how to test conductive rubber on PCB pads

Today, with the increasing progress of PCB, some traditional printed board testing methods have been unable to meet the testing requirements, and many new testing methods have emerged as the times require, and conductive adhesive needle bed is one of them.

The molds (also called needle beds) for testing printed boards are generally divided into universal and dedicated types. The conductive rubber needle bed is composed of conductive glue on the basis of the special type.

1. Structure and principle

Conductive adhesive is a special conductive medium. It is characterized by good conductivity, but it can be bent and elastic, and has a certain degree of wear resistance. He is almost dedicated to testing densely arranged regular quad flat pack (QFP) surface pads (Figure 1), or QFP-shaped pads for short

Because this type of pad is very common in PCBs, it is also the most difficult type of pad to test.

As far as the current technical conditions are concerned, the minimum diameter of the test needle used in the special needle bed is about 0.7mm. Some factories can produce 0.65, 0.55, or even 0.4mm or smaller test pins. But these needles aren't just expensive. And it is very difficult to make. When it reaches about 0.4mm, it is almost impossible to use it in large quantities. This is especially true for the above-mentioned densely-arranged pads, which tend to be dense and narrow. It usually requires cross-selecting stitches to make. Now, with the increasingly high requirements of high-tech products, such as computer boards and mobile phone boards, it is more common to use test needles with a size of 0.7mm or even less than 0.5mm, and the production of special needle beds is not competent.

Therefore, the conductive needle bed is produced, and its structure is as follows

Image 2

Structure

As the picture show, it was made of dedicated probe bed, air cylinder, air tube, connected copper pillar, conductive rubber copper block etc. the probe bed work as the base, on the probe bed, no need to drill the test hole for “QFP” pad, but dig a big hole on the upper layer of QFP pad, the hole size was equal to QFP size, to place the conductive rubber copper block, then drill a smaller hole on next base board to connect the copper pillar. Under the probe bed, it connect one small cylinder. The other part of conductive rubber jig are same as normal dedicated probe bed, need install general test probe. You should be very careful, do not effect other test probe when making so many install hole for copper pillar.

The conductive rubber probe bed need the support of electric test machine. Such kind of electric test machine should have the special air-supply valve and test program for conductive rubber .’

2, test theory 

The conductive adhesive test is a method that utilizes two tests.

During the first test, the small cylinder does not supply air. When the needle bed is pressed down and tested, all the pads in the QFP pads are equal to no needles, and all the corresponding lines are equivalent to open circuits, but the other one of these lines or There are test pins in more than one pad, which can also be detected if there is a short between those lines. Therefore, this test measures the short circuit phenomenon of all pads in the QFP pad.

In the second test, the small air cylinder provides air pressure. When the needle bed is pressed down to test, the conductive copper block is pushed up, so that the conductive glue shorts all the pads in the QFP-shaped pad together, and also through the circuit. Test pins at one end to test all open circuits

3. Classification

The conductive glue needle bed can be divided into strip conductive glue needle bed and sheet conductive glue needle bed according to the shape of the electric glue

According to the conductive glue copper block mode, it can be divided into: split type and conjoined type

Image 3

Different types of conductive adhesives have their own advantages and disadvantages. For example, the split type test is more ideal, but the one-piece type can measure a smaller area of square pads. When making a needle bed, it should be judged according to the actual situation of the pads on the PCB. What type of conductive glue needle bed is best to use?

3. Fabrication of conductive glue needle bed

The conductive adhesive copper block is made according to the actual size of the pad on the PCB. The more complicated one is the copper block using strip conductive adhesive.

The shape and size of the conductive adhesive copper block is the size of the square-shaped pad, which can be square or rectangular. There is a screw hole in the middle, and along the frame, a slot equivalent to the diameter of the conductive adhesive strip is opened, and the conductive adhesive is embedded in the groove.

It should be noted that after the screws are installed, the nut should not be higher than the copper block. After the conductive adhesive copper block is installed on the needle bed, the conductive adhesive copper block should not be higher than the needle bed substrate, and the conductive adhesive copper block and the square on the PCB. The pads must be aligned accurately.

Sometimes the square pads are so small that the pads cannot be measured if there is a screw hole in the copper block. Then how to do? now we can use conjoined type conductive rubber copper block（like image 3）。This kind of copper block is relatively simple, it is a regular rectangular strip, but there is a screw hole on the bottom, a smooth surface on the top, and then a piece of "sheet conductive glue" is attached.

After the conductive glue copper block is made, some other parts (such as the connected copper pillar, etc.) are not special, as long as it is controlled to a certain size, and then connected to the cylinder, the needle bed is completed. Generally, after the test needles on the needle bed are made, the installation of the conductive glue copper block is finally carried out.

4. tips

1. Although the advantages of the conductive glue needle bed are obvious, there are also many defects and deficiencies. The following issues need attention:

A, independent pad

Shorts between individual pads and other pads cannot be detected.

B. There are two or more lines connected to the QFP-shaped pad（like image 5A）

3       There are two situations:

A, the whole line is in the "QFP" shape, in this case, the short circuit is all undetectable

B. There is a line connected to the outside of the "QFP". In this case, the short circuit can be tested, but the open circuit between the two pads in the "QFP" cannot be tested.

4        Two or more of the QFP-shaped pads are connected to the same inner-layer large copper surface through vias（like image 5b）

This is as same situation  like 5a <B>，But it is often ignored because the inner layer is in other layers

5，There are two places in the same unit of the PCB to be tested with conductive glue

This situation should be avoided as much as possible, because if there is a line that happens to be connected from this conductive glue test place to that conductive glue test place, then this line will not be tested.

When encountering the above problems, and can only test with conductive glue needle bed, we can only make another small needle bed to deal with these pads specially