1、- -_ Y EIA 251-A 70 = 3234600 0063528 5 H - - EIA STANDARD Test to Determine the Temperature Rise as a Function of Current in Printed Conductors a e; a cy m 8 - Y RS-251 =A (Revision of RS-251) Engineerng Department ELECTRONIC INDUSTRIES ASSQCiATiON EIA 251-A 70 m 3234600 00b3529 7 m NOTICE EIA engi
2、neering standards are designed to serve the public interest through eliminating mis- understandings between manufacturers and purchasers, facilitating interchangeability and improve- ment of products, and assisting the purchaser in selecting and obtaining with minimum delay the proper product for hi
3、s particular need. Existence of such standards shall not in any respect pre- clude any member or non-member of EIA from manufacturing or selling products not conforming to such standards, nor shall the existence of such standards preclude their voluntary use by those other than EIA members whether t
4、he standard is to be used either domestically or internationally. Recommended standards are adopted by EIA without regard to whether or not their adoption may involve patents on articles, materials, or processes. By such action, EM does not assume any liability to any patent owner, nor does it assum
5、e any obligation whatever to parties adopting the recommended standards. Published by ELECTRONIC INDUSTRIES ASSOCIATION Engineering Department 2001 Eye Street, N.W., Washington, D. C. 20006 Electronic Induatries Adsociation 1970 All fights reserved Price $1.00 Printed in U.S.A. -i EIA 251-A 70 = 323
6、4600 O063530 3 RS251-A Page 1 TEST TO DETERMINE THE TEMPERATURE RISE AS A FUNCTION OF CURRENT IN PRINTED CONDUCTORS (From EIA Standard RS-251 and Standards Proposal No. 1033, formulated under the cognizance of EIA Working Group P-7.6 on Printed Wiring Boards.) 1. SCOPE 1.1 This test method covers a
7、procedure for comparatively determining the effects of printed con- ductor materials, conductor cross sections, substrate materials, and processes on the temperature -D.C. current characteristics of printed wiring boards. A standard test sample is recommended and defined. . 2. GENERAL 2.1 The test s
8、ample shown in Figure 1 shall be vertically suspended by the upper comers and cen- trally located within an enclosure free of forced air movement. The sample shall be so oriented that the conductors are horizontal. Only one test specimen shall be tested at any one time, with current passing through
9、only one conductor. 2.2 The current leads for connection to the printed conductors shall be #12 AWG stranded wire and shall be secured by soldering to the terminal area. Pretinning of parts prior to soldering is advisable. 2.3 The potential leads shall be #26 AWG magnet wire and shall be secured to
10、the test tabs by soldering. Pretinning of parts prior to soldering is advisable. 2.4 The ambient air temperature measured at approximately 4 inches perpendicularly from the center of the conductor side of the board shall be 25C I 5C. -The temperature shall be recorded at each measurement. 2.5 Extran
11、eous surface coatings shall be removed without affecting the dimensions of the conductor. The sample shall represent the materials and processes under investigation. 2.6 To achieve 3-figure significance in temperature rise measurements, all potentials should be meas- ured accurately to four signific
12、ant figures. 2.7 Caution should be exercised in the choice of the current adjust “Resistor”. Resistors of low wattage rating will give erratic readings due to resistance fluctuations resulting from overheating. 2.8 In reporting results it is recommended that the temperature rise for each conductor m
13、aterial be given for a particular value of current fr a specified conductur cross-sectional area, approximate (b - - -. -_ EIA 251-A 70 rn 3234b00 00b3531 5 rn e- ,. - i- n RS-251 -A Page 2 - -1 Ir1 geometry and substrate material. Likewise, the results may be reported as a plot of temperature rises
14、 versus current for each of the conductor materials, cross sectional areas, approximate geometry, and substrat e mat erial. 3. TEST METHOD 3.1 The conductor current shall be passed through an external series resistor (R,) having a maximum temperature coefficient of resistance of 0.00002 per degree C
15、 at 25C. Though its accuracy is not as important as its insensitivity to heat, a commercial standard resistor of 0.1 ohm f 0.01% has been found to be satisfactory. The function of this resistor is to provide a reference for measuring the current. Its value should be of the same order of magnitude as
16、 that of the conductor under test to minimize scale changes in meas- urements. The test circuit is shown below- CURRENT ADJ. I STD. CELL LI I I ?. I F +-)-r / I l I- I I I POTENTIOMETER - 3. The voltage drops across the external series resistor and the test section of the current carrying conductor
17、(R) are to be measured with a laboratory type potentiometer having an accuracy of at least 0.02% (for example, Leeds and Norhrup type K3 or equivalent). Measurements at specific currents are to be made after thermal stabilization. The elapsed time between these two measurements should be as small as
18、 possible. 3.3 The resistance of the conductor at the reference temperature (Rtl) should be determined with a test current not to exceed 100 milliamperes applied for as short a time as possible. EIA 251-A 70 3234b00 O063532 7 Ra-25 7-A Page 3 3.4 The value of the temperature coefficient of resistanc
19、e (a) shall have been determined and identi- fied. Temperature coefficients for various electrical conductor materials are given in American Society for Testing Materials, B 193-65, and American National Standards Institute-C7.24-1966. e When the temperature coefficient of resistance of the conducto
20、r is unknown it may be determined by measuring the resistance of the conductor at different oven temperatures and calculated by the formula and adjusted to the desired ambient temperature by 1 at = 1 + (t - t,) 3.5 Determine the cross sectional area of the conductor under test by use of the formula
21、for volume resistivity. A L Pv = -R where pv = volume resistivity in ohm-circular mil/ft A = cross sectional area in circular mils L = gage length, used to determine R in feet R = measured resistance in ohms (R, I , see calculations) PVL A = - where pv = 1 1.529 ohm cir mil/ft 25C for ?h oz. copper
22、= 10.827 ohm cir mil/ft 25C for 1 oz. and over R L= 0.5 ft. (Fig. 1 -test pattern) A = Area in cir mils. A x 0.7854 = Area in square mils. 4. CALCULATIONS The temperature rise of a conductor is determined by measuring the change in resistance of the test length of conductor and using the relationshi
23、p Rt = Rt 1 Rt Rt at tl = Resistance of conductor at the desired current. = Resistance of conductor at reference temperature (ti). = Temperature coefficient of resistance of conductor at reference temperature (ti ). = Reference temperature; that ambient temperature at which Rtl was measured. EIA 251
24、-A ?O m 3234b00 OOb3533 9 m .O75 1 I l I RUN WIDTHS 4 14.4 b RS251-A Page 4 4 The following relationship can be derived from the foregoing using voltage drops: ERt Rs Rt = Est ER = Voltage drop across test conductor at the desired current. ER Voltage drop across test conductor at the reference curre
25、nt. = Voltage drop across the external series resistor at the desired current. = Voltage drop across the external series resistor at the reference current. = Resistance of external series resistor. = Resistance of test conductor at the reference temperature. Determined by measuring voltage drop with
26、 less than 1 O0 MA current passing through con- ductor and determined from I1 RS .O50 I I 1.2 FIGURE 1 -TEST PATTERN P . e EIA 251-A 70 = 3234600 0076047 3 = RELATED EIA STANDARDS In addition to this Standard, the following EIA standards are available on printed circuits: RS-161 Unit Standards for C
27、eramic Based Printed Circuits . $1.40 RS-162 Test Standard for Ceramic Based Printed Circuits .60 RS-208 Definition and Register, Printer Wiring SO RS-2 13 Test Point Locations for Printed Wiring Assemblies . SO RS-3 19-A Solderability of Printed Wiring Boards SO RS-216 Standard Method of Test for Adhesion of Printed Wiring 1.00 MINIMUM ORDER FOR ALL ENGINEERING PUBLICA TIONS AND STANDARDS $I. ou For a free and complete listing of Engineering Publications and Standards write: EIA Engineering Department 2001 Eye Street, N.W. Washington, D.C. 20006 o
