Manual Inspection for PCBs
Defining the best microscope configurations for accurate, repeatable, and consistent PCB inspection. What are you looking for, what configuration suits the application best, and what are the appropriate lighting techniques.
TABLE OF CONTENT
- What are we looking for
- Best Microscope configurations
- Best Methods for lighting
Printed circuit boards (PCB) have been inspected for many years with a host of different methods including the naked eye, magnifiers, microscopes, video/digital cameras, and automated tools using digital vision systems. The most prevalent methods have been the naked eye and magnifiers. Microscopes are also one of the major tools used, and in fact as components become smaller the need for higher magnification systems becomes more relevant. The Stereo Microscope which has been around for over 100 years and invented by Horatio Greenough is finding new relevance in today’s PCB inspection environment. The major reason for this profound resurgence of the microscope has been the miniaturization of electronic components, new optical designs, new illumination technologies, and reduced cost of optical components. The purpose of this white paper is to introduce you to the new technologies that will enable higher quality inspections at faster rates. Define the best configurations of the pertinent optical system, and identify the ideal lighting technique needed to ensure repeatable quality inspections.
Inspection of PCB’s is getting more difficult as we see a wide variety of components that are more densely packed, have high complexity, and are getting smaller. In general, the inspection requirements for PCBs tend to center around a few specific areas:
- Presence and Absence of Components – are the components that are supposed to be there actually there and if so are they the correct components.
- Solder Joint Quality – this encompasses many things but two of the most important are;
- Is there enough solder?
- Is the quality/structure of the solder joint good enough?
- Rework – using a tool to repair damaged or non-working PCB, change components, and touch up after inspection.
Of course there are many issues to consider when inspecting PCB’s but for the purposes of this paper we will only consider the above in defining the appropriate tools needed for the inspection. Experience shows that the tool configuration required for the above issues would also be appropriate for many of the other issues surrounding PCB inspection.
Some of this work can be done with magnifiers which typically magnify the area of interest from 2-4X. This magnification is appropriate for large area work but when the requirement is beyond 4X typically the stereo microscope is the best alternative. The stereo microscope magnifies images from 6X to around 40X with the most useable range being between 10X and 25X. The final magnification differs slightly from manufacturer to manufacturer but generally all cover the useable spectrum mentioned above. There are some misunderstandings about magnification that need to be clarified. In our experience many users ask for 40x, 50x, or even 60x magnification, but the actual relevant range for a PCB is 10X to 25X even for fine pitch devices. This is because technicians like to see a large area on the PCB in order to be easily able to find and categorize the defect. Higher magnifications would make inspecting slower and the likelihood of finding issues that are not relevant becomes greater (these are what would be considered false calls). Additionally, a good quality microscope will allow you to see more than low quality instruments at lower magnificationsA high resolution image at a lower mag will allow the operator to see more detail over a larger area(field of view), thus scanning for defects and issues is faster and consequently more productive. Most inspection technicians appreciate good quality optics and can easily tell you the difference between what is good and bad. Unfortunately, this tends to be an area in the plant that adds little value to the assembly process and this there is a huge emphasis on purchasing the least costly tool for the job. In most cases the least costly tools don’t perform as well and the ultimate cost to the assembly and inspections processes are much higher than the cost differential between the best tool and the least expensive tool.PRESENCE AND ABSENCE OF COMPONENTS
The field of view refers to the entire visible area you see when looking through the microscope. It is desirable to see as large an area as possible in one view A high quality optical system will allow you a larger field of view with high clarity, minimizing the need to increase magnification. The result will be a quicker and more productive scan of the PCB for basic inspection. The key is to find any anomalies such as missing components, missing solder, too much solder, and incorrectly mounted components quickly, and then have the ability to increasingly magnify (zoom) the problem area to see if it really is an issue. Having the appropriate field of view is critical. Below are some examples of images with smaller and larger fields of view. Notice how much more can be seen with larger fields. The larger field translates into faster inspections and productivity increases from 5% to as much as 20% with the proper tools.
Larger fields also have an added benefit. Because the field is larger the technician will minimize the movement of the PCB under the microscope when searching for defects. Less repetitive movements minimizes the onset of carpel tunnel syndrome which today costs manufacturers millions of dollars in worker’s comp claims and lost productivity.
Solder joint quality is typically the most important reason for doing optical inspections. Unfortunately the solder joint is also one of the most difficult surfaces to inspect due primarily to the fact that solder joints are typically highly reflective surfaces which produce glare when illuminated. Often times the glare can mask problems with the solder quality. It is therefore essential to use the correct light technique and tools. Technology is helping us significantly in the inspection arena. LED lights produce bright light, last for years rather than hours, are energy efficient, allow lighting angle control, and maintain a constant color temperature at the viewing surface. Bright lights are great, but for solder joint inspection it is important to diffuse the light. Diffused light will provide even illumination and minimize glare. Even illumination, minimized glare, constant color temperature reduce eye strain and help foster repeatable inspection processes. This translates directly into faster and more consistent inspection and rework which results in tremendous cost savings. Below are some comparisons of lighting systems for microscopes and what you can expect from this new technology.
One of the other common uses of the microscope is for rework and repair. When doing rework or repair you run across the same issues as you would in solder joint inspection processes with a couple of additional complications. First, you typically need room between the bottom of the microscope and the PCB so that you can get your rework and repair tools into position. This is reffered to as “working distance” and allows the technician to get his/her tools into position at the correct and comfortable angle. Having enough working distance will minimize hand movements during rework tasks and allow a comfortable working angle when using soldering irons or other rework tools. The typical working distance is from ~120mm(~4”) to ~160mm(~6) depending on the tool being used. Of course shorter working distances are appropriate for inspection but may be problematic when using most rework tools. On the other hand working distance that are too tall can cause the operators hands to be in an ergonomically poor position and thus could create more problems. The best optical instruments have a variety of options that allow adjusting the working distance according to the application the microscope is being used for. These options add cost but more than pay for themselves in short order.
The tool of choice for rework tends to be the soldering iron. Aside from being extremely hot, the soldering process generates flux fumes. These fumes are created under the microscope and work their way up into the optical system. If the microscope optical system is not protected the flux fumes will gum up the inside of the optics potentially destroying the instrument or creating an expensive repair. The best optics today account for both the working distance and flux fume issues by providing options for protective covers, or objectives that prevent fumes from reaching the internal optics. These options should be heavily considered when choosing an instrument. Below are some examples of working distance of common repair and rework microscopes.
Lighting in and of itself is a complete white paper on its own because lighting is what makes the optic work well or work poorly. Today there are 3 major lighting methods including fiber otpics, fluorescent, and LED. Fiber optics have been available for many years provide lots of light, use halogen bulbs, require a power supply with a cooling fan and a fiber to transfer light from the power supply to the microscope. Fluorescent lights have also been available for many years, provide even diffused illumination (similar to most factory floor lighting), and require bulbs that are generally more costly than halogen bulbs. Todays choice for most inspection and rework applications should be LED lights which produce bright light, last for years rather than hours, are energy efficient, allow lighting angle control, and maintain a constant color temperature at the viewing surface. Unfortunately, the low cost nature of LED’s has created confusion in the microscopy and inspection markets as not all LED’s are not created equal. In fact there are so many LED microsope lighting systems and the costs have been driven down so much that once again the user can easily be easily confused and ultimately purchase inappropriate products that have a lower level impact on the process. The right lighting could easily significantly advance productivity. LED’s are a small point source of light and are configured in an array to increase the volume of light. LED’s for microscope lighting are generally configured in a circular array with the array having as many as 80 LED’s in some cases.. PCB’s and solder joints are highly reflective and the one problem with LED’s is that that point source of light will be reflected back into the microscope which when viewed by the technician will appear as hot spots. This creates eye strain, uneven illumination, and in general a poor inspection environment. The best LED lighting systems account for this through the use of diverging lenses and diffusers which will scatter and diffuse the light eliminating the hot spots and minimizing the glare.
LED’s output very specific wavelengths of light. Theses narrow bands of light come in many colors but the one we are most concerned with is the wavelength that approaches white. Why? When using white light the colors of the surface you are inspecting will be true to their actual color. True colors are what technicians expect thus allowing them to focus on the inspection process rather than the color of the surface. Many low cost LED systems generate a blue light that is not very realistic and thus confuses technicians. White light or day light (which is only slightly blue) is preferred in order to get an accurate rendition of the product through the microscope.
LED’s output a very narrow spectrum of light and changing the power to the LED will not change that spectrum. An increase in the voltage to an LED will simply provide more intense light, but the color spectrum remains constant. Consequently, because of the constant color spectrum the images you see from one microscope to the next will be homogenous allowing for consistent and reliable inspection processes.
Lighting is one of the most critical factors when considering inspection tools, but often also the most overlooked. A few dollars spent here will have significant impact for predictable, dependable, and consistent inspection processes.
Ergonomics are what we use to define the level of comfort someone experiences during a repetitive task. The inspection process tends to be one of the most repetitive functions on the PCB assembly line consequently having a comfortable (ergonomic) position is critical. Discomfort leads to frequent breaks, headaches, neck aches, and eyestrain ultimately costing the company lost time, lost productivity, workers comp claims and ultimately lost profitability. It is important that inspection and rework technicians sit with their backs straight, have adequate and appropriate lighting, have enough room under the microscope for the rework tools, and have their arms in a comfortable position. The most effective inspection instruments can be configured for ideal ergonomic positions but this of course comes at a price, so you must make every effort to understand the requirements for inspection and should include answers to the following questions:
- What will you be looking for or doing under the microscope?
- How much time will be spent on this process?
- How critical is this process to my production flow?
- Is it a gating factor in my production line?
Small changes can have tremendous positive impact on productivity.
The best microscope configuration should take into account all of the issues discussed above and fortunately with today’s technology one optical system can support all of the major issues associated with inspecting and reworking PCB’s. There are many considerations when choosing an optical system for your application. In the PCB assembly area you should consider the following as requirements needed to perform the highest quality work:
- StereoZoom Body with 5X to 30X magnification using 10X widefield eyepieces
- Field of View of at least 45mm (large viewing for faster scanning leading to higher productivity)
- Depth of field of at least 13mm (ability to see tall objects completely in focus, reducing focusing time thereby increasing productivity)
- Mounted on a boom stand or Articulated arm stand (more flexibility)
- Diffuse LED lighting
- A configuration that is ergonomically correct for the task at hand
Below are some typical configuration examples
Imaging, Digital Imaging, and Digital Microscopes all refer to the same idea of creating either live or still pictures of the inspection surfaces you are working with. The world has become digital and this technology has invaded the inspection business as well. There are many system configurations including the ones above that can be made available with digital cameras. This technology allows manufacturers to share quality issues visually, generate training videos, and conference interactively and immediately all over the world. However, digital cameras have different sensitivities than does the human eye, so lighting is even more critical in order to generate high quality, reproducible images. This subject is prevalent and important in many areas of the production floor and in order to give it justice is covered in a separate white paper located at the following link. www.jhtechnologies.com or you can reference the link below which discusses the needs for imaging PCBs.
PCB inspection and rework is a function seen on every manufacturing floor at every contract manufacturer or OEM in the world. It is a cost center and many manufacturers what to minimize any of the work being done in these areas. And they do that by refining their production processes in such a way that first pass yield and quality is very high. Additionally, most manufacturer’s will gladly invest heavily in expanding their production capacity while neglecting their inspection capability. This is evident as you tour a manufacturing plant and see microscopes that are sometimes 30 or 40 years old, technicians hunched over their instrument trying to do quality rework, and lighting that is either dim or too bright. As with everything, technology has made the old microscope products obsolete and replaced them with tools that provide much higher functionality thereby reducing overall inspection and rework costs. Embracing the inspection and rework processes and understanding the types of functionality available in todays microscopes will bring a higher degree of quality shipping out the back door. Pay attention to your technicians, see how often they get up from their work or if they are massaging their own necks and look for other clues that would lead you to believe they are uncomfortable. Make them more comfortable and both the quality and quantity of their work will grow.