PCBA Tech - About PCB process solder bead probe technology

Bead Probe Technology

In the processing of PCBA electronic products, because the density of the parts on the circuit board is getting denser, but the space is getting smaller and smaller, especially for the board of a mobile phone, the first thing that is sacrificed is the test point that does not have any function. Many bosses believe that: quality is manufactured, so as long as the quality of the circuit board assembly is good, there is no need for follow-up electrical testing. Up to now, BGA packages have been big enough for SMT and process engineers. Now a bunch of new IC packages (such as QFN) have appeared, and the entire communication module is built on a small circuit board. The finished product factory needs to put this The whole module circuit board is regarded as an SMT part and is soldered on the circuit board.

The traditional ICT test method uses a pointed probe to contact a circular test point to form a loop. This method requires a large area of test point, and then the probe must be shot to the target just like an archery. In the range of the target, it needs to use a lot of circuit board space; while the bead probe technology is just upside down. It hopes that the test point does not occupy the space of the circuit board as much as possible, but in order to contact the probe to form a loop, So printed solder paste to make the test point higher, and then use a larger diameter flat-head probe (50, 75, 100 mils) to increase the chance of contact with the test point, just like knocking an iron nail with a hammer.

In theory, this is really a breakthrough in the rebirth of test points, but there are still many technologies that need to be overcome in the real environment:

The solder paste printed on the PCB wiring easily affects the problem of poor contact between the probe and the test point due to the residual flux. In response to this problem, a number of probe manufacturers have designed probes for use with bead probe technology.

Solder paste printing process

The printing of solder paste must be very precise. In particular, the cohesion of lead-free solder paste is worse than that of tin-lead solder paste, and more precise solder paste printing is needed, because the high tin print volume will determine the height of the solder. If the solder height on the test point is not enough, ICT misjudgment The rate will increase. This involves the printing process of the solder paste, the precision of the steel plate, and the tolerances when the circuit board is assembled.

Circuit board

If the width of the wiring is too small, it is easy to be inadvertently inferred by probes or other external forces due to insufficient adhesion. It is generally recommended that the minimum wiring width should be more than 5mils. It is said that 4mils has been successfully tested, but as the width of the wiring is smaller, the false positive rate of its ICT is higher. It is recommended to increase the width of the wiring and cover it with green paint (mask) to make it stronger.

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Whether the use of bead probe technology will affect the quality of high-frequency.

Will there be capacitive effect or antenna effect when using bead probe technology? Neither the current test nor the customer's response has heard of any problems in this regard.

What is the reliability of the bead probe technology? There is no problem after testing 200 cycles.