Common PCB assembly defects, How to prevent them ?

The need to chase the market very often forces the manufacturers of electronic systems to neglect some fundamental aspects of the quality of supply, paying attention  mostly  to the low price. Unfortunately, this type of approach has hidden costs which  will  be  revealed   to be significant in the mass production phase.

Furthermore, the rapid advancement of technology also makes it difficult to keep up with the times, so very often a supplier does not have time to standardize a process that already becomes obsolete, forcing it to new investments or to succumb to more financially gifted competitors.

In fact, if we look at the PCB assembly technique it has gone from the PTH of the ‘80s where a wave soldering machine and a bit of space in the garage  was  enough to be able to present itself as an assembler to the latest SMT technology of the 2000s and today it is increasingly difficult to be able to experiment with assembly techniques without having sophisticated assembly machinery and in-line analysis systems of the production process.

Based on our production experience, we can  affirm  that many of the problems related to assembly defects depend on two  essential reasons:

  1. a) correct design of the printed circuit
  2. b) the efficiency of the production process
Item Cause Percentage
1 Bad welding 45%
2 Missing components 10%
3 Wrong components 15%
4 Broken components 15%
5 Defective printed circuit 5%

1)Bad welding

First of all, the PCB designer must use standard foot prints suitable for the component that has been chosen to use and providing the right opening masks in the solder and avoiding  the  placing   of tiny components in the shadow of the larger ones.

Avoiding holes in the solder pads is an essential condition to ensure successful soldering

Provide a  Burn-Inphase for printed circuits and components from long storage in the warehouse

Use alloy solder pastes  according  to  required  surface finishing process for which the PCB was thought ,

Use  alloy solder pastes recently produced and / or in any case well preserved in the  warehouse.

Use printed circuit boards produced by first class suppliers, free from warping, variations in substrate

thickness and / or with defects in process contamination.

Keep work areas clean,  using  of  Clean Area  is strongly  recommended

Keep under control the temperature and  welting  times in the welding process

Use the correct thicknesses of stencil sheets when applying the solder pastes.

2) Missing components

The more automated the process, the less this type of assembly error occurs

Usually the modern Pick & Place machines are precise and reliable and are able to inform in time when the carousel   has emptied or if a component is blown away, usually it is human  errors that affect the final yield of a production process, even an inadequate fastening of the PCB    conveyor  plays an important role in the production yield if the fingers do not fix the plate well it can vibrate during transport and go into oscillation losing  blows during positioning.

If a component  feeder  is not well fixed in its housing ,machine fails to pick components accurately.

In any case, having an automatic optical inspection  AOI  system is the best remedy to identify problems of this type.

Mobile probe circuit testing techniques are very useful in identifying these types of defects.

3) Wrong components

The prevention of this type of errors passes through the establishment of an adequate and well-studied quality system which will have to provide simple and effective controls over of the whole  production process phases ,where everyone can check the correctness of the previous phases and interact with  each other   in order to  correct the error detected in the document. The control procedures must be simple and upgradable  with daily experience rather than entrusted to an expert external consultant who does not live the reality of the factory.

Obviously in the case of manual assembly, any type of error among those listed above can occur randomly or systematically, visual inspection and functional testing are essential in this case.

4) Broken components

Rarely can a top quality electronic component come out of its original packaging broken. Nowadays, the major electronics brands are able to produce with zero defects at the end of the construction process. Instead, we must be  aware  of imitation attempts and beware of the risks of buying non-genuine parts that also exist in circulation and come from the refurbishing market because in this case there is no guarantee of operation.

Typically, during the assembly process, an electronic component is destroyed due to mechanical damage or electrostatic discharges occurring during handling operations.

Here the establishment of protected areas against ESD risks is essential  although  it is  not enough.

In fact, most of the damage caused by ESD occurs outside the protected areas, as it is impossible to imagine a world made entirely of  “fresh cheese”, when the product ends up, for example, in the reworking phase or in the hands of the warehouse worker or final transporter.

The separation of PCBs from the multiple panel also represents a very frequent cause of component breakage, it would be better  to use a special equipment for the final cut of the PCBs rather than relying on

the manual snap of the board or using cutters and shears that end up breaking the electronic components arranged on the border of the pcb.

5) Defective   PCB

Here, too, once  again, the main recommendation for preventing assembly defects is to use first-class suppliers. Modern electronics does not admit partial results ,those  manufacturers  who are  not adequately equipped to meet the new  frontiers  of this  market  would  do  better not to apply in this fascinating sector, the craftsmanship  of electronics now concerns the past years.

Today’s  PCB manufacturer must guarantee inside equipment such as Flying Probe ,IR test, Xray Microscope, Thermography Camera ,Visual System Inspector ,   AOI   and  so on .

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