of Additive Particle Ratio Packing group two Very similar result as the packaging group one. Packing of particles not only affect the thermal conductivity, but also on viscosity ~33% (fine/total) ratio has the lowest viscosity, indicating best packing 4:1 course:fine 2.3:1 course:fine 1.5:1 course:fine 11

12 SEM of two packing groups Smaller particle packing group Larger particle packing group 12

13 Test Methods- High Position Tack Measure of Green Strength 100 mm x 25 mm x 2mm test area g test weight Aluminum substrate Test time- up to 10 minutes Use uncured adhesive High Position Tack Test 13

14 High Position Tack Identified a good modifier Very low loading level is needed The high position tack increase significant with the increase of the modifier The modifier does have negative impact. It slightly reduced the thermal conductivity of the adhesives 14

15 SEM Results No Modifier Low load of modifier Mid load of modifier 15

16 Conclusions RTV silicone can be made to have fast-fixture by adjusting green strength via low level modifier and packaging of thermally conductive fillers RTV silicone can be formulated to have W/mK TC without use of expensive fillers Size and ratio of course and fine particles has pronounced affect on viscosity and thermal conductivity, optimum ratio of 30% small particle is the idea loading for our adhesive systems 16

17 Thank you!