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How to solve the problem of PCB missing parts

Print circuit board assembly, on in short - PCBA, refers to the whole process of blank
PCB board assembled using SMT or DIP types of components.
 PCBA is an essential part of all the worldwide electronic products
components, as most of the electronic devices used in our lives, are inseparable from

the use and applications of PCB assembled boards.

The PCBA process requires a vast amount of resources and machinery. If there wasn’t
sophisticated printing machines, SPI, placement machine, AOI equipment and
systematic, standardized process management way of doing things, the result will be a
mass disruption in the process of PCBA production and processing.

In this article, we will discuss why PCBA parts go missing by sharing a

case study and the solution we came up with to resolve the issue we faced. By
the end of this article, you’ll be able to find the cause of missing PCBA parts and figure

out a solution quickly and easily.

Check the machine for PCBA components.

What is the main reason for PCBA missing parts

When we say “parts go missing” we refer to a situation where a product in the
production process will have some productions mistakes, resulting in the customer
receiving the PCBA with missing welded parts.
Let’s start by understanding the main factors that lead to PCBA parts go missing:

1. The problem of PCB incoming material deformation

pcb testpcba test

If the deformation occurs when the SMD component is mounted using the placement
machine, the result will be uneven and flat PCB. The component will not stick to the
solder paste during the concave surface attachment process (The accuracy of the
placement machine is ±0.01mm, very precise if placed and used correctly.)

Preventive measures:

Select high-quality PCB suppliers;
Incoming materials should be strictly monitored and tested according to IQC
incoming inspection standards;
Follow the PCB handling and storage guidance.

2. Solder paste printing problem

Pcb Solder paste printing problem     Solder paste printing problem pcb

In the process of applying solder paste to PCB pads, in a situation where the stencil

plugs holes are too small, or the amount of solder paste is insufficient, the board will end
up with no solder paste applied on the PCB pads or less solder paste than required for
healthy PCBA board.

Preventive measures:

  •  (1) Configure the stencil auto-cleaning settings to clean once every 2 assembled pieces.
  •  (2) Manually check the amount of solder paste every 20 assembled boards.
  •  (3) Regularly check the SPI solder paste inspection machine, in case of bad printing quality, it will not affect the next batch.

3. The components braid charging holes are too large

AOI machine for pcb test

The components might be turned over due to vibration, in such case when the braid is
placed, the direction of the placed material will end up reversed.

Preventative measures:

  • (1) Maintenance of Feeder, ensure proper work at all times;
  • (2) Configure AOI device check before use;

4. Suction nozzle maintenance

Suction nozzle maintenance pcb
Ensure the air path of the component nozzle isn’t blocked, the nozzle isn’t damaged, the
nozzle height is placed correctly and that there is no visible stains or cracks in the
nozzle port, which might result in gas leakage.

Preventative measures

Ensure the Suction nozzle is regularly maintained.

5. Faulty equipment vacuum circuit

In some cases the vacuum circuit of the equipment is faulty, and the blockage causes
parts to go missing.

Preventive measures:

maintenance of the equipment every day and every week

6. SMT engineer error of patch program

SMT patch program

pcb test

The SMT engineer made mistakes during the patch program configuration stage or by
selecting wrong component thickness parameters during the programming stage. This
will result in the X-Y axis coordinates of the components to be incorrect which will lead

to a faulty batch of PCB assembled components.

Preventive measures:

Training of SMT engineers.
Purchase or develop intelligent programming software.
SMT first piece inspection machine inspection.

7. SMT hand-pasted materials related problems

SMT hand-pasted materials related problems

During the SMT process, some human-made mistakes can occur by the SMT assembly line
workers that paste the materials by hand. 

This usually affects small batches in the premass production stage where samples of products are being made.

Preventive measures:

Minimize personnel contact with PCBA during the production process before reflow
soldering taking place.
Make a standardized operation process when clearing and attaching bulk materials,
and focus on special labeling and quality control by quality trained staff.

8. Process loopholes and human errors in PCBA production

MES system for pcba process

Preventive measures:

Optimize the fool-proof function of the MES system,
Strengthen the management of training personnel execution.

9. Damaged parts due to packaging and transportation

10. Cases of PCB missing parts

At this stage, we learned a lot about possible mistakes that might occur during the mass
production process. Let's analyze real scenarios of PCBasic PCBA customers' missing
parts: studies of factors that cause PCB missing parts.

The customer found few U12 and U16 parts went missing during the “line test”

PCB missing parts

At first, we had to take quick action which was to repair all the products with missing
components. This is a temporary but quick fix that will give us enough time to look into
the real cause of such a problem.

Root cause analysis 

The problem was not carefully addressed when dealing with the customer's
demands, which led to a misunderstanding with the production staff in charge of the
BOM information remarks. Eventually, leading to the components not being included
in the final BOM.

Engineers and IPQC erased materials according to the BOM remarks during
production, and no questions are raised to validate whenever there is a problem in
the BOM or this is the customer's demands. Finally, resulting in fewer assembled

Long-term corrective action and implementation

We learned from our mistakes and here is the steps we took in order to correct the
mistakes and improve the quality of our Assembly line:
it’s now required that when processing customer demands, the information first
needs to be confirmed with the superior staff before being entered into PCBasic’s
production system. The description should be clear and easy to understand, and no
questions should be left unanswered.

The serial numbers of the components will be merged into the system only after the
project manager confirms if there are alternative materials during the BOM
reviewing process.
In case of abnormality during the production process, once found out that the
remark information is not posted but there are incoming materials, the technician
and IPQC need to provide feedback. Only then, The production can be continued
after everything was confirmed and received clearance.

Preventive actions to prevent recurrence

Substitute materials cannot have two different serial numbers. They should be
classified into two lines with the same serial number. The material need to include a
note of related substitute materials, so that for this serial number, we can choose
one of the two, instead of not welding or not sending materials at all.


In this article, we discussed several major factors for the reason behind PCBA parts go
missing or are not welded, many of which may be caused by PCB manufacturers,
engineering data, SMT process, human errors, equipment, systems, and other factors.
By paying attention to these problems in advance, we can prevent problems from
happening and re-assure the process goes smoothly as planned.
PCBasic provides a one-stop service for all PCBA processes and uses big data for
intelligent management techniques to control the whole process better which reduces
most if not all of the above possible mistakes.

Whether your project is in the prototyping stage, small-batch, or mass production - feel

free to write to us, we will give you a fast quotation, sincere service, and good quality
product. We firmly believe that if good product quality will be used in PCBA, The
processing industry will grow stronger and further, and we hope to be there with you and
to serve you when that happens.


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