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Lead-free soldering technology in the detection and testing

Lead-free soldering technology in the detection and testing

Worldwide, the major industrial countries are rapidly reduce lead-welding manufacturing process, including the PCB components. North America, the European Union and Japan are planning to adopt the "lead-free" technology, and many companies give up as quickly as possible lead-free soldering processes. Some companies take advantage of this situation, to strengthen its lead-free technology as the primary means of the consumer market.
Turned to lead-free soldering technology is almost to make all aspects of PCB components have been affected, including the testing and inspection methods. Here, we focused on a number of related technical issues, as well as lead-free solder to the automatic optical inspection (AOI), automated X-ray inspection (AXI), as well as online testing (ICT) and other major testing and detection techniques impact.
The concept of a new welding technology
Prohibit the use of lead solder trends have prompted electronics manufacturers and industry organizations, such as NEMI, IPC, etc. began to consider changing the traditional tin - lead solder chemical methods to seek new way out. Teradyne companies to participate in the NEMI's "lead-free soldering component technology blueprint in North America," "lead-free hybrid components and rework projects" to develop, and joined the IPC 7-32 welding detection capability Standards Committee.
The concept of a new lead-free solder includes tin - silver - copper and tin - copper welding technology. Most of the electronics industry to achieve the same behavior of lead-free soldering, are the tin - silver - copper alloy family of transformation. NEMI's recommended a method for reflow soldering of Sn3.9Ag0.6Cu (+ / -0.2%) and for the wave of the Sn0.7Cu "industry standard" lead-free alloys. However, with Business Cardthe number of process changes, should seriously consider using a more appropriate ratio of ingredients to suit a wider range of applications, make this alloy are consistent with the specified product marketing requirements, but also economical and practical.
Lead-free solder has a high melting point ingredient, may cause component and / or component damage. In accordance with the concept of lead-free solder, SnAgCu The melting point temperature from 183 íŠ to nearly 217 íŠ, the peak temperatures as high as 260 íŠ. Longer period of time by pre-heating temperature can be appropriately reduced. Fixed temperature also affected, and some parts of the restoration temperature of up to 280 íŠ.
This high temperature components used must be eligible for certification, without the certification requirement for manual assembly of components.

Optical detection problem
Detection of lead-free soldering and testing are basically the conventional leaded solder is no different. Lead-free solder joints shape looks like a traditional tin - lead solder joints are very similar. Detection of belonging to the type of welding, the key is to find the right shape to determine the visual characteristics of each type of detection mechanism.
However, lead-free and lead-soldered joints, or from the appearance of some differences and affect the accuracy of AOI systems. Lead-free solder joints of the stripes more apparent, and the leaded solder joints than the corresponding rough, which is due from liquid to solid phase transition caused. Therefore, these joints appear to appear to be more rough and uneven. In addition, the higher the surface tension of lead-free solder, unlike leaded solder so easy to flow, the formation of rounded shapes are also different. These visual differences called for AOI equipment and software re-calibration. For example, some lead-solder AOI system, set the "automatic adoption of value" may be related to lead-free soldering there is a slight difference.
If you currently are using manual testing instrument, and to consider converting to AOI systems, it is the right opportunity, because at this time manual testing instrument must be "re-calibration."
Lead-free solder testing of industrial research findings
Teradyne in 2002 funded the establishment of the National Physical Laboratory (NPL), on the AOI system, lead-free solder carried out an independent assessment of detection capabilities. NPL is the UK national standards laboratory, an independent measurement and materials science research, development, and knowledge conversion center, enjoyed a high international reputation.
NPL on the "lead-free surface mount components comparison of automated optical inspection system," the subject has been studied, and in July 2002 published a study designed to determine lead-free soldering components automated optical inspection system if there are problems.
For research use for the test object is a specialized production of monochromatic components of the study, one produced a lot, and some defects, some defects. Components, including many different types of welding. Each component contains nearly 100 components, as well as more than 1400 lead-free solder joints. The design of components types include 0.4mm pitch 256 pin QFP, 0.5mm pitch TSOP, as well as the 0402 resistance. Defect types include lack of weld components, misalignment components, but the dimensions are correct parameter error components, bad solder joints, wrong polarity components, welding bridges, as well as welding uneven elements.
AOI system to participate in the evaluation study are six different manufacturers, including Teradyne. Lead-free components and conventional leaded components test using the same software algorithms. Research has shown that the evaluation PCB lead-free solder leaded PCB with the same or even better. Both error detection rates are very similar. The number of tests needed to be undertaken whether or lead has nothing to do with the test object.
Studies have shown that, although the test results on different devices, slightly different, but most AOI systems can be used for lead-free detection of surface-mount components. Some algorithms and rely on the use of color monochrome camera system in the assessment of lead-free solder joints will experience problems. Practice has proved that, using AOI system for solder analysis, no need to use color images, monochrome images have been included in welding all the information necessary for analysis. However, the use of color image better lead-free defect detection, such as welding bridges and bad solder joints and so on.
Automatic X-ray detection problems
We found that lead-free solder ball joints in the cold solder joint increase. High density of lead-free solder welding, you can detect cracks in welding, and cold solder joint. Copper, tin and silver should fall within the "high density" material, therefore, as the lead in these materials impede X-ray irradiation. Therefore, it is necessary to re-calibrate X-ray systems, but all of the X-ray inspection companies - regardless of their production of manual or automatic X-ray inspection system - say their own devices for the detection of lead-free soldering is no problem, but in order to fine welding Characterization, monitoring assembly processes, as well as the most important solder joint structural integrity analysis, test equipment, demand increased.
Lead-free solder on the impact of ICT
As mentioned earlier, tin and lead-free solder alloy is an option, however, tin solder will appear "metal to be" phenomenon - that is, a small metal bumps, solder or pad extending beyond. Such tilting may grow to be very long, so that the two current welding area is too large, short-circuit, causing equipment failure. Line test can be easily used to find the problem, but the tin whisker growth may take some time, this may be a long-standing reliability problems. Many organizations are active efforts, such as NEMI companies are using different tin alloys, trying to minimize the occurrence of this phenomenon.
In order to optimize lead-free reflow soldering process, we have increased the use of flux in a non-clean environment, with the increase of contact resistance, could contaminate probe head, damage to the device performance. Therefore, called for the strengthening of equipment maintenance work, or to probe the first change for the more acute types. However, a more pointed probe head may be related to lead-free solder brittleness of conflict, causing damage. Lead-free solder due to the brittleness of the components in the test equipment to limit the bending properties to be cautious when.
Rework & Repair problems
Finally, consider the problem is lead-free solder rework and repair of the impact of the work. Lead-free alloys require higher melting temperature. If the components of the components of a larger size, there will be a high heat dissipation, and hence, pre-heating components. As a result of lead-free soldering, and in addition to some sort of bare PCB film to flame retardant, high temperature requirements of repair may damage the components and / or components. Mentioned above, NEMI is working through "lead-free hybrid components and rework the project" study these issues. Many companies have also been making efforts to reduce or eliminate defects in PCB assembly line, to establish a "zero defect" assembly line.

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