-
- News
- Books
Featured Books
- smt007 Magazine
Latest Issues
Current IssueComing to Terms With AI
In this issue, we examine the profound effect artificial intelligence and machine learning are having on manufacturing and business processes. We follow technology, innovation, and money as automation becomes the new key indicator of growth in our industry.
Box Build
One trend is to add box build and final assembly to your product offering. In this issue, we explore the opportunities and risks of adding system assembly to your service portfolio.
IPC APEX EXPO 2024 Pre-show
This month’s issue devotes its pages to a comprehensive preview of the IPC APEX EXPO 2024 event. Whether your role is technical or business, if you're new-to-the-industry or seasoned veteran, you'll find value throughout this program.
- Articles
- Columns
Search Console
- Links
- Events
||| MENU - smt007 Magazine
Advances in Lead-free Soldering and Automatic Inspection
December 31, 1969 |Estimated reading time: 5 minutes
By Jim Fishburn
One popular notion is to use lead-free solder as a way to eliminate one of the most harmful chemicals from manufacturing's outflow. Longer-term are the issues of safe lead disposal as landfills, incineration and other methods come under increasing scrutiny. For the industry's part, perfecting the composition, application and reflow of lead-free solder represents a triumph over many technical obstacles across many scientific disciplines. However, the question remains: How does this affect the automated inspection systems developed for conventional lead-solder applications?
Figure 1. The eutectic Sn/Pb solder joint's surface (a) is compared to its lead-free solder counterpart (b). Note the rough surface of the latter, which accounts for difficult inspection.
null
null
Why Lead-free Solder Inspection Is Difficult
There are two characteristics of lead-free solder that make optical inspection challenging:
- The reflowed shape of the joints, a function of wettability, generally are uneven, prompting inconsistent inspections.
- Their surfaces tend toward dull, matte finishes.
There are three reasons for these characteristics:1. Fillet-shape Dispersion. Final fillet shapes depend on several variables, including circuit design, assembly-line conditions, various component-plating technologies, flux selection and wavesolder reflow temperature. Thus, as lead-free solder melts, the alloy's surface tension is stronger than that of conventional tin/lead (Sn/Pb) materials. The result is that it has more difficulty spreading along surfaces. Its capacity for soldering, or wettability, depends on its attraction to the land materials, i.e., integrated circuit (IC)-lead plating, making fillet shape a factor of even more variability.2. Matte Solder Surfaces. Eutectic Sn/Pb solder generally bears a reflective surface and appears smooth. In contrast, lead-free solder joints usually display a dull or matte surface that looks rough (Figure 1). This is because lead-free solder is a non-eutectic alloy. After reflow, it drops in temperature neither quickly nor uniformly.Generally, lead-free solder has a liquid- to solid-state-temperature difference of about 10°C. Because of a slow cooling process, the metal composition starts to solidify non-uniformly such that the liquid and solid states coexist — the source of the dull surfaces. On the other hand, with quick cooling down periods, surfaces become comparatively smoother. In short, whether dull or shiny and smooth, much depends on the cooling temperature conditions. Moreover, the range of dull surface types varies widely.
3. Reflection Characteristic of Solder Surfaces. There is a correlation between solder surfaces and their reflective characteristics, which depends on the time it takes the solder to solidify. An Sn/Pb solder fillet surface is smooth and its surface comparatively even, the conditions prompting a reflective surface. Occasionally, lead-free solder will exhibit a reflective surface similar to that of Sn/Pb solder only to become dull and non-reflective, depending on the cooling temperature conditions. When lead-free solder hardens and becomes totally non-reflective, it appears both dull and slightly white in color.
Lead-free Solder and Automatic Inspection
To an automated optical-based solder inspection system (AOI), the dull surface of a lead-free solder joint appears white or features a white mask that obscures the fillet's actual shape. However, new systems that use a color-highlight technology have proven to be effective in overcoming reflection problems to inspection.*
Figure 2. The color-highlight system works by projecting red, green and blue light onto a PCB surface at different angles to produce a 2-D image that yields 3-D data.
null
Color Highlighting, to inspect for solder quality, works by using a source to project red, green and blue lights onto a printed circuit board (PCB) surface at different predetermined angles. The camera "captures" the colors as they reflect off the board, its components and solder, producing a 2-D image that, nevertheless, conveys 3-D information (Figure 2). The red color reflects off near-flat surfaces; the green off medium angles and the blue off steep angles — all captured by the color camera. In this manner, the color-highlight system can process the reflective patterns to identify the solder shape, even if whitening masks it.
Three Examples. The color-highlight system uses algorithms to convert the red flat angles, green medium angles and blue steep angles into a topographical 3-D picture of the solder joint. Figure 3 graphically shows the distribution of color information in RGB (red/green/blue) for the leaded-solder joint, lead-free solder joint and white diffusion board, respectively. In 3a, the leaded-solder surface features sharp peaks for each of the colors, i.e., the color highlights are clear. In 3b, however, the non-reflective, lead-free solder graph shape spreads and the ratio of mixed color between R and G and between G and B is increased. However, the maximum values of each RGB remain the same, i.e., the color-highlight system is able to convert the color to shape information. Finally, 3c shows that, even with a white diffusion board, the color-highlight system will pick up reflections that can be separated into maximum values of each RGB. Thus, even if the reflection is low, the correlation between the color and the angle is the same and a fillet shape emerges.
"De-white" Processing
Low reflection levels of lead-free solder surfaces challenge all AOI systems by appearing whitish in color. If the white becomes stronger and the image more faded, it is difficult to program an inspection system for a lead-free soldered PCB. In response, an algorithm has been developed to "de-white" the image and correctly amplify the colors for easier inspection programming and color-image processing (see sidebar).
To study the whitish solder surface, the same solder shape is formed of both a reflective surface using leaded solder and a non-reflective surface using lead-free solder; both images are captured using the color-highlight system (Figure 4). In the case of a reflective surface (Sn/Pb solder), a clear red color appears in the center whereas with a dull (lead-free solder) surface, a whitish color appears instead of a vivid red.
Figure 3. Color data distribution in red, green and blue for a leaded-solder joint (a), a lead-free fillet (b) and a white-diffused board (c), which the color-highlight system uses to yield solder joint shape information.
null
After measuring the pixel values out of the whitish area, each RGB color is measured. As stated, a red typically appears in the whitish area. Hence, the ratio of red is greater than the other colors. However, the ratios of the green and the blue are of almost the same value, which provides the evidence to confirm fading to a whitish color.
Jim Fishburn may be contacted at Omron Electronics LLC, 1 Commerce Dr., Schaumburg, IL 60173; (847) 843-7900; E-mail: jim.fishburn@omron.com. Figure 4. A color-highlight system captures the reflective surface of an Sn/Pb solder joint (left), which produces a clear red color. With a non-reflective joint, the red is obscured by the whitish surface.
null
VT-WIN is a trademarked product of Omron Electronics LLC.