Testing and inspection issues in PCB lead-free soldering technology

The process requirements of circuit boards are a very important factor in the production. They directly determine the quality and positioning of a PCB board, such as tin spraying, gold plating, and immersion gold. Relatively speaking, immersion gold is for high-end boards. Since immersion gold The quality is good but the cost is relatively high, so many customers choose the most commonly used tin spraying process.
Spray tin is the most common method of circuit board surface treatment. It is generally divided into lead-spray tin and lead-free tin spray.

There is a big difference between the two:

1. The lead content of lead-free tin does not exceed 0.5, Those with lead tin can reach 37.

2. From the surface of spray tin, lead tin is relatively bright, while lead-free tin (SAC) is relatively dull.

3. Leaded tin is brighter, lead-free tin (SAC) is darker, but the wettability of lead-free tin is a little worse than that of leaded tin.

4. Lead will increase the activity of the tin wire during the welding process. Lead-based tin wire is relatively easier to use than lead-free tin wire. However, lead is toxic and is not good for the human body after long-term use. Moreover, lead-free tin has a higher melting point than lead-based tin. This makes the welding point much stronger.

5. Lead in lead is harmful to the human body, but lead-free is not. The eutectic temperature of lead is lower than that of lead-free. The specific amount depends on the composition of the lead-free alloy. For example, the eutectic of SNAGCU is 217 degrees, and the soldering temperature It is the eutectic temperature plus 30~50 degrees. It depends on the actual adjustment. The leaded eutectic is 183 degrees. Leaded is better than lead-free in terms of mechanical strength and brightness.

Across the world, major industrial countries are rapidly eliminating lead soldering manufacturing processes, including PCB assemblies. North America, the European Union, and Japan all plan to adopt “lead-free” technology, and many companies are abandoning lead-based soldering processes as quickly as possible. Some companies are taking full advantage of this situation and using lead-free technology as a primary means of strengthening their consumer markets.
The move to lead-free soldering technology has affected nearly every aspect of PCB assemblies, including testing and inspection methods. Here, we focus on some related technical issues and the impact of lead-free soldering on major testing and inspection technologies such as automatic optical inspection (AOI), automatic X-ray inspection (AXI) and in-circuit testing (ICT).

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1. The concept of new welding technology

The trend of banning the use of lead solder has prompted electronics manufacturers and industrial organizations, such as NEMI and IPC, to consider changing the traditional tin-lead soldering chemistry and seek new solutions. Teradyne participated in the formulation of NEMI’s “North American Lead-Free Soldering Component Technology Blueprint” and “Lead-Free Hybrid Component and Rework Project”, and joined the IPC 7-32 Soldering Inspection Capability Standards Committee.
The new lead-free soldering concept mainly includes tin-silver-copper and tin-copper soldering technologies. In order to achieve lead-free soldering, most of the electronics industry peers are transitioning to the tin-silver-copper alloy family. NEMI recommends an “industry standard” lead-free alloy of Sn3.9Ag0.6Cu (+/-0.2%) for reflow soldering and Sn0.7Cu for wave soldering. However, with many process changes, careful consideration should be given to adopting a more appropriate proportion of ingredients to suit a wider range of applications so that the specified alloy not only meets the marketing requirements of the product, but is also economical and practical.

2. Reflow temperature

Lead-free solder ingredients have higher melting points and may cause damage to components and/or assemblies. According to the lead-free soldering concept, the melting point temperature of SnAgCu increases from 183°C to nearly 217°C, and the peak temperature is as high as 260°C. The high temperature can be appropriately reduced by preheating for a longer period of time. Repair temperatures are also affected, with some parts reaching repair temperatures of up to 280°C.
Components used at these higher temperatures must be qualified, and uncertified components require manual assembly.

3. Optical detection issues

Testing lead-free soldering is basically no different from testing conventional lead-based soldering. The solder joint appearance of lead-free soldering looks very similar to that of traditional tin-lead solder joints. The key to detecting which type of welding it is is to find a detection mechanism that correctly determines the visual characteristics of each shape.
However, the appearance of solder joints between lead-free and lead-based soldering is still somewhat different, which affects the accuracy of the AOI system. Lead-free solder joints have more pronounced streaks and are rougher than their leaded counterparts, caused by the phase transition from liquid to solid. Therefore, such solder joints appear rougher and uneven. In addition, lead-free solder has a higher surface tension and does not flow as easily as lead solder, resulting in different fillet shapes. These visual differences require recalibration of the AOI equipment and software.
For example, the “automatic pass value” set in some leaded soldering AOI systems may be slightly different from that of lead-free soldering.
If you are currently using manual detectors and are considering converting to an AOI system, this is the right opportunity because the manual detectors must be “recalibrated” at this time.

4. Industrial research conclusions on lead-free solder testing

In 2002, Teradyne funded the establishment of the National Physical Laboratory (NPL) to independently evaluate the lead-free solder inspection capabilities of the AOI system. NPL is the British National Standards Laboratory, an independent center for measurement and materials science research, development and knowledge transfer, with a high international reputation.

NPL conducted research on the topic “Comparison of automated optical inspection systems for lead-free surface mount components” and published the results in July 2002, aiming to determine whether there are problems with the automated optical inspection systems for lead-free soldered components.
The test object used for the study was a single-color component made specifically for this study. Many were made at one time, some with defects and some without defects. Components include many different welding types. Each assembly contains nearly 100 components and more than 1,400 lead-free solder joints. The designed component types include 0.4mm pitch 256-pin QFP, 0.5mm pitch TSOP, and 0402 resistors. Types of defects include missing solder components, misaligned components, components with correct dimensions but wrong parameters, bad solder joints, wrong polarity components, solder bridges, and uneven solder components.
Six different manufacturers participated in the AOI system evaluation study, including Teradyne. The same software algorithm is used for detection of lead-free and conventional lead-containing components. Studies have shown that the evaluation results of lead-free solder PCBs are the same as or better than leaded PCBs. The false detection rates of both are also very similar. The number of tests required is independent of whether the test object contains lead.
Research shows that although test results vary slightly on different devices, most AOI systems can be used for inspection of lead-free surface-mounted components. Some systems that use color algorithms and rely on monochrome cameras have problems evaluating lead-free solder joints. Practice has proved that when using the AOI system to analyze welding, it is not necessary to use color images. Monochrome images already contain all the information necessary for welding analysis. But using color images is better for detecting lead-free defects, such as solder bridges and bad solder joints.

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5. Automatic X-ray detection issues

We found that the number of false soldering increased in the spherical solder joints of lead-free soldering. Lead-free soldering has a higher soldering density and can detect cracks and weak soldering during soldering. Copper, tin, and silver are considered “dense” materials, so materials like lead block X-ray exposure. So, it is necessary to recalibrate the X-ray system, but all the X-ray inspection companies – whether they make manual or automatic X-ray inspection systems – say that their equipment has no problem detecting lead-free soldering, but for good soldering Characterization, monitoring of assembly processes, and most importantly structural integrity analysis of solder joints have increased the testing requirements for equipment.

6. The impact of lead-free soldering on ICT

As mentioned earlier, tin alloy is a lead-free soldering option. However, tin soldering can cause “whiskers” – small metal bumps that extend beyond the solder joint or pad. Such whiskers may grow very long, causing excessive current in the two welding areas, causing a short circuit and causing equipment failure. This problem can be easily caught using in-circuit testing, but the growth of tin whiskers can take some time, which can be a long-term reliability issue. Many organizations are actively working, such as NEMI, which is using different tin alloys to try to minimize the occurrence of this phenomenon.
In order to optimize the reflow process of lead-free soldering, we have increased the amount of flux used. In a non-cleaning environment, as the contact resistance increases, it may contaminate the probe head and damage the equipment performance. Therefore, it is required to strengthen the maintenance of the equipment or change the probe head to a sharper type. However, sharper probe tips may conflict with the brittleness of lead-free solder, causing damage. Due to the brittleness of lead-free solder, extra care must be taken when limiting the bending characteristics of components in test equipment.

7. Repair & Repair Problems

A final issue to consider is the impact of lead-free soldering on rework and repair work. Melting of lead-free alloys requires higher temperatures. If the component size on the assembly is larger, higher heat dissipation will occur, so the components should be preheated. Due to the use of lead-free soldering and the removal of certain flame retardants from the bare PCB, the high temperatures required during repair may damage components and/or assemblies. As mentioned above, NEMI is studying these issues through the “Lead-Free Hybrid Components and Rework Project.” Many companies are also working hard to reduce or eliminate defects in PCB assembly lines and establish “zero-defect” assembly lines.

Prevention and improvement of lead-free soldering plate explosion problems
Choosing the best materials and the most expensive manufacturing process can prevent and improve the occurrence of bursting problems. For example, sheets with high Tg are generally more expensive, and PCB hot pressing time generally requires a longer time, which increases the cost. According to the golden triangle model of management, enterprise competition mainly revolves around three aspects: service, quality, and cost. Especially in the current financial crisis environment, when it comes to the issue of bursting, we have to make choices and refine the influencing factors in all aspects to seek prevention and improvement.

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