PRINTED CIRCUIT BOARD ELECTRONIC TESTER

Abstract: An automated testing apparatus which performs shorts and continuity testing on a printed circuit board or ceramic substrate (2) submerged in a bath of ionic conducting solution. The apparatus comprises a central controlling device (12) which interfaces with an electrical measuring instrument (8), a controlled table (16) which positions a movable probe (4) over the board or substrate (2) under test so as to measure its electrical characteristics, and a recorder (14) which displays results.

Description: PRINTED CIRCUIT BOARD ELECTRONIC T STER Background Of The Invention 1. Field Of Tπe Invention The present invention relates to a shorts and continuity tester, and more specifically to one which utilizes an ionic solution to measure the electrical characteristics of various portions of a multilayered printed circuit board or ceramic substrate. 2. Description Of The Prior Art A multipoint "bed of nails" type fixture has been one way in which shorts and continuity checking has been performed on multi- *short-circuits Q MPI ■ . WIPO layered printed circuit boards and ceramic substrates. With such a structure each "nail" is connected to at least one other "nail" by way of an electrical measuring instrument such as an oh rπeter. As the "bed of nails" fixture is positioned over the circuit board to be checked, the resistance value is measured bet egi the two"nails" which come into contact with two points on the circuit board. The measured resistance value is then compared to the resistance value expected between those two points in the circuit. A difference between the two values is an indication of a malfunction which necessitates further investigation. One disadvantage of this type of fixture is that it requires contact with all points on the circuit board under test. Further¬ more, as the state of the art advances in the miniaturization of printed circuit boards, their conductor paths can be printed closer and closer together. As the conductor paths are fabricated closer together, it is physically more difficult to configure a "bed- of nail" type testing fixture that will operate reliably due to the close proximity required for adjacent "nail" probes. Accordingly, it is an object of the present invention to provide a relatively simple, lower cost and more reliable means in which to accomplish electrical measurements on printed circuit beards. It is a further object of the invention to eliminate the long turn around time required to design and construct prior art test fixtures . O...PI Y - v.ipo Brief Description Of The Drawinαs Various other objects, features and attendant advantages of the present invention will be more fully appreciated from the following detailed description when considered in connection with the accompanying drawings, in which: Figure 1 represents a block diagram of the present invention. Figure 2 is a top view of the ionic solution bath shown in Figure 1. Description Of The Preferred Embodiment To provide an insight to the basic principle of the testing technique a brief explanation of ionic conduction is in order. When two electrodes are placed in an ionic (conduction solution), an AC measurement device will display impedance (resistance and/or capacitative reactance) values. These values are related to the conductivity of the solution, the surface area and shape of the electrodes and the distance between the electrodes. If one electrode is mechanically fixed and the solution conductivity held consτant, while the other electrode moves then the measured impedance will increase as the distance between the electrodes increases. Referring now to the drawings, and more specifically to Figure 1, thereof, the instant invention utilizes the conduction properties of an ionic solution to perform shorts and continuity testing. The unit under test, element 2, is placed in the solution where it is examined by a single point probe 4. This probe in conjunction with reference electrode 6 and electrical measuring instrument S co- operate to measure the electrical properties of unit 2. When a ceramic substrate or multi-layered printed circuit board is the unit under test and is placed in an ionic solution, each connection point on the unit can be considered as a different electrode. Figure 2 illustrates a typical printed circuit 5 board having conductor paths 20, 22 and 24. When movable probe 4 contacts points PI or P2, P3 or P4, and P5 or P6, respectively, the points and/or conductors 20, 22 or 24 act as one of three different electrodes in conjunction with the reference electrode to form a unique conductivity cell. M-ovab e probe 4 is insulated from the lOionic solution except for a minute point at its tip which makes electrical contact with each respective electrode on the unit under test. If probe 4 were in contact with PI or P2 when electrical measuring instrument 8 is an AC ohm eter, the impedance measured I5by instrument S would be unique to PI, P2 and conductor 20. In an analogous manner, conductors 22, P3, P4 and 24, P5, P5 would each have a unique or a characteristic impedance between each respective eler.εnt through conducting solution 10 to reference electrode 6. 0 To simply state the basic testing principle, those points on a unit under test which are electrically connected will have the same characteristic resistance and capacitative reactance value. The instant invention as configured in Fig. 1 is a fully 25 automated testing apparatus. Ccnouter 12 is the central controlling device which is interfaced wit.n electrical xeasuring instrument 8, numerical control table 76. and strip printer 14. The basic OM Y IP - 5 - approach is for numerical control table 16 to move probe 4 by way of support means 18 over the unit under test and make impedance measurements utilizing an AC ohmmeter as electrical measuring instru- π__nt 8. These __easuraτιents are thei ssit to cocputer or centralcσntrol- 5 lingdevice 12 overahardware interface where they are processed. The objective istomake measuretnents on the unit under test 2, in order to determine which paths are connected, open or shcrted, thus performing a shorts and continuity test. In some situations, two totally separated (not electrically lOconnected) points on a unit under test will have characteristic impedance readings that are close in value. One way in which to eliminate any ambiguity is to use more than one reference electrode 6 so that more than one characteristic reading can be taken for each point on a- unit under test. By having reference dectrode 9 positioned 5 adjacent to reference electrode β, the distances between eitherelectrode and any conductor pattern on a printed circuit board would be unequal, thereby yielding two different characteristic impedance readings. There are two operational approaches to testing a product. In most cases, a product design and knowledge of expected continuities 0 would allow a computer programmer to direct the automatic tester to first record all characteristic impedances of a unit under test and then have computer 12 automatically compare all characteristic impedance values to check for un esirεd matches. Such matches would be shorts. In this mode, strip printer 14 would print either a PASS or a FAIL plus failure data. The second approach is to direct the computer to make character¬ istic impedance measurements and process the data to determine what points are electrically connected. In this mode, strip printer 14 lists all continuities on a unit under test. It should be noted that whatever mode of testing is used, the programmer must input in advance to the computer the x and y co-ordinates for each move of probe 4. When electrical measuring instrument 8 is configured as an AC ohmmeter, it consists of an AC signal source. The signal source could be a function generator which is stable in amplitude and frequency to four significant figures. It is used as a stimulus for the "characteristic impedance" measurements in the conducting solution. All testing is done in thermally insulated and temperature stable bath 10. The temperature stability is necessary because the solution conductivity changes approximately 2 percent per degree centigrade. One type of conducting solution is potassium phosphate monobasic sodium hydroxide, a buffer reagent with a pH of 7. Submerged in the solution is a non-metallic adjustable holder to accomodatε a wide range of units under test. It can also be moved in the horizontal plane for alignment with movable probe 4. Strip printer 14 is a high speed small printer used to display testing results. Recorded data strips can be torn off and attached to the unit under test as it travels down a production line. The numerically controlled table IS is a high speed (25απ/sec) xy table used to move an electrically insulated overthe unit under test. The table has a rravel of 25cπ_ x 25cm and is controlled by local electronics which interface to controlling computer 12. OMPI_ Yλ There are also local manual controls which allow an operator to manually move insulated probe 4 in order to make simple measuraiEnts , program position coordinates, or to reference the XY table to the product. 5 Insulated probe 4 is attached to numerical control table 16 by way of a light weight probe arm 18 which extends over the testing bath. Arm 18 allows for the positioning of insulated probe 4 by the computer controlled numerical control table 16. Probe 4 is a small tungsten needle of a diameter that tapers frcm 100.050cm to 0.013cm at the tip. The probe is completely insulated using a polymer such as polyamideimide, with the exception of a very minute area at the end of a 45° tip. The vertical motion of probe 4 is controlled by a small solenoid which is controlled by the computer. 5 A modification of the basic approach described above can be made to perform shorts and continuity testing by means of voltage rather than impedance measurements. In this mode electrical measuring instrument 8 would operate as a voltmeter so that characteristic voltage measurβnents can be taken by probe 4 0with respect to a reference potential. For most applications the reference will be ground, but it could be any desired value. An electric field is required to be imposed across the unit under test submerged in the ionic solution by means of either horizontal electrodes 28 and 29 or vertical electrodes 26 and 27. 5 T e utilization of horizontal and vertical electrodes is necessary to avoid the possibility of any two unconnected points on the unit under test having the same characteristic voltage. In this way, o_._?ι _? an independent voltage reading could be taken by movable prcbe 4 corresponding to each of the two electric fields established by electrodes 26, 27 or 28 and 29. One of electrodes 26 or 27 would act as a source electrode while the other would act as a sink electrode so as to impress an electric field between them. In an analogous manner one of electrodes 28 or 29 would act as a source electrode, while the other would act as a sink electrode. When using this approach those points on a unit under test which are electrically connected will have the same characteristic voltage value. The computer and strip printer can be programmed in a manner analogous to that when characteristic impedance measurements are taken so that the approach can be as automatic as possible.

Claims: CLAIHS 1. Apparatus for testing for faults on a print¬ ed circuit board (2) or a ceramic substrate charac¬ terized by. a) a bath composed of an ionic conducting solution, said board or substrate being submergeable within said solution; b) an electrically insulated movable probe (4) positioned within said bath, said probe having an electrically conductive tip c) at least one reference electrode (6) positioned within said bath; d) means (16,18) for moving said tip of said probe (6) into electrical contact with at least one point on said board (2) or sub- strate when submerged in said bath; e) means (8) electrically coupled to said movable probe (4) and reference electrode (6) for providing record data represent¬ ative of a measurement of an electrical characteristic; and f) means (12) responsive to the record data representative of the measured electrical characteristic fordetermining if there is an electrical fault on the submerged board or substrate. 2. Apparatus according to Claim 1, wherein said bath is thermally insulated to provide temper¬ ature stability. OMPI 3. An automated testing apparatus for perform¬ ing shorts and continuity tests on a printed circuit board (2) or ceramic substrate, c h a r a c t e r i z e d by a) a bath composed of an ionic conducting solution into which said board (2) or substrate is to be submerged; b) an electrically insulated movable probe (4) positioned within said bath so as to have its tip in electrical contact with a point on said board or substrate (2) , when submerged; c) one or more reference electrodes (6,9) positioned within said bath; d) an electrical measuring instrument (8) capable of measuring electrical character¬ istics, said instrument having a first lead connected to said movable probe (4) , and respective second leads connected to each one of said reference electrodes (6,9) e) a controlled table (16) ; f) a support means (18) connected between said controlled table (16) and said movable probe (4) ; g) a central controlling device (12) receiving values of electrical characteristics measured by said electrical measuring in¬ strument and outputting control information to said controlled table (16) ; and h) means for recording (14) data generated by said central controlling device (12) . OMPI 4. Apparatus according to Claim 3,. c h a r a c t e r i z e d in that said measuring instrument (8) is an A.C. meter capable of measuring the impedance between said movable probe (4) and each one of said one or more reference electrodes (6,9), and that said central controlling device (12) receives from said A.C. meter impedance values as the aforesaid electrical characteristics values. 5. Apparatus according to Claim 5 , c h a r a c t e r i z e d by: a source electrode (26) and a sink electrode (27) which are so positioned in said bath, that when said board (2) or substrate is submerged in said bath, said source and sink electrodes are respectively on one side, and the other thereof, said source and sink electrodes each having a different voltage so as to impose an electric field between them; as said measuring instrument (8) : a voltmeter capable of measuring the voltage between said movable probe and each one of said one or more reference electrodes (6,9); said central controlling device (12) receiving characteristic voltage values from said voltmeter as the aforesaid electrical characteristic values. 6. Apparatus according to Claim 5, c h a r a c t e r i z e d by further comprising a second source electrode (28) and a second sink electrode (29) which are positioned in said bath in spatial quadrature relative to.the first source and sink electrodes (26,27), so S also to be respect¬ ively on one, and the other side of the board (2) or substrate when submerged, said second source and sink electrodes each having a different voltage so as to impose an electric field between them. 7. Apparatus according to Claim 3, 4 or 5 wherein said bath is composed of a buffer reagent with a pH of 7. 8. Apparatus according to Claim 7 wherein said bath is further composed of potassium phosphate monobasic sodium hydroxide. 9. Apparatus according to Claim 3, 4 or 5 wherein the control table (16) is a numerical controllable. 10. Apparatus according to Claim 3, 4 or 5 wherein the means for recording (14) is a strip printer. 11. Apparatus according to Claim 3, 4 or 5 wherein the central controlling means (12) is a computer. Q?-r_ \VI? * 2 *

Inventors: GEISEL DONALD JOSEPH, GEISEL, DONALD, JOSEPH
Applicants: GEN ELECTRIC, GENERAL ELECTRIC COMPANY
Application year: 1981
Main classification: G01R31
IPC codes: G01R31