Electrical circuit test probe

Abstract: A probe for providing electrical contact to electrical circuits during the testing thereof. There is a tubular housing (40) having an open end and a generally closed end with a main portion extending from the generally closed end towards the open end and a retaining and sliding bearing portion adjacent the open end having a diameter smaller than the main portion. A probe plunger (12) is disposed in the tubular housing and has a cylindrical inner portion (14) which is a slide fit and disposed within the main portion of the tubular housing and a cylindrical outer portion (18) of a diameter smaller than the inner portion and forming a shoulder (20) with the inner portion at a point of meeting. The outer portion of the plunger passes through the retaining and sliding bearing portion and has an outside diameter which is a slide fit to the inside diameter of the retaining and sliding bearing portion. A longitudinally compressible bias spring (26) is disposed between the generally closed end of the tubular housing and the inner portion of the probe plunger under a pre-loading for biasing the probe plunger towards an extended position.

Description: Background of the Invention : The present invention relates to probes used for testing electrical circuits and, more particularly, to a probe for providing electrical contact to electrical circuits during the testing thereof comprising, a tubular housing having an open end and a generally closed end having a small cleaning hole therethrough; a probe plunger disposed in the tubular housing, the probe plunger comprising a cylindrical inner portion being a slide fit within the tubular housing and a cylindrical outer portion of a diameter smaller than the inner portion and forming a shoulder with the inner portion at a point of meeting; a longitudinally compressed bias spring disposed between the closed end of the tubular housing and the inner portion of the probe plunger for urging the probe plunger to an extended position; and, a cylindrical retaining and sliding bearing region formed adjacent the open end of the tubular housing and having the outer portion of the plunger passing therethrough, the retaining and sliding bearing region having an internal diameter which is a slide fit to the outside diameter of the outer portion. Test probes are used in groups of hundreds or thousands for contacting a printed circuit board under test. Computer-based testers are used to supply signals and analyze outputs from the board under test. One hundred percent production testing of computer boards and other electronic equipment board is the typical industry standard. For such tests to be efficient and accurate, the probes employed must be very reliable. In prior art test probes, there are several points of probe failure. One is the failure of the probe's tip to pierce the flux or oxide films on the solder-coated pads of the circuit board. This problem typically applies to in-circuit testing after components are assembled to the board and the board itself has been wave soldered, or the like, to affix the components to the board. Another major point of probe failure is a result of internal wear of the probe itself, which generates contaminating particulates. These particulates, in turn, prevent good electrical contact between the probe plunger and the tubular housing. This wear can be caused by spring fatigue (which allows sharp broken coils to scrape particles from the tubular housing's internal surfaces) or as a result of probe side loading. Side loading occurs if probe points engage the sloped sides of solder lumps or bent component leads, or if single pointed probes enter misaligned holes in test boards. The probe generally indicated as 10 in Figure 1 represents an early design in the art. It was manufactured in strokes up to 4mm for 2.5 mm mounting centers. The probe plunger 12 comprises an inner portion 14 which has a diameter sized to be slide fit in the tubular housing 16. An outer portion 18 extends outward concentrically from the inner portion 14. The outer portion 18 is of a smaller diameter than the inner portion 14 thereby forming a shoulder 20 at their point of joining. The probe plunger 12 is retained in the tubular housing 16 by crimping the housing around the outer portion 18. As depicted in Figure 2, the tolerances must be kept quite close to prevent problems. If the tolerance between the outside diameter of the inner portion 14 is not very close with respect to the inner diameter of the tubular housing 16, the plunger 12 scrapes the sharp edge of the crimp, as indicated by the arrow 22, generating wear particles which rapidly degrade the performance of the probe 10. If the tolerance is too close, the plunger 12 can bind in the housing 16, causing probe failure. Likewise, excessive side loading can cause rubbing and wear regardless of the tolerances held during assembly. Also, as mentioned above and as indicated by the arrow 24, breaking of the outward bias spring 26 can cause sharp edges that produce undesired and damaging particles through scraping. Industry testing requirements developed in the late 1970's created a demand for longer stroke probes; that is, the distance the tip 28 carried by the end of the outer portion 18 can move inward towards the housing 16. In response to these requirements, the industry soon adopted the probe configuration of Figure 3 (as e.g. known from Jahrbuch der deutschen Gesellschaft für Chronometrie e.V., Vol.30, 1979, page 272, Figure 2), which resolved the wear problems of the prior art probes of Figures 1 and 2 while offering the required longer stoke. The probe 10′ of Figure 3 remains the most common configuration throughout the industry to date. Examples can be seen in U.S. Letters Patent No. 4,397,519 of Cooney and U.S. Letters Patent No. 4,659,987 of Coe et al., the latter patent being assigned to the common assignee of this application. While solving the wear problem and offering a longer stroke (relatively speaking as compared to the prior art), a continuing problem with the prior art probe design of Figure 3 is that the evolved standard industry dimensions for the equipment in which it is employed do not permit adequate space for a long life spring which will also provide enough force to reliably pierce board contaminants as described above. Long spring fatigue life becomes particularly important in universal test beds. Such beds use 100 test probes per 6,45 cm² (square inch) and frequently use 30 or 40 thousand probes per bed. In this style of tester, the same probes are used over and over again for all the types of boards tested. Probes in such a tester will often encounter 250,000 test cycles per year. Spring breakage is a serious concern using the probes 10′ of Figure 3. Another problem with the prior art probe 10′ of Figure 3 is that it is unstable during assembly by the automated apparatus employed for the purpose. Note that the probe plunger 12′ of this design has an inner portion 14′ and an outer portion 18′ of the same diameter separated by a middle portion 30 of reduced diameter. The plunger 12′ is retained within the housing 16 by a crimp 32 disposed along the length of the housing 16 at a point which will intercept the shoulder 34 formed by the junction of the inner portion 14′ and the middle portion 30. The plunger 12′ is guided longitudinally within the housing 16 by the combined effect of the outer portion 18′ and inner portion 14′ sliding within the tubular housing 16. As can be appreciated by those skilled in the art, however, during assembly the outer portion 18′ must fit within the housing 16 a sufficient distance following insertion at one station so as to remain stable during movement to the next station when the plunger can be depressed to pre-load the spring 26 and the crimp 32 made in the housing 16. This forces the spring 26 to be shorter than would be desirable. In order to get the required piercing force at the tip, the spring 26 must, therefore, be quite stiff. This results in a higher propensity to breakage of the spring 26. The prior art probe 10′ of Figure 3 typically provides full stroke spring life below 100,000 cycles. As can be appreciated, in apparatus such as the test bed described above performing 250,000 cycles per year, this means that each of the 30 or 40 thousand probes in the bed only has a spring life expectancy of 4.8 months -- following which, the formation of particles through scraping and deterioration of probe performance begins. Document EP-A-0257832 discloses a telescoping pin probe including a tubular member one end of which houses a fixed pin, said tubular member being reduced in diameter in order to fix said pin. Wherefore, it is an object of the present invention to provide a test probe for use in electrical component test beds, and the like, which has a design which resists scraping action between the components which can produce probe-deteriorating particles. It is another object of the present invention to provide a test probe for use in electrical component test beds, and the like, which has a design which provides increased shut height for a given spring length. It is still another object of the present invention to provide a test probe for use in electrical component test beds, and the like, which provides increased spring life expectancy without breaking. Other objects and benefits of the present invention will become apparent from the description which follows hereinafter when taken in conjunction with the drawing figures which accompany it. Summary : A probe according to the invention is defined in Claim 1. A method of assembling such a probe is defined in Claim 3. Description of the Drawings : Figure 1 is a cutaway elevation drawing of a prior art test probe in a first embodiment. Figure 2 is a drawing of the probe of Figure 1 illustrating some of the problems associated therewith. Figure 3 is a cutaway elevation drawing of a prior art test probe in a second embodiment. Figure 4 is a cutaway elevation drawing of a test probe according to the present invention. Figure 5 is a partial, cutaway drawing of the probe of Figure 4 showing the probe plunger being inserted into the tubular housing during assembly to pre-load the spring. Figure 6 is a partial, cutaway drawing of the probe of the present invention following the insertion step of Figure 5 and showing the probe plunger inserted into the tubular housing with the spring pre-loaded and prior to forming the retaining end thereon. Figure 7 is a partial, cutaway drawing of the probe of the present invention during assembly following the position of Figure 6 showing the manner in which the probe plunger is held captive in the tubular housing with the spring pre-loaded by the forming of a retaining and sliding bearing region on the end of the tubular housing. Description of the Preferred Embodiment : The probe of the present invention is shown in Figures 4-7 wherein it is generally indicated as 10″. As those skilled in the art will readily recognize and appreciate, probe 10″ is a modification of the early prior art probe 10 described herein previously with respect to Figures 1 and 2. Like the prior art probes which it replaces, the components are all of electrically conductive materials. It should be pointed out immediately, however, that the modifications to the prior art probe of Figures 1 and 2 that result in the present invention also produced unexpected benefits which were not apparent or obvious. For example, while the prior art probe 10 suffered from a snort stroke and scraping of the crimp, the probe 10″ of the present invention has increased the stroke length within the industry standard shut height from 0,407 cm (0.16 inch) to 0,636 cm (0.25 inch) which was the stroke length increase achieved by the changeover to the probe 10′ of Figure 3. Moreover, in tested embodiments, the springs within the probe 10" of the present invention achieved a minimum cycle life of 3.6 million (3,600,000) deflections at full stroke. This life is stated as "minimum" because, as of this time, no springs have been broken in one hundred test probes tested to that level. A separate test of sixty probes also manufactured in accordance with the present invention resulted in probes tested having a resistance below 30 milliohms with 14 milliohms being a typical resistance. No known commercial probes according to the prior art are known to achieve such characteristics. Because of the unexpected achievements in performance produced by the present invention, it is also worthy of note that the probe can be produced in a miniaturized version which is equally reliable; yet, capable of mounting on 1,27 mm spacing. The benefits relative to testing of electronic circuit boards which also are becoming increasingly miniaturized are readily apparent. The reason for this increase in performance and reliability as achieved by the present invention is, of course, a product of the change in the spring 26 made possible by the design of the probe 10". This can be seen with reference to the following table which compares various prior art probes to the probe 10″ of the present invention: The modifications to the prior art probe 10 of Figures 1 and 2 made to achieve the objectives and non-obvious benefits of the present invention can be seen by comparing the drawings of Figures 1 and 2 to the drawings of Figures 4-7. The tubular housing 16 is of extended length to hold the extended length spring 26 that is possible with the configuration and method of assembly. As compared to the prior art design of Figure 3, this invention provides 50% more shut height for the spring. The probe plunger 12 is of substantially the same design as the plunger 12 of the prior art probe 10; however, the outer portion 18 is longer and the inner end 36 of the inner portion 14 is angled as shown to create a sideward force that provides improved electrical contact from the housing 16 through to the plunger 12 in a manner known in the art. During assembly, as depicted in Figure 5, the spring 26 is first inserted followed by the inner portion 14. Contrary to the design of probe 10′ of Figure 3, the plunger 12 is stable with a relatively short engagement. Thus, the spring in this design can not only be longer; but, can be pre-loaded much more than the prior art designs. This additional pre-loading provides more uniform plunger force in the operating range and, additionally, the longer spring fatigue life that provides the above-reported vast increase in cycle life without spring breakage. With the plunger 12 inserted as shown in Figure 5, it is depressed as shown in Figure 6 to pre-load the spring 26 and position the shoulder 20 away from the open end 38 of the housing 16. As depicted in Figure 7, the area of the housing 16 between the shoulder 20 and the open end 38 is then closed by swaging, or the like, to form a reduced diameter retaining and sliding bearing region 40. As can be appreciated, because of the increased spring strength possible in the greater space afforded, in combination with the close clearance elongate sliding bearing of region 40, this design offers extended low electrical resistance life as well as excellent tolerance to side loading forces. Since the area of contact is an extended one with no exposed corners or edges of the bearing surface of region 40 or the tubular housing 16 in contact with the probe plunger 12 (as compared to the prior art probe 10) there is no potential for the edge scraping produced by that design. Thus, it can be seen that the probe of the present invention has met its stated objectives by providing a probe that has increased spring length, increased spring loading, increased spring life, and no potential for particle-producing scraping action between any of the components, even in the presence of side loading forces, all with the same stroke length and within the same shut height space as the prior art probes with which it is interchangeably replaceable.

Claims: A probe assembly for providing electrical contact to electrical circuits during the testing thereof of the kind having a probe plunger (12) disposed in a tubular housing (16) and urged outward through an open end (38) of said tubular housing under the force of a preloaded compression spring (26) contained within said tubular housing, said probe plunger having a cylindrical inner portion (14) which is a slide fit and provides electrical contact with said tubular housing, and a cylindrical outer portion (18) of a diameter smaller than said inner portion and forming a shoulder (20) with said inner portion at the junction of the portions, said tubular housing (16) being of a deformable, electrically conducting material and the probe being assembled by inserting the inner portion (14) of the probe plunger (12) into the open end of said tubular housing, said open end being subsequently deformed to retain the shoulder within said housing, characterised in that part of the length of said tubular housing from its open end (38) is compressed around said outer portion (18) of the plunger to produce an elongate cylindrical retaining and sliding bearing region (40) which is a close clearance slide fit with said outer portion (18), thereby maximising the length of said compression spring and hence its ability to preload said probe plunger, and in that the inner end (36) of said inner portion of the plunger probe is formed with an angled face, whereby pressure exerted on said face by said compression spring urges said inner end into contact with said tubular housing to ensure good electrical contact. A probe according to Claim 1, characterised in that compression of said open end (38) of the tubular housing (16) is effected by swaging, or rolling. A method of assembling a probe assembly for providing electrical contact to electrical circuits during the testing thereof of the kind having a probe plunger (12) disposed in a tubular housing (16) and urged outward through an open end (38) of said tubular housing under the force of a preloaded compression spring (26) contained within said tubular housing, comprising forming, from a deformable, electrically conducting material, the tubular housing (16) with an open end (38), forming the probe plunger (12) with a cylindrical inner portion (14) to be a slide fit within said tubular housing and a cylindrical outer portion (18) of a diameter smaller than the inner portion to provide a shoulder (20) with said inner portion at the junction of the portions, inserting the compression spring (26) and said inner portion (14) of the probe plunger (12) through said open end (38) into said tubular housing (16), and deforming said open end (38) to retain the shoulder within said housing, characterised by compressing part of the length of said tubular housing from said open end (38) around said outer portion (18) of the plunger after said probe plunger has been inserted into said tubular housing to produce an elongate retaining and sliding bearing region (40) which is a close clearance slide fit with said outer portion, thereby maximising the length of said compression spring and hence its ability to preload said probe plunger, the inner end (36) of said inner portion of the probe plunger (12) having been formed, prior to the insertion into said tubular housing (16), so as to present an angled face. A method according to Claim 3, characterised in that said elongate retaining and sliding bearing region is effected by a swaging, or rolling operation.

Inventors: COE THOMAS D, COE, THOMAS D.
Applicants: QA TECHNOLOGY CO INC, QA TECHNOLOGY COMPANY INC.
Application year: 1989
Main classification: G01R1
IPC codes: G01R1