Design Guidelines for the Caddock MP725 Surface Mount Precision Power Resistor

Transcription

1 Design Guidelines for the Caddock MP725 Surface Mount Precision Power Resistor 1.0 General Discussion of SMT Power Technology 2.0 Thermal Properties and Power Dissipation for the Caddock MP Board Layout Footprint and Design Considerations 4.0 Pulse and Surge Energy 5.0 High Frequency Characteristics 6.0 Solder Reflow Assembly 7.0 Typical Solder Heating Profile 8.0 Appendix 1

2 While the pc board interface is 100 C, the element temperature of the MP725 dissipating 1.88 watts will be less than 115 C which is well below its 150 C recommended maximum continuous operating temperature. Better power dissipation can be achieved on printedcircuit boards with the use of heavier copper, larger pad areas, thermal vias, and heat sinks. Caddock MP725 Surface Mount Precision Power Resistors Caddock Electronics has extended their MP800 and MP900 family of heat sink mountable power resistors with the addition of a surface mount package in a style similar to the DPak and D 2 Pak. The MP725 resistor has a power rating of 25 Watts at 25 C case temperature and provides cool operation on FR-4 or G-10 printed-circuit boards, insulated metal substrates (IMS) and various ceramic substrates. Standard off-the-shelf resistor values are available from 0.020Ω ±5% to 1.00KΩ ±1%. 1.0 General Discussion of SMT Power Technology Until recently, SMT (surface mount technology) has been limited to lower power applications due to limitations in printed-circuit board technology, availability of power components, and heat sinking methods. Standard FR-4 and G-10 printed-circuit boards have a maximum longterm operating temperature of 125 C (100 C recommended) and are poor conductors of heat (typical thermal resistance ranging from 20 C/W to 100 C/W). Due to this high thermal resistance, the printed-circuit board is frequently the limiting factor in a SMT design. Two to three watts is usually an upper limit for power without special heat sinking. The following example illustrates the effect of the pc board as the limiting factor in power dissipation. Assuming a thermal resistance of 40 C/W for the printedcircuit board, 25 C ambient temperature, and 100 C maximum board temperature, it can be seen that power dissipation is limited to 1.88 watts by the board thermal characteristics, even for ideal SMT power devices with low thermal resistance. IMS (Insulated Metal Substrates), or Thermal Clad 1, is an alternative to standard glass-epoxy pc boards that offers considerable improvement in thermal transfer. IMS is a copper clad polymer/ceramic blend dielectric layer which can be laminated to various heat sink materials. The copper layer can be etched like printed-circuit board material. The most common heat sink materials used are steel, copper and aluminum. Using a 1.4 X 3.2 FR-4 pc board and standard 0.10 square inch pad size, the thermal resistance from the resistor element to ambient air ( Røja ) of the Caddock MP725 resistor is approximately 65 C/W. With an IMS aluminum board of the same size and natural convection cooling, Røja is reduced to approximately 25 C/W. This results in a power dissipation increase from 1.25 W on the FR-4 board to 3.7 W on the IMS aluminum board for the same 100 C attachment interface temperature. Since the IMS board can also operate at higher temperatures, the Caddock resistor can dissipate over 5 Watts before the maximum 150 C element temperature is approached. Additional increase in power can be achieved by further heat sinking of the metal core. 2.0 Thermal Properties and Power Dissipation for the Caddock MP725 Caddock rates all leaded heat sink mountable resistors in the same manner as power semiconductors for consistency and ease of design. A maximum case temperature is specified and thermal resistance values are provided to enable the customer to estimate appropriate temperatures under various operating conditions. Table I: MP725 Series Specifications Power at 25 C case Pd 25 watts Thermal Resistance (resistor film to case / tab) Røjc 5 C/W Element Temperature (max) Tj 150 C. equation 1: PD= (TS - TA) / Røsa PD = (100 C - 25 C) / 40 C/W = 1.88 Watts 1 Trademark of The Bergquist Company... see reference at end of this documentt 2

3 CADDOCK MP725 TJ RESISTOR FILM TEMPERATURE (MAX. 150 C) RØJC THERMAL RESISTANCE OF RESISTOR FILM TO HEAT SINK MOUNTING TAB TC CASE TEMPERATURE (MOUNTING TAB) RØCS THERMAL RESISTANCE MOUNTING TAB TO MOUNTING PAD (SOLDER) TS PAD TEMPERATURE RØSA THERMAL RESISTANCE PAD TO REFERENCE TEMPERATURE (IMS, CERAMIC, OR PC BOARD) P D TA AMBIENT OR REFERENCE TEMPERATURE A simplified model of heat transfer is the analogy to Ohm s law. In this model, temperature can be related to voltage; thermal resistance can be related to electrical resistance; and power is analogous to current. The following equation results: equation 2: TJ = PD (Røjc+Røcs+Røsa) + TA PC board manufacturers recommend the interface temperature (Ts ) should not exceed 100 C for standard FR4 or G10 printed-circuit boards. To determine the maximum power dissipation for an MP725 attached to an FR-4 printed-circuit board with a design maximum board interface temperature of 100 C at a 25 C ambient temperature, the following calculation would be applicable. Solder attachment and standard sized pads have been assumed. Røsa = 40 C/W typical ( see Board Layout section ) from equation 1: from equation 2: TC - TA = PD * Røsa C = PD * 40 C/W P D = 1.88 W TJ = 1.88 W ( 5 C/W C/W + 40 C/W) + 25 C = 110 C An interesting comparison is provided by an SMT X power resistor chip which is rated by its manufacturer as a 1 Watt device at 70 C ambient temperature. When this chip resistor is soldered to 2 X copper traces on an FR-4 pc board, while dissipating only 1 watt at 25 C ambient temperature, it has a solder pad temperature of 100 C and an element hot-spot temperature of 180 C. With 1 watt power dissipation, the Caddock MP725 resistor will have an element hot-spot element temperature of only 80 C, or 100 C less than the chip resistor. For preliminary design calculations, typical thermal resistance (Rth) and thermal resistivity (rth) for various materials is listed in Table II. As as guideline to calculate maximum power ratings and pertinent temperatures, thermal resistance versus heat sink pad area on singlesided FR-4 / G-10 printed-circuit board is provided in chart 1. As an example, a sample calculation is provided to determine maximum power dissipation of an MP725 resistor mounted to an IMS board by solder reflow with the back surface of the aluminum IMS board attached with thermal grease to a 50 C heat sink. To obtain a value for Røcs using Table II for reference, Røcs is equal to 0.1 C/W for solder plus 0.4 C/W for IMS dielectric plus 0.1 C/W for aluminum plus 0.3 C/W for thermal grease, or a total of 1.10 C/W. from equation 2: TJ = PD (Røjc+Røcs) + TA 150 C = PD ( 5 C/W C/W) + 50 C. PD= 100 C / 5.9 C/W = 16.9 Watt Caddock suggests actual measurements under worst case conditions to provide a basis for conservative ratings and optimal reliability. More power efficiency can be gained by the use of heavier copper, double sided boards, and thermal vias or by the use of IMS or ceramic substrates. 3

4 TABLE II Material (thickness) rth ( C in/w) Rth ( C/W) Solder (0.010 ) Epoxy (0.010 ) Filled Epoxy (0.010 ) Thermal Grease (0.003 )* Q-Pad II (0.006 )* SIL-PAD 400 (0.007 )* SIL-PAD 2000 (0.015 )* Thermal Clad dielectric (0.003 ) FR-4 / G-10 printed-circuit board (0.063 ) to 65 Aluminum (0.063 ) Copper (0.063 ) Steel (0.063 ) % Alumina (0.030 ) Aluminum Nitride (0.030 ) Berylia (0.030 ) *Rth varies with compression Thermal Resistance from Solder Pad to Ambient Air of 6" X 6" X.063" FR-4 Single-Sided Printed Circuit Board Versus Pad Area for 1 Oz & 2 Oz Copper oz. 2 oz Total Pad Area (sq. in.) Chart #1 4

5 Caddock MP725 Power Ratings at 25 C Ambient for Various Assemblies... Solder Reflow Attach on PC Board with Convection Cooling (except where otherwise noted) Table III Description P(W) Røca FR-4 pc board sq. in. pad size Same as above / Double sided... 2 oz. copper Wakefield 216 heat sink... convection Wakefield 216 heat sink LFM airflow 5 20 IMS on Aluminum (1.4 X 3.2 X ) IMS on Aluminum on 25 C heat sink reference Røca is thermal resistance from tab to ambient air or reference in C/W 1.2 W for 100 C board temperature 1.5 W for 100 C board temperature The relatively high thermal resistance of pc boards as shown on chart 1, and their low maximum operating temperatures illustrate the dominant effect which they have in determining the maximum power dissipation. For long term reliability, pc board manufactureres recommend a 100 C maximum operating temperature. Special high temperature pc board materials are available that will safely operate up to 135 C, but ceramics and IMS boards are the usual choices for higher power applications. 3.0 Board Layout Footprint and Design Considerations Figure 2 provides the nominal dimensions for heat sink pads and lead pads. These should be used as a minimum dimension and may vary with customer design practices. All power ratings are based on this standard pad size and some improvements can be obtained with larger pad dimensions. The major portion of heat transfer will be through the heat sink, but considerable heat can also be transfered through the leads. For this reason, larger lead attach pads and conductors will also help with heat dissipation. The use of SMOBC (solder mask on bare copper) may be helpful as a means of increasing pad size while maintaining proper alignment through surface tension of the solder area Figure 2 MP725 SMT Power Resistor Nominal Solderable Pad Dimensions 4.0 Pulse and Surge Energy In addition to design criteria for average power, the instantaneous energy delivered to the MP725 SMT resistor must be considered in applications where high energy and low duty cycle establish the average power or where any large energy surges may exist. Examples of this are snubber applications, PWM (pulse-widthmodulated) applications, and where inductive surges or capacitor discharges may be present. When using an MP725 resistor in a design with high pulse or surge energy, the heat sink should be specified for the worst case average power that the device will experience in normal or overload conditions. For PWM applications, the energy of an individual pulse can be determined by dividing the average power by the frequency. For other applications, the individual pulse energy must be determined and verified to be within the limits specified in Table II. For pulses of very short duration, the resistor element must dissipate a major portion of the pulse energy due to the thermal time constant of the resistor. As pulse duration increases, a greater portion of the thermal energy can be dissipated by the resistor ceramic, copper mounting pad, leads and the circuit board or heat sink. Chart 2 provides a guideline of the instantaneous energy which can be absorbed in an individual pulse by a MP725 resistor without additional heat sinking at an ambient temperature of 25 C. Above 0.30 seconds, a heat sink will begin to provide assistance in dissipating heat energy and should become a design consideration. The overload rating above 0.30 seconds is 1.5 times the rating in application for up to 5 seconds. For example, if the case (tab) temperature is 75 C at maximum specified ambient temperature, the continuous power rating for the MP725 can be determined from the derating curve on the data sheet. The continuous power rating is 60% of maximum, or 15 watts. Thus, the overload rating is 1.5 times 15 watts, or 22.5 watts for up to 5 seconds. 5

6 . 10 Single Event Pulse Energy at 25 C Case Temperature for Caddock MP725 Resistor Chart 2 Energy (Joules) µs 18µs 32µs 56µs 100µs 180µs 320µs 560µs 1ms 1.8ms 3.2ms 5.6ms 10ms 18ms 32ms 56ms 100ms180ms300ms Pulse Width Determine the average and maximum power dissipation of the resistor and the maximum ambient temperature. If the resistor is being used in a pulse application, or if surges are present, the following procedure can be used. A. Do not exceed a peak voltage of 400 volts. B. Estimate the single pulse energy for 25 C for the MP device for the pulse duration in your specific application. Energy formulas for typical waveforms are shown in chart 3. Chart 3 Voltage Waveform Across Resistor (R) V Energy E = V2 t R D. Using the Single Event Pulse Energy Chart ( Chart 2 ) compare the energy in your pulse to the rating on the chart and make certain that it falls below the maximum for the device you have selected for the specified pulse width. t E. Average power is an important consideration when multiple pulses occur. The average power dissipation can be calculated from the following formula: V E = C V2 2 t = RC equation 3: P = E f P = average power dissipation in Watts E = single pulse energy in Joules f = frequency ( pulses per second ) t V E = V 2 t 3 R t V E = V2 t 3 R t 6

7 Example 1: An MP Ω-1% is used as a snubber discharging a.1 µf capacitor from 300 volts at a frequency of 1 khz. The resistor is solder attached to a ceramic board which is capable of maintaining a tab temperature of 75 C. Equation4 w=2πf Z = R + J ( wl + w 3 R 2 LC 2 - R 2 C ) 1 + (wrc) (wrc) 2 1. The 300 volt peak is well below the 400 volt rating of the MP The energy stored in a capacitor is: E = 1/2 C V2 ( from waveform chart 3 ) = 0.5 X 0.1 E-6 X 3002 = 4.5 mj t = R C (from waveform chart 3) = 100Ω X.1 E-6 = 10 µs Per chart 2, the MP Ω-1% is rated 15 mj for a pulse width of 10 µs, so the 4.5 mj pulse is well below the maximum rating. 3. The average power is: P = E f ( from equation 3 ) = 4.5 E-3 X 1 E 3 = 4.5 watts With a 75 C tab temperature, using the derating curve it can be determined that the MP725 is capable of 60% X 25 watts, or 15 watts. Based on this information, the MP Ω-1% is a good choice for the application. 5.0 High Frequency Characteristics A general model of the MP725 resistor has a series inductance of 6 nh and a shunt capacitance of 1.18pF. If the heat sink tab is electrically common to one of the leads, the equivalent shunt capacitance increases to 2.25 pf. An MP Ω-1% resistor will have VSWR below 1.1 to frequencies in excess of 400 MHz and this frequency range can be extended with compensation. The low inductance makes this resistor an ideal choice for snubbers and high frequency sense applications. L C 6.0 Solder Reflow Assembly The most common method of attaching SMT components to board assemblies is solder reflow. This can be done by wave, vapor phase reflow, infrared reflow, or hot plate. In all methods, the temperature and time profiles must be carefully controlled. The following general mounting precautions should be considered in the assembly process to minimize thermal stress and to obtain reliable solder connections: Temperature profiles should be optimized to the board size and components used, solder paste properties, flux action, pad sizes and other factors. The preheat cycle should be optimized to assure uniform heating of components and gradual evaporation of solvents The temperature gradient from preheating to soldering should be gradual. Gradual and natural cooling of at least 3 minutes should occur after soldering. Do not apply mechanical stress or shock to the part while cooling. This may damage the part and/or create cold solder joints. Do not exceed a case or lead temperature of 220 C. Always preheat the device Surface Mount Technology Magazine (IHS Publishing Group ) has provided a helpful article which provides more in-depth study of SMT reflow phases and process dynamics by Charles L. Hutchins in their Step-by-Step Guide to Surface Mount Technology published in R The equation for impedance (Z) of the MP725 provides a good model for the resistor from dc to 500 MHz: 7

8 7.0 Typical Solder Heating Profile Each circuit board will have a particular temperature versus time profile that will provide the best assembly results. The profile will vary with the board size and type, components, solder type, and reflow system parameters. Motorola has provided a good starting point for a high mass assembly using 62Sn/36Pb/2Ag solder which has a melting point between 177 C and 189 C. It is important that the board and solder joints should heat first and the lead or body temperature of the Caddock resistor should not exceed 220 C. Many oven profiles exceed this temperature for a brief period of time. This is acceptable as long as the case and lead temperatures of the Caddock resistor do not exceed the 220 C maximum temperature. RECOMMENDED SOLDER INTERFACE TEMPERATURE VS. TIME PROFILE FOR SMT ASSEMBLY OF CADDOCK MP725 RESISTOR FOR 62Pb/36Sn/2Ag SOLDER 200 C STEP 1 PREHEAT "RAMP" STEP 2 VENT "SOAK" 150 C STEP 3 HEATING "RAMP" STEP 4 HEATING "SOAK" 160 C STEP 5 HEATING "SPIKE" 170 C STEP 6 VENT STEP 7 COOLING 205 TO 219 C AT PEAK 150 C 100 C SOLDER IS LIQUID FOR 40 TO 80 SECONDS (DEPENDING ON MASS OF ASSEMBLY) 50 C TIME (3 TO 7 MINUTES TOTAL) TMAX PROFILE BASED ON MOTOROLA RECOMMENDATIONS FOR DPAK AND D2PAK SMT ASSEMBLIES PUBLISHED IN MOTOROLA DISCRETE PRODUCTS FOR SURFACE MOUNT TECHNOLOGY, FALL Appendix (1) More information on IMS boards may be obtained by calling the following companies: Bergquist USA Bergquist Netherlands Denka Corp Thermagon, Inc (2) Information on SMT heat sinks is available from: EG&G Wakefield Aavid