Why 3D Optical Inspection should be standard in SMD-Production

Recently we spoke to Dr Julia Traut, Head of Marketing and Product Management at SmartRep. SmartRep is a technical distributor of manufacturing equipment and process systems for electronics production. Its headquarters are in Hanau, near Frankfurt in Germany. She elaborates on SMD production lines and products and why automated optical inspection (AOI), and 3D measurement technology is so important in modern SmartFactories.

At first glance, SMD production and products seem straightforward: apply solder paste, place components, and send the board to the soldering oven. But the devil is in the details. While naked-eye inspection was possible in the 1980s, today's rapid SMD lines and miniaturised components require more advanced methods like automated optical inspection for high-speed SMD lines.

The human eye is too slow and imprecise, even by a microscope, to be useful.  

Automated optical inspection (AOI) for high-speed SMD lines: Since high-speed SMD lines produce multiple circuits every 15 to 20 seconds - manual inspection, even by a microscope, has become all but impossible for manufacturers. By the time you have inserted the board in the microscope, five other boards have already come off the line. In this context, the human eye is too slow and imprecise to be useful.

That’s why automatic optical inspection systems have been developed to operate within the micro-metre range. Since 2010, a 3D measurement standard has been established for this purpose. To guarantee the quality of solder joints, evaluations are made based not only on the presence of solder and components but also on the precise flow of the solder, which is assessed according to different classifications.

Two optical inspections

In an effort to reduce production costs, it's now standard practice to incorporate two optical inspections within the SMD process. The first evaluates the solder paste application, and the second checks the component placement later on. This strategic approach ensures that poorly printed boards aren't fitted with valuable components; instead, they're immediately identified and either set aside or redirected to the rework phase.

Solder Paste Inspection System (SPI) and Automated Optical Systems (AOI) 

The first inspection unit in the process is called the Solder Paste Inspection System (SPI), the latter is for automated optical inspection (AOI). These systems determine numerous measurement data in real-time. In a modern SmartFactory, these are not only used to document and monitor the production process but also to optimise and continuously improve the process window. AI technology is also used for this purpose. 

3D vs 2D image analysis

3D analysis, in contrast to the conventional 2D image analysis, offers a more comprehensive insight. It doesn't just confirm the presence of solder paste but also quantifies the exact volume applied. A top-down view can sometimes mislead; an area might seem abundant in solder paste, yet the quantity might be inadequate for a secure connection. Furthermore, subtle discrepancies like a slightly raised lead might go unnoticed in a 2D image, a challenge that 3D analysis can address more effectively. 

In addition, the tilt of a component, for example, can be determined. This occurs when there is much more solder under one side of the component than under the other side. Solder bridges, component twists, offsets and headers can also be determined more easily with 3D reconstruction. 

Another parameter that a 3D analysis provides that 2D analysis can’t is the component height: this can be used to reliably check the presence of a component. 

Korean machine builder Koh Young brought 3D measurement technology to the SMD industry in 2003: The first 3D solder paste measurement system opened up new analysis possibilities through a patented form of phase shift technology. In 2009, the technology was transferred to the AOI. Since then, solder joints and components on printed circuit boards can be detected in 3D.

The devil, and magic, are in the detail

The ESCATEC Group has incorporated this advanced technology into its manufacturing workflows. In 2015, ESCATEC launched its first 3D SPI system in Penang after a benchmarking exercise. By March 2016, a Koh Young SPI was operational in Heerbrugg. Following its successful field tests, a series of machine integrations ensued. Presently, Koh Young's SPI and AOI systems stand as the global standard within the ESCATEC Group. This standardised machinery not only streamlines operations but also simplifies the process of transferring production from Europe to Asia. Serving as an electronics expertise hub, ESCATEC Switzerland focuses on the design and launch of new products, ensuring a smooth handover to their Asian manufacturing units. 

Being a medically certified entity, ESCATEC values the impeccable measurement precision and precise 3D data provided by the Koh Young systems. These systems allow ESCATEC to monitor the printing and component placement processes with minute detail.

This ensures that if a board is identified with an error, the production line doesn't halt instantly. Instead, the problematic board shifts into the buffer.

ESCATEC Switzerland has also invested in an SPI buffer from SmartRep. "Following a suggestion from Koh Young's distributor, SmartRep, we integrated a buffer unit with 20 slots into the line right after the SPI," shares Production and Engineering Manager Dr. Martin Mündlein. "This ensures that if a board is identified with an error, the production line doesn't halt instantly. Instead, the problematic board shifts into the buffer. As the line remains operational, faulty boards can be effortlessly removed." He adds that pre-printing is feasible, which can expedite changeovers. 

Dr Mündlein observed after the 3D SPI system's initiation, "Before investing in the SPI, our printing process was primarily scrutinised under a microscope, and we felt we had a good handle on it." The 3D data now reinforces this belief. However, the current system allows for far more intricate analyses. Given the swift and straightforward programming of the Koh Young SPI system, every product now undergoes this rigorous inspection.

2D and 3D AOI

In 2018 and 2019, the implementations of Koh Young 3D AOI systems took place: "The 3D AOI makes an actual geometric measurement from which the inspection criterion is derived - and not just an image comparison," Dr. Mündlein explains the 3D AOI technology. He adds that these 3D AOI measurement results considerably reduce the number of pseudo defects compared to the 2D AOI previously used: "The 3D AOI conducts a genuine geometric assessment, forming the basis for the inspection criterion, rather than just relying on image comparison," elaborates Dr. Mündlein on how the 3D AOI technology works.

Pseudo defects

He highlights that by using 3D AOI, there's a significant decrease in pseudo defects compared to the previously employed 2D AOI: "The operator deals with far fewer incorrect alerts." An incorrect alert, or false call, refers to the machine mistakenly flagging a component during inspection, which, upon manual review, is deemed satisfactory. The precision offered by the 3D AOI measurement technology in the Koh Young system results in a reduced number of these errors.

Component libraries and an auto-programming feature further ease the burden on the operator, minimising the need for extensive troubleshooting and optimisation. As a result, the adoption of 3D measurement technology at ESCATEC has elevated quality standards and lightened the load on production staff.

In conclusion

To ensure quality and save costs, modern SMD production today cannot do without inspection systems. The miniaturisation of electronics and line cycles of just a few seconds make reliable optical inspection integrated into the production line necessary. The inspection of components and solder joints in 3D has become the standard. SPI and AOI systems from Koh Young provide measured values for quality control and traceability. These are the cornerstones of Industry 4.0. By integrating SPI and AOI systems, ESCATEC has not only been able to improve and ensure the quality of its products but has also future-proofed its machinery for use across multiple sites.