Plastic and nonmetallic article shaping or treating: processes – With severing – removing material from preform mechanically,... – Removing surface portion of composite workpiece to expose...
Patent
1989-04-10
1991-04-09
Lowe, James
Plastic and nonmetallic article shaping or treating: processes
With severing, removing material from preform mechanically,...
Removing surface portion of composite workpiece to expose...
29407, 29412, 295271, 29559, 51165R, 264 401, 264156, 264160, 264261, 264263, 26427217, B29C 3912
Patent
active
050062956
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The present invention relates generally to quality control testing of printed circuit boards and especially to preparation of samples to facilitate testing of the quality of printed circuit boards with metallized holes. During hole plating of printed boards, levelness and thickness of the metal plating are most important.
Up to now, the only certain way to be able to make an acceptable estimation is to cut the holes through a center plane. Then, the metal thickness is measured using a microscope. Thus, it is very important that the measuring be made at a plane as near the center plane of a metal plated hole as possible. The error from the center plane to a radius which intersects the measuring plane must not exceed 5 degrees, i.e. at a hole diameter of 1 mm about 50-60 .mu. and at 0.3 mm about 10-320 .mu..
One of the disadvantages with this method is that it requires considerable amounts of plastic, of which the major part has to be ground away which is both time consuming and expensive. Testing according to this technique is mentioned in Danish patent No. 125 217.
A considerably better way of practising this principle is described in the Swedish patent application No. 8301136-1. Here, the embedded samples are milled to approximately the center of the holes to be tested. Thereafter, material is milled on both sides of the holes so that a narrow raised part is formed which then can be ground for investigation.
This means considerable savings both in time and material compared with the method of the Danish patent.
In principle, this method can be exact enough for holes with a diameter of about 1 mm or larger, but when the hole diameter is 0.3 mmm or smaller, the deviations are too large. Besides, IEC, IPC and US military rules stipulate that three holes in a row are to be tested. The tolerances when drilling printed boards may be around .+-.50 .mu.. It might be nearly impossible to get three random holes in a row with a linear deviation of .+-.10-20 .mu..
SUMMARY OF THE INVENTION
This invention proposes an acceptable solution of the above described problem.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will now be described in detail in connection with the attached drawings, wherein
FIG. 1 shows a printed circuit board having a row of metal plated holes, the quality of plating in which is to be investigated.
FIG. 2 shows the location of a test piece which is to be cut from the printed circuit board of FIG. 1.
FIG. 3 shows the placing of a plurality of the test pieces cut as in FIG. 2, into a fixture.
FIG. 4 shows the casting fixture for casting test blocks incorporating the test pieces.
FIG. 5 shows the final test blocks which are to be precision cut along a center plane which runs between the index grooves provided at opposite sides of the test block.
FIG. 6 shows an arrangement with a minimized grinding surface and with only one metalized hole circumscribed by the perimeter of the test piece.
FIG. 7 shows a test piece in which the cutting of holes, index grooves and contours are carried out by a laser. Here, the work-piece is made as a so-called break card.
DETAILED DESCRIPTION
FIG. 1 shows an arrangement in which a printed board 1 is placed on a support layer 4 and is held in place by a movable and rotating fixture 2 having guiding pins 3.
The printed board is here shown with a row of seven metal-plated holes. The invention proposes, for rather large holes, with a diameter of 1 mm and bigger, to use 7 holes. In such a case, the two end holes 5 of board 1. Are entered by guiding pins 3 on fixture 2 for fixing the location of the row of holes relative to the support layer 4.
If a cutting of, for instance, a test piece having a circular perimeter is made so that the perimeter intersects both of the end holes 7 at such a location that one half or less of the diametrical extent of each remains inside the test piece, these holes 7 can be used for further registration and positioning in instances where the holes have a diameter of 1 mm or more.
If a greater accuracy is nec
REFERENCES:
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patent: 3763611 (1973-10-01), Duhring et al.
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patent: 4592138 (1986-06-01), Hesselgren
patent: 4648211 (1987-03-01), Dawson et al.
patent: 4648212 (1987-03-01), Dawson et al.
patent: 4748773 (1988-06-01), Dawson et al.
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