Apparatus and method for testing printed circuit boards

Apparatus and method for electrical testing of a printed circuit (PC) board having a circuit layout wherein circuit paths interconnect selected plated through holes and components are mounted on the board by insertion of component leads through predetermined plated through holes in the board. The test apparatus includes a test board having a circuit layout identical to the circuit layout of the PC board. The test board does not have components mounted on it, but has instead test probes extending through predetermined plated through holes on the test board. The test probes are soldered to the circuit paths on the test board establising electrical connections between predetermined test probes via circuit paths on the test board. To test the PC board, test probes are brough into contact with components leads on the PC board and since electrical interconnections already exist between certain test probes, electrical interconnections between components on the PC board are established via the test probes and circuit paths on the test board allowing electrical testing of the PC board before the component leads are soldered to the PC board.

TECHNICAL FIELD 
This invention relates to automated manufacturing of printed circuit boards 
and more particularly to automated testing of such boards. 
BACKGROUND OF THE INVENTION 
Many complex electronic equipments are constructed of a number of discrete 
circuit modules in the form of printed circuit boards having circuit paths 
formed on each side to which may be connected various electrical 
components. Interconnections between circuit paths on opposite sides of 
the boards are accomplished by plated through holes. The plated through 
holes also function as receptacles for leads emanating from the electrical 
components. In the automated manufacturing of the boards, electrical 
components are inserted into the boards through the plated through holes 
by a component insertion machine. The component leads are then cut and 
cinched by the insertion machine, and then typically a visual check is 
made to ensure that component leads are present in predetermined holes. 
The boards then go through a soldering stage wherein the component leads 
are soldered to the board. Presently, only after the component leads are 
soldered, i.e. positive electrical connections are established between 
component leads and circuit paths, can electrical circuit testing be done 
to determine whether the right components have been inserted, whether the 
component is correctly oriented, whether the component is operable and 
whether the component meets the required circuit specifications. 
When a defect is detected during the electrical testing, the defective part 
or parts have to be unsoldered which may damage the board and other 
components which are located nearby. Such a repair operation is very labor 
intensive and requires a relatively high labor grade personnel to perform. 
Because of the difficulty of repair of the boards after soldering, it has 
been recognized for a long time that it would be advantageous to perform 
electrical circuit testing on the board before the components are soldered 
to the board. 
The problem, however, in performing electrical testing before component 
leads are soldered to the board is that there is no way to ensure that the 
components on the board are in fact electrically interconnected by 
predetermined circuit paths on the board. Consequently, a test result may 
indicate a defect with a component or circuit where in fact the problem 
may be caused by a poor or a complete absence of electrical connection 
between component leads and circuit paths on the board. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, we provide test apparatus and 
method for testing a printed circuit board before the component leads are 
soldered to the board. Apparatus includes a test board which has a circuit 
layout identical to the circuit layout of the board under test, but in 
place of components, it has a plurality of test probes which project from 
the major surface of the board through the plated through holes and are 
adapted to contact leads of components mounted on the board to be tested. 
The test probes are soldered to the test board establishing electrical 
connections between selected test probes by circuit paths on the test 
board whereby electrical interconnections between components on the board 
are ensured via the test probes and the circuit paths on the test board.

DETAILED DESCRIPTION 
Referring now to FIG. 1 there is shown a printed circuit (PC) board testing 
apparatus 10 comprising the present invention. The invention will be 
described with respect to a PC board 11 having an electrical circuit as 
shown in FIG. 3. Board 11, as illustrated in FIG. 1, is typically 
constructed of a plastic material which is coated on both sides with a 
thin copper film that is photo-etched to define various circuit paths. 
Circuit paths on one side may be connected to circuit paths on the 
opposite sides by means of plated through holes. The plated through holes 
also function as receptacles for leads emanating from various circuit 
components such as resistors, transistors, inductors, capacitors or 
intergrated circuits. To illustrate the principle of the invention, the PC 
board 11 as shown has plated through holes 12, 13, 14 and 15 and printed 
circuit paths 16 and 17 connecting holes 12 and 14 and 13 and 15, 
respectively. Two resistors 18 and 19 have their leads 21 and 22 and 23 
and 24 inserted through holes 12 and 13 and 14 and 15, respectively. In 
automated manufacturing of PC boards, a component insertion machine (not 
shown) is used to insert the component leads of the two resistors. After 
insertion, the machine cuts and cinches leads 21 through 24 on the side of 
the board opposite to the one upon which the components are located. The 
board 11 is then placed on a support 30 which is mounted in a test 
enclosure 31. 
Support 30 has guide pins 47 which mate with locating holes 48 on board 11 
to position the board 11 directly above a test board 40 which is mounted 
inside the enclosure 31. Coil springs 49 keep the board 11 a predetermined 
distance above support 30. The test board 40 has a circuit layout which is 
identical to the circuit layout of the board 11. The plated through holes 
12' through 15' on test board 40 correspond to plated through holes 12 
through 15 on the board 11. Circuit paths 16' and 17' correspond to 
circuit paths 16 and 17 on board 11. In place of component leads 21 
through 24, test probes 41 through 44 are positioned in the plated through 
holes 12' through 15', respectively, and are soldered to the circuit paths 
16' and 17' establishing electrical interconnections between test probes 
41 and 43 and 42 and 44. 
To perform the electrical testing of the board 11 before soldering, a 
flexible cover 45 made of a material impervious to air is placed over the 
board 11 and air is evacuated from the enclosure 31 such that a vacuum is 
created which holds the cover 45 against board 11 whereby the components 
18 and 19 are held in place during the test. As illustrated in FIG. 2, at 
the same time board 11 resting on springs 49 moves down until test probes 
41 through 44 contact component leads 21 through 24, respectively. Because 
test probes 41 and 43 and 42 and 44 are electrically interconnected via 
circuit paths 16' and 17', it does not matter whether or not component 
leads 21 through 24 are electrically interconnected via the circuit paths 
16 and 17 on board 11. A meter (not shown) connected across test probes 41 
and 42 will read the value of resistors 18 and 19 in a parallel 
arrangement.