Temperature testing apparatus for an electronic circuit substrate

United States Patent [19]

Gilley

[li] Patent Number: [45] Date of Patent:

4,515,484 May 7, 1985

[54] TEMPERATURE TESTING APPARATUS FOR AN ELECTRONIC CIRCUIT SUBSTRATE

[73] Assignee: AT&T Technologies, Inc., New York, N.Y.

[21] Appl. No.: 561,697

[22] Filed: Dec. 15,1983

[51] Int. C1.3 G01K 11/00

[52] U.S. Q 374/137; 374/102;

374/103; 136/230; 339/17 C; 339/254 R

[58] Field of Search 29/593, 574, 405, 705

236/DIG. 6; 40/11 R, 11 A, 16.6, 20 A, 26 374/102, 103, 158, 194, 137, 153; 267/151, 158 361/400; 339/17 C, 17 M, 17 LM, 254 R, 254 M; 136/230, 232, 233, 235

[56] References Cited

U.S. PATENT DOCUMENTS

2,282,441 5/1942 Whitlock 236/DIG. 6

3,001,173 9/1961 Swengel 339/254 R

Primary Examiner—Steven L. Stephan

Assistant Examiner—Thomas B. Will

Attorney, Agent, or Firm—W. G. Dosse; S. I. Rosen

ABSTRACT

To test the soldering temperature on the upper surface of an electronic circuit ceramic substrate (16) that is conveyed on a moving belt (12) over a plurality of heater platens (10), a thermocouple (20) is yieldably pressed to the upper surface of the substrate by a pressing mechanism (22). The pressing mechanism (22) comprises two flat springs (54 and 56) that press an insulator (52) down onto the thermocouple (b 20). The flat springs (54 and 56) are in turn pressed toward the upper surface of the substrate (16) by a coil spring (62) which pushes down onto a pressure plate (60). The pre-load of the coil spring (62) is manually adjusted by a nut (64) that has a groove (66) around which the wire (24) of the thermocouple (20) is wrapped for strain relief, as the wire extends to a digital readout device (26) that rides on the belt (12) along with the substrate (16).

7 Claims, 3 Drawing Figures

TEMPERATURE TESTING APPARATUS FOR AN ELECTRONIC CIRCUIT SUBSTRATE

GOVERNMENT CONTRACT

The Government has rights in this invention pursuant to Contract No. NOOO39-78-C-0OO6 awarded by the Department of the Navy.

TECHINCAL FIELD

This invention relates to apparatus for testing a timedependent temperature profile and more particularly to an apparatus including an electronic circuit substrate having a surface exposed to a source of heat and a test surface, the temperature profile of which is to be tested, and for holding a temperature sensor onto the test surface.

BACKGROUND OF THE INVENTION

In the manufacture of electronic circuits on ceramic substrates, circuit conductors and tantalum resistances and sometimes many other types of circuit components are formed on the surface of a ceramic substrate. Land areas at each end of a tantalum resistor or at each end of a conductor form the contact points on which separate electronic component leads are bonded to the circuit elements that have been formed on the surface of the substrate. These land areas are sometimes solder plated but are preferably gold plated. Solder and flux are then placed on the land areas. The substrates carrying the separate components that are as yet not solder-fused to the land areas are then heated to reflow or fuse the solder at the land areas and thus bond the separate components to the circuit elements formed on the substrate to form a complete electronic circuit pack. Substrate heating is accomplished on a hot belt in a linear reflow soldering system sold by the Browne Corporation of Santa Barbara, Calif, under the model designation LR-6.

In the linear reflow soldering machine, a thermo-conductive belt is continuously moved over a series of heated platens, which are heated to the desired range of temperatures. The substrates carrying the separate circuit components, that are as yet not soldered to the surface of the substrates, are placed on the belt and travel over successive platens to heat the upper surface of the ceramic to the various desired soldering temperatures, in a particular sequence. The heat transfer is from the underlying platen, through the belt and through the ceramic to the solder-coated connector surfaces of the tantalum circuit elements formed on the surface of the substrate and then to the leads of the separate circuit components. Too low a temperature applied to the ceramic substrate will result in improper soldering. Too high a temperature could damage the separate components being soldered to the surface of the substrate.

In order to test the temperature profile of the upper surface of a substrate carried on the belt of the Browne Reflow Soldering Machine, at the various locations along the path of the belt, a thermocouple is held against the upper or test surface of a ceramic substrate used as a test fixture. The leads of the thermocouple are connected to a digital display unit. The substrate, with the thermocouple attached, and the digital display unit are placed on the belt and move along the belt as would a normal ceramic circuit pack being reflow soldered. The digital display unit, which stands on heat-resistant legs to protect the unit from the heat of the belt, is then

read from time-to-time as the thermocouple-bearing substrate moves along the belt of the Browne Machine so that a series of time-dependent temperature readings can be obtained.

5 It is known to hold the thermocouple to the ceramic substrate with putty in order to attach the thermocouple to the top or tested surface of the ceramic substrate. However, this tends to give spurious readings due to the excessive insulating effect of the bonding material and,

10 when the bonding material becomes loose, give oppositely spurious readings due to the poor contact between the thermocouple and the substrate. The bonding materials tend to have difficulty accommodating the wide temperature ranges encountered during the re

15 peated heating and cooling cycles going from room temperature to the soldering temperature on the conveyer belt and then back to room temperature. Typically, putty would break up and fall off within a week.

The present invention relates to an apparatus for testing a time-dependent temperature profile on a test surface of an electronic circuit substrate having a heated

2^ surface. A temperature sensing means is held to the test surface by a flat spring oriented yieldably to press, with substantially one end thereof, the temperature sensing means against the test surface of the substrate, with a force proportional to the amount of flexure experienced

30 by the flat spring. The flat spring is insulated from the temperature sensing means, and the other end of the flat spring is pressed toward the substrate with a force that is manually adjustable.

Referring now to the accompanying drawings and more specifically to FIG. 1, several heater platens 10 are shown schematically. A belt 12 is arranged to move

50 continuously in the direction of an arrow 14 at a controllable velocity over the tops of the platens 10, by means of some appropriate drive mechanism (not shown). The belt 12 is made of a relatively conductive material so as to conduct heat from the platens 10 in

55 contact with its lower surface to the bottom of any object resting on its upper surface. An electronic circuit substrate 16, arranged for temperature testing as described below, is shown riding on top of the belt 12 with the bottom or heated surface thereof in direct contact

60 with the upper surface of the belt. A thermocouple 20is pressed against the upper or test surface of the substrate 16 by a pressing mechanism 22 which is described in greater detail below in conjunction with FIGS. 2 and 3. Two electrical wires 24, of the thermocouple 20, prefer

65 ably running together in parallel, are connected to a digital readout device 26, which gives to the operator a visual display of the temperature sensed by the thermocouple. To avoid exposing the digital readout device 26