REG NASA-LLIS-0828-2000 Lessons Learned Thermographic Mapping of PC Boards.pdf

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1、Best Practices Entry: Best Practice Info:a71 Committee Approval Date: 2000-04-17a71 Center Point of Contact: GRCa71 Submitted by: Wil HarkinsSubject: Thermographic Mapping of PC Boards Practice: Use thermographic mapping methods to locate hot spots on operating PC boards.Programs that Certify Usage:

2、 This practice has been used on Space Acceleration Measurement System (SAMS), Isothermal Dendritic Growth experiment (IDGE), and STDCECenter to Contact for Information: GRCImplementation Method: This Lesson Learned is based on Reliability Practice number PT-TE-1403, from NASA Technical Memorandum 43

3、22A, Reliability Preferred Practices for Design and Test.Benefit:Quick find of electronic components operating at or above recommended temperatures. Also, this technique can validate the derating factors and thermal design via low cost testing versus analysis.Implementation Method:Using an infrared

4、camera and the flight PC board, make thermographic pictures of the prototype PC boards in operation. Verify the thermograph and determine the delta T to the actual use environment Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-with thermocouples. Sh

5、ut down the equipment and prepare it for a vacuum test.The board to be tested is placed in a mother board with the appropriate +5 V and 12 V power supplies. Power is applied to the board, and after a short period, a video recording of the board is made with an infrared camera.Technical Rationale:The

6、 following procedure is used to determine the temperature of each component:Junction temperature:TJ= TA+ TJAWhere:a71 TJ= Junction Temperaturea71 TA= Ambient Temperaturea71 TJA= Junction to Ambient Temperature Risea71 TJ= 40oC for this exampleThe case temperature, TC, which is measured on the bench

7、at room ambient is given by Equation(2):TC= TJ- QCAP Where:a71 TC= case temperaturea71 QCA= case to ambient thermal resistancea71 P = Power dissipatedFor reliability purposes, it is necessary to keep junction temperatures for Provided by IHSNot for ResaleNo reproduction or networking permitted witho

8、ut license from IHS-,-,-CMOS devices at or below 49 degrees C. The case temperature to be measured on the bench comes out to be TC= 34 degrees C for this application.Infrared pictures are made of the PCB mounted outside the package on extended connectors while the equipment is operating on the bench

9、. The logic IC temperature is determined from the infrared picture. If less than or equal to 34 degrees C, the junctions are at the desired operatingtemperature. If greater than 34 degrees C, the reason for the higher temperature is determined.Corrective action is worked out and approved by the Engi

10、neering Review Board.Figure 1 is a drawing of the component layout of the SCSI card, and Figure 2 is a thermographic photograph of the board. Thermographic pictures are usually in color, but in this monochrome reproduction, the cross hairs are at the hottest location (128 degrees F), black represent

11、s 108 degrees F, white is 98 degrees F, dark grey is 88 degrees F, and light grey is 78 degrees F.Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-refer to D descriptionD Figure 1. SCSI Card Provided by IHSNot for ResaleNo reproduction or networking p

12、ermitted without license from IHS-,-,-refer to D descriptionD Figure 2. ThermographThe operating temperature with the board back in the case is checked by several thermocouples attached to the hottest observed components. This is done in a simulated use environment, perhaps during the thermal enviro

13、nment tests. The resultant delta-T is added to the measured case temperature as a final check of the junction temperature, TJ, in the end-use environment. For sample logic IC, the delta-T was 5 degrees C so the resultantjunction temperature is 45 degrees C in the package. This is below the guideline

14、 of 49 degrees C.References:1. Crall, R. F., “Thermal Imaging Benchtop Analysis for Reliability,“ Evaluation Engineering, December 1989.Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-2. Masi, C. G., “What Can Thermal Imaging Do for You?, “ Test & Me

15、asurement World, May 1988.3. MIL-HDBK-217E, “Reliability Prediction of Electronic Equipment,“ Rome Air Defense Center, October 27, 1986.4. Foster, W. M., “Thermal Test Report for the Space Acceleration Measurement System Circuit Boards“, NASA LeRC Code 6730 Internal Report, November 1987.Impact of N

16、on-Practice: Allowing undetected hot spots to exist in flight hardware can be very expensive since the later a problem is detected in a flight program, the more it costs to repair. Using thermography to verify system engineering models is a fast, low-cost technique.Related Practices: N/AAdditional Info: Approval Info: a71 Approval Date: 2000-04-17a71 Approval Name: Eric Raynora71 Approval Organization: QSa71 Approval Phone Number: 202-358-4738Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-