Environmental Hood for Testing Printed Circuit Cards

This invention relates to testing in general and more particularly to an environmental hood for use in probing printed circuit cards under test in a cold controlled environment.

In order to meet certain specifications, electronic equipment must be capable of operating to extremely cold temperatures, normally -55.degree. C. In order to carry out such tests it is necessary that various parts of the circuit be probed while in a cold environment. Previous attempts to carry out such testing have not met with complete success. In one method, a clear plastic hood is brought to the test console and placed around the printed circuit board. Cold nitrogen gas is then admitted to the inside of the clear plastic hood in order to bring the circuit board down to the required temperature. With the plastic hood, frosting occurred and it was impossible to observe the printed circuit under test. Furthermore, probing required lifting the hood and thereby allowing the printed circuit card to come above the required temperature.

Another method which has been used is a cold chest in which the printed circuit board has been placed. In such an arrangement, a cable must be brought from the printed circuit board to the test sets. Again, cold nitrogen has been used as the cooling medium. Probing with the cold chest required a long cable run, in addition to making it difficult to probe. The long cable runs in some cases completely destroyed circuit operation, i.e., some printed circuit boards are such that they will not operate through more than a few inches of cable. Thus, there have been serious difficulties in running such tests particularly where automatic, i.e., computer controlled, card test sets were used. Such testing does require probing under temperature extremes. It is thus evident that there is need for an improved system which permits carrying out such testing.

It is thus an object of the present invention to provide an environmental hood which permits probing printed circuit cards under test in a cold control environment without problems of frosting of the hood and also avoiding long cable runs.

This object is obtained in an environmental hood which can be disposed over a printed circuit card under test which includes an inner wall containing at least one probe hole through which a probe can be inserted, an outer wall surrounding the inner wall, with a spacing therefrom, at least one ring containing at least one opening in the wall thereof disposed between the inner and outer walls and bonded thereto, the ring surrounding the probe holes, means to supply a hot gas to the space between the inner and outer walls. With this arrangement the hot gas will exit through the hole in the ring, be mixed with the cold gas escaping from the probe hole and raise its temperature above the dew point so as to prevent frosting. In this manner an operator is able to continuously view the printed circuit card for probing purposes.

In addition, in the illustrated embodiment of the present invention an adapter plate is provided which receives thereon a printed circuit card connector at which point the electrical interface with the printed circuit card is established. The hood, which is closed on 5 sides is placed in abutting relationship with the adapter plate. The adapter plate contains a plurality of holes in communication with the space between the inner and outer walls. Baffles are disposed on the adapter to direct hot gas escaping through the holes toward the connector at the electrical interface. This prevents frosting of the electrical interface, which frosting could interfere with the circuit operation.

FIG. 1 is a perspective view of the environmental hood of the present invention.

FIG. 2 is a cross-sectional perspective view through a portion of the environmental hood in the area of one of the probe openings.

FIG. 3 is a cross-sectional view through the hood of FIG. 1 illustrating its manner of support on an adaptor plate.

FIG. 1 is a perspective view of the environmental hood of the present invention. As illustrated, it is closed on 5 sides, leaving one side open for placing over a printed circuit card. The hood includes an inner wall 11 and an outer wall 12. The inner wall contains a plurality of holes or openings 13 for the insertion of probes. Essentially concentric with each of the holes 13 is a mixing ring 15, spacing the inner wall 11 from the outer wall 12. The ring is bonded to both the inner and the outer walls. The inner wall, at least, and preferably both walls will be made of a clear plastic material. This permits viewing the printed circuit card under test through the environmental hood.

The rings 15 contain a plurality of holes or openings 17 therein, in the preferred embodiment four holes 17, spaced 90.degree. apart. The outer wall 12 contains opening 19 of a diameter equal to the inner diameter of the mixing ring 15 at the location of each mixing ring to permit access to the probe holes 13. A first inlet tube 21 is brought through the outer wall 12 and the inner wall 11 to permit admitting a cold gas to the space inside the inner wall. A second tube 23 is provided for bringing a hot gas into the space between the inner wall 11 and outer wall 12.

FIG. 2 is a cross-sectional perspective through a portion of the environmental hood in the area of one of the probe openings 13. Naturally, cold gas will exit through the probe hole 13. With nothing further done, this cold gas would cause frosting and interfere with viewing the printed circuit board for probing. However, the openings 17 in the mixing ring permit hot gas to exit into the space within the mixing ring 15 and to mix with the cold gas coming out of the hole 13. The hot gas indicated by arrows 25 mixes with the cold gas indicated by arrows 27 to raise it above the dew point and prevent frosting. Similarly, the hot gas in the space between the inner wall 11 and the outer wall 12 prevents frosting of these walls. The spaced inside the mixing ring 15 thus forms a mixing chamber 29.

A cross-sectional view of the hood of FIG. 1, disposed over a printed circuit board 31 and its manner of support on an adapter plate 29 is illustrated on FIG. 3. FIG. 3 also indicates the manner in which probing is carried out. As illustrated, openings 13, mixing rings 15 and openings 19 are provided on the two long sides of the environmental hood. The adapter plate 29 supports a connector 33 into which the printed circuit board 31 is plugged. Leads for a cable 35 are brought out from the connector. Again, if nothing further was done, since the leads become quite cold, frosting of the electrical leads could take place and interfere with operation of the circuit. To prevent this, the adapter plate 29 contains a plurality of openings 37 which are in communication with the space 39 between the inner wall 11 and outer wall 12. Thus, hot gas will exit through these openings 37. Disposed adjacent the openings 37 are baffles 41 which direct the hot gas to the electrical interface leads 35. The hot gas thus keeps the leads warm and prevents frosting at that point. As is best illustrated by FIG. 3 and as is also evident from FIG. 2, the openings 13 are of a conical shape with the large part of the cone toward the outside of the inner wall 11. As illustrated, this permits a probe 43 being inserted at various angles to contact any desired point on the printed circuit card. Since two rows of holes 13 are provided, one above the other, and since the holes 13 are provided on both sides of the printed circuit board, and also spaced along the length direction of the hood, access to any point on the printed circuit board is possible.

It is also apparent that the size of the rings 15 and the size of the openings 19, along with the angle of conical opening 13, is selected so as to permit access to all points on the board.

As indicated above, the tubes 21 and 23 are provided for supplying a cold gas and hot gas respectively. In both cases the gas supplied should be a dry gas. Also, in both cases it will be under pressure to insure escape through the openings 13 and 17. Dry nitrogen is suitable both for the cold and hot gas. However, any other dry hot or cold gas can also be used. Furthermore, although the mixing rings 15 are shown to be of circular shape and the holes 13 of a conical configuration, it will be recognized that the rings could equally well be square, rectangular, etc., and the openings 13 take the shape of a truncated pyramid for example. Thus, when the word ring is used it simply means a closed spacing member. Similarly, the openings when referred to as tapered toward the outside, can take any possible shape, not only the conical shape shown. Also, although the hood is shown if of a rectangular construction, it should be recognized that hoods of a bubble shape, i.e., with rounded surfaces are also possible. The only requirement is that the hood be closed on all but one side and capable of being placed on an adapter place.