Thermal Modeling and Analysis of Device-Contactor ... - Johnstech

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WhereA = area in square inches (including both sides)P = power in Watts (heat load)If the loadboard or printed circuit board is coated with ½ ounce copper (low mass), spreading resistance(lateral resistance to heat flow) becomes a consideration, whereby the heat flow through the copper foildecreases with distance thereby causing radiation to the air, and also the temperature, to decreasemeasurably with distance from the heat source. In the case of ½ ounce copper, the heat sinking iseffectively limited to less than a ½ inch circle. Increasing the thickness of the copper to 1 ounce willincrease the heat sinking to the diameter of a 2 inch circle. For high performance, e.g. RF circuit boards, acommon practice is to make the upper and lower outer layers of microwave PCB material such as RogersRT/duroid ® 5880. They are glued to interior layers of standard FR-4 material. The latter layers sustain thepower and low frequency circuits associated with the DUT. Heat transfer from the loadboard to theenvironment is by convection with an excellent explanation of this process for loadboards in anyorientation given by Remsburg [1] in his text on advanced thermal design.As mentioned earlier in this paper, the ultimate goal of a test system of the nature described herein is toensure that the temperature of the die in the DUT in operating within safe limits and that any heat generatedis efficiently transferred to the environment. Once the thermal resistance of the system in has beendetermined and the amount of heat calculated, then it is possible to calculate the final temperature of the dieand conversely, if the maximum die temperature is specified, it is possible to determine the maximumpower the device can handle. Equations 13 and 14 can be used to calculate the aforementioned factors.Temp. Die = T amb + (P Diss * R θ, system ) °C (13)WhereandWhereTemp. Die = the final operating temperature of the die (°C)T amb = Ambient temperature of the environment (°C)P Diss = Power dissipated (heat) by the DUT (Watts)R θ, system = Overall thermal resistance of the system (cf. Fig. 2) (°C/W)P Diss = (T j - T amb )/ R θ, system (Watts) (14)T j = junction temperature of die (°C), upper limit is generally 150°CAn example of what can be achieved with the processes and methodology that is the subject of this paper isgiven in Table 3. The genesis of this table was to determine and tabulate the thermal resistance vs. contactcount vs. power dissipation of various size QFN packaged devices using two forms, namely,1. Pad ROL100 contacts in a Torlon housing, and2. Pad ROL100 contacts in a copper ground insert in a Torlon housingPad ROL100 contacts are a product of Johnstech International Corp., Minneapolis, MNAssumptions made when creating this table include:a) Maximum die junction temperature = +150°Cb) Ambient temperature = +25°Cc) Total system thermal resistance = (25 + Factor) °C/W6
Table 3. Summary of RTH and RCI Thermal Characteristicsfor Pad ROL100 Contacts and InsertsPackageSizeRTHGrounding# of ContactsRTHGrounding(°C/Watt)RTHGroundingPower Diss.(Watts)RCIGrounding# of Contacts3x3 N/A N/A N/A Insert Only(CI)5x5 5 Contacts 46.6 2.68 3 Contacts(RTH)(RCI)7x7 12 Contacts 34.8 3.59 12 Contacts(RTH)(RCI)9x9 12 Contacts 34.8 3.59 12 Contacts(RTH)(RCI)RCIGrounding(°C/Watt)RCIGroundingPower Diss.(Watts)33.07 3.7830.47 4.1029.73 4.3528.33 4.41III.Example of a Case StudyA. Example of Thermal Resistance and Heat Transfer Capability of DUT-Contactor-Loadboard SystemAs with many theoretical studies, it is always instructive to include an illustrative example to highlight theuse of the equations and material involved. In this case one is asked to determine the thermal dissipationcapability of a 1mm contactor with an 8- QFN 5X5 package footprint for the DUT. The DUT is a Ka –Band, Microwave Power Amplifier with an RF output of 1 Watt and a DC input power of 5 Watts. A listof specifications and parameters for this device and setup is shown immediately below.• Contactor is a Pad Series ROL100 product• Three ROL100 contacts in a copper insert for ground/thermal• Eight ROL100 peripheral contacts• Plating material on DUT pads is matte tin• Test Temperature Range: Ambient (+25°C)• Operating Voltage (for each device): +5.0 VDC• Input current: 1000 MA DC• Power (Watts) of each device: 5 W DC• RF output power: 1 Watt• DC-to-RF conversion efficiency: ~20%• Energy dissipated as heat: ~4 Watts• Test Time Duration: > 1 minute• Duty Cycle: Steady-State (100%)• Possible topology of PCB• 2 Layers with the top layer dedicated to ground• FR-4 PCB’s are 4.5” x 4.5” x 0.010” thick• Copper traces are ½ ounce with 50 µinches of gold over 200 µinches nickelB. Solution and General CalculationsThis analysis is based on a Pad ROL100 contactor with a 2.6mm x 2.8mm copper insert having three (3)ROL100 BeCu contacts, 0.254mm wide inserted therein. Additionally, there will be a total of eight (8)similar contacts around the periphery of the contactor for additional heat transfer to the load board and theenvironment. The 1mm, 0.254mm (0.010”) wide ROL100 contact is fabricated from a beryllium-copperalloy (98%Cu, 2%Be) and has the following characteristics. Refer to Table 2 for the properties ofberyllium-copper alloy.Applying the formulas Section II, parts A through F and using the numerics listed above, one can formulatea list of values as shown in Table 4. With respect to convective heat transfer from the loadboard-to-7
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Page 5: R θ,insert-Ifs = R θ,insert + R
Page 9: IV.ConclusionsA thermal system and

Thermal Modeling and Analysis of Device-Contactor ... - Johnstech

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