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Sunken Joint
Sunken joints on the base of the board are most commonly due to outgassing from the printed board. They are seen as another process indicator like small voids in the solder fillets referred to as pin holes or blow holes. If the copper plating thickness remains a minimum of 25um in the barrel of the hole any moisture will not outgass through the copper during soldering. The following procedure for testing through holes will be useful.

NON DESTRUCTIVE EVALUATION OF PCB HOLES
The test is used to evaluate printed circuit boards with plated through holes for outgassing. It indicates the incidence of thin plating or voids present in through hole connections. It may be used at goods receipt, during production or on final assemblies to determine the cause of voids in solder fillets. Provided that care is taken during testing the boards may be used in production after test without any detriment to the visual appearance or the reliability of the final product.

TEST METHOD
A sample board or part of a board is selected for examination. Each of the holes for examination are filled with oil; provided the oil is optically clear both visual inspection and photography may be undertaken. The oil is injected into each hole using a hypodermic syringe which makes it easy to perform and limits excess oil from being applied to the board. For effective examination it is necessary for the oil to form a concave meniscus on the surface of the hole to be examined. The concave form allows an optical view of the complete plated through hole. The easy method of forming a concave meniscus on the surface and removing excess oil is to use blotting paper. In the case of any air entrapment being present in the hole further oil is applied until a clear view of the complete internal surface is obtained. The sample board is mounted over a light source; this allows illumination of the plating through the hole. A simple light box or illuminated bottom stage on a microscope may provide suitable lighting. A suitable optical viewing aid will be required to examine the hole during test. For general examination 5X magnification will allow viewing of bubble formation, for a more detailed examination of the through hole 25X magnification should be used. The next stage of the test method is to reflow the solder in the plated through holes which also locally heats the surrounding board area. The easiest method is to apply a fine tipped soldering iron to the pad area on the board or to a track connecting to the pad area. The tip temperature can be varied but 500oF is normally satisfactory. The hole should be examined simultaneously during application of the soldering iron. Seconds after the complete reflow of the tin lead plating in the through hole bubbles will be seen emanating from any thin or porous area in the through plating. Outgassing is seen as a constant stream of bubbles which indicates pin holes, cracks, voids or thin plating. Generally if outgassing is seen it will continue for a considerable time, in most cases it will continue until the heat source is removed. This may continue for 1-2 minutes; in these cases the heat may cause discoloration to the board material. Generally assessment can be made within 30 seconds of application of heat to the circuit. After testing the board may be cleaned in a suitable solvent to remove the oil used during the test procedure. The test allows fast and effective examination of the surface of the copper or tin/lead plating. The test may be used on through holes with non tin/lead surfaces, in the cases of other organic coatings any bubbling due to the coatings will cease within a few seconds. The test also provides the opportunity to record the results both on video or film for future discussion.

TEST EQUIPMENT
1. Sample Printed Circuit Boards For Evaluation

2. Microscope with top and underside lighting. Alternatively a suitable magnification aid of between 5 to 25x magnification and a light box.

3. Canada Bolson oil or suitable alternative which is optically clear for visual inspection and can be easily removed after test

4. Suitable hypodermic syringe for application of oil in each hole and blotting paper for removing excess oil.

5. Soldering iron with suitable temperature control.