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Visual Inspection a Shortcut to Cost Reduction and Reliability Improvement of Electronics Manufacturing

Electronic products are widely applied in consumer electronics, telecommunication electronics, industrial control electronics, military and aerospace electronics all of which play an essential role in all fields of people's life and even some of which are concerned with people's life. Although electronics products can be classified into a couple of categories as what's mentioned above, they fundamentally share the same manufacturing process. Basically speaking, electronics manufacturing is primarily composed by four elements: Components, PWB (Printed Wiring Board), SMT (Surface Mount Technology) and AI (Auto-Insert) technology, and Assembly. Electronic components are first stuck to PWBs through the application of SMT and/or AI technology and then a couple of electronic sections are combined together to make final products.

It's necessary to claim that the so-called visual inspection mentioned in this article refers to manufacturing inspection with the help of visual inspection devices like image capturing devices. Through image amplification, character capturing or image processing, comparison will be made between practical products and input data so that product quality can be guaranteed. Up to now, typical visual inspection devices include microscope, video magnifier, solder paste inspection equipment, AOI (Automated Optical Instrument), to name a few. However, visual inspection achieved by naked eyes is excluded.

Necessity of Visual Inspection in Electronics Manufacturing

Gradual development of technology leads electronic manufacturing technologies to non-stop improvement. As a result, electronic products tend to become miniaturized with densification being increasingly protruding. Moreover, as components become downsized and PCBs (Printed Circuit Boards) have to accept increasingly higher density interconnections, mounting density has rocketed to a high extent, leading to increasingly higher product quality requirement. Therefore, inspection on product quality during electronic manufacturing process is never insignificant.

Apart from potential trend of modern electronics, visual inspection with the help of visual inspection devices demonstrates more merits than traditional naked-eye inspection.
• Naked-eye inspection is tiring with low efficiency.
• Naked eyes fail to meet the inspection requirement of miniaturized component inspection.
• Naked-eye inspection is bad for working staff's eyesight and difficult to capture accurate inspection result.

Nevertheless, depending on visual inspection devices, labor intensity will be greatly decreased with inspection efficiency upgrading. Up to now, visual inspection under inspection devices has been widely applied in all phases of electronics manufacturing.

Application of Visual Inspection in Component Quality Control

Component manufacturing has long seen much significance of visual inspection because it works for silkscreen, line loop resistance, inductance, polarity and orientation. All the aspects mentioned above are inspected by staff in naked eyes and higher subjectivity tends to be generated and staff are easy to suffer from fatigue. As a result, component quality accuracy and reliability are hard to be captured. By amplifying image through inspection devices prior to inspection, defects will become clearer and staff will be confronted with less fatigue.

Application of Visual Inspection in PCB Quality Control

Even the most simplified version of PCB manufacturing process still contains 10 steps at least.

When it comes to PCB quality control inspection, most attention has to be paid to the following inspection items: foreign matter, copper exposion, scratch, insufficient plating, incorrect silkscreen, ineven solder paste and pads, residual copper, and imaging. All those defects are hardly observed through naked eyes. At present, visual inspection methods or devices for PCB quality control include:
• AOI equipment for bare PCB inspection. AOI ranges for bare PCB boards cover short circuits, open circuits, residual copper, vacancy etc.
• Magnifying glass. Magnifying glass is only capable of inspecting defects that are exposed on the surface of PCB. This method is seldom leveraged in modern electronics manufacturing.
• Naked-eye inspection. Naked-eye inspection is an old-school inspection method although it seldom meets constant improvement of quality control and PCB design density. As a result, reliability and stability of naked-eye inspection are hard to be ensured.
• Optical video display system combining naked-eye inspection and AOI. This system displays PCB images on monitor through magnifying them to make inspection easier.

Application of Visual Inspection in SMT/AI Technology

SMT is now the most prevalent assembly technology in electronics manufacturing. Printed Circuit Board assembly process with the application of SMT mainly include: solder paste printing, mounting, reflow solder, cleaning, inspection and rework. All inspection devices applied in all phases of manufacturing include: magnifying glass, microscope, ICT (In-Circuit Test), flying probe test, AOI, AXI and function test among which magnifying glass, microscope, AOI and AXI belong to the range of visual inspection.

Visual inspection will cause different inspection results and targets by placing on different stations during SMT assembly process.

• After Solder Paste Printing

Solder paste printing is the first procedure of SMT assembly and its quality directly determines that of final products. Thus, it's of much necessity to arrange visual inspection after solder paste printing. Up to now, solder paste printing inspection is carried out through the following methods:
a. On-line or off-line solder paste inspection, which mainly displays 3D configurations of solder paste that has been printed on pads on the monitor so that defects can be exposed in terms of thickness, volume, displacement, vacancy, excessive or insufficient solder paste.
b. Magnifying glass.
c. CCD inspector. It displays magnified images on the monitor so that operators will be aware of printing quality.

• After Chip Mounting

Partially-assembled PCBs go over visual inspection after chip mounting before reflow soldering, leading mounting defects to be exposed at an early time. As a result, volume faulty products can be avoided and maintenance and rework cost will be dramatically reduced as well. This phase depends on the following visual inspection devices:
a. AOI prior to reflow oven. This helps defects to be exposed including missing components, displacement, polarity and orientation.
b. Magnifying glass.
c. CCD inspector.

• After Reflow Soldering

Inspection after reflow soldering shares equivalent visual inspection devices with those before reflow oven, mostly AOI, magnifying glass and CCD.

Apart from the stations mentioned above calling for visual inspection, a lot more stations need visual inspection as well such as maintenance station, barcode inspection, and stations for special components inspection including BGAs, ICs, connectors etc.

Comparison between Visual Inspection and Manual Inspection

Electronics manufacturing industry has seen naked-eye inspection as a traditional inspection method, also called manual inspection. Below table indicates comparison between visual inspection and manual inspection.

Item Visual Inspection Manual Inspection
Inspection validity Clear
Suitable for mini defects
Suitable for surface defects
Sometimes inaccurate
Not suitable for mini defects
Visual scope Visual scope can be magnified to an agreeable range. Limited visual scope
Influence on operators Hardly harmful to operators' eyes Extremely harmful to operators' eyes

Modern electronics manufacturing has witnessed essential significance of visual inspection for full considerations of quality control. Visual inspection with devices participated in is beneficial for manufacturing defects to be exposed at an early stage so that volume yield pass rate can be increased.

PCBCart Implements Multiple Kinds of Inspection to Guarantee Your PCB Quality And Performance

We fully understand printed circuit boards are the core of almost all electronics, that's why we implement multiple PCB inspection methods throughout PCB manufacturing and assembly process. Our commonly used test methods include Electrical Test, AOI, AXI, Flying Probe Test. We also offer custom test options based on specific requirement from our clients. You may reach us to discuss your precise PCB test demands.

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Helpful Resources
Full Feature PCB Manufacturing Service
Advanced PCB Assembly Service
Electrical Test Introduction
Automated Optical Inspection Introduction
Automated X-ray Inspection

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