Let There Be Light: Tools and Illumination Strategies for Visual Inspection of PCBs

Oct. 8, 2008
Let There Be Light: Tools and Illumination Strategies for Visual Inspection of PCBs By Charles Kempf, Metron Optics, Inc. Automated inspection equipment is usually efficient and cost-effective for high volume runs of

Let There Be Light: Tools and Illumination Strategies for Visual Inspection of PCBs

By Charles Kempf, Metron Optics, Inc.

Automated inspection equipment is usually efficient and cost-effective for high volume runs of PCBs. The simple pass/fail electrical tests performed by machines work well enough when speed is more important than anything else. When quality control is paramount, however, human visual inspection with proper tools and illumination techniques is a more cost-effective, error-proof way to go.

When Automation Is Not the Answer

For decades automation has been touted as a cure-all for everything. It’s no wonder engineers and engineering managers have become accustomed to relying on expensive, complicated machines to perform just about every task in the manufacturing environment. But even the newest technology is not necessarily always the quickest and most cost-effective way to accomplish a task. When it comes to inspection, for instance, machines may be able to give a pass/fail result, but they cannot distinguish a part as marginal, nor can they make decisions on how to correct a problem. Human visual inspection may be old-fashioned, but in some areas, it is more efficient and cost-effective than automation.

For starters, automated inspection equipment such as X-ray, laser and other electronics, is expensive. Moreover, the expense doesn’t stop after the machine has been purchased. There is costly software to purchase, maintain and update. Qualified staff to operate, trouble-shoot and fix machinery is highly paid and may require repeated training to stay up-to-date. Furthermore, when a machine breaks down or newer technology becomes available, replacement costs can be exorbitant, only to begin the process over again a few years later.

Another drawback relates to the costly production stoppages that follow the breakdown of machinery or the detection of an error. When that happens, humans must problem-solve and find the source of the problem. Often an error cannot be corrected, making it necessary to discard an entire run of expensive PCBs.

The Right Tools

On the other hand, human inspection, given the right tools, allows inspection of PCBs in a simple, cost-effective manner, with minimal training and without the worry of machine downtime. There is no software to purchase and upgrade, no highly paid personnel to train and no expensive machines to repair or replace. If a human eye detects an error, rather than a machine, it can often be corrected quickly and inexpensively with little downtime.

However, with human inspection, ergonomics becomes an important consideration. Unless the proper tools are supplied, human inspectors are likely to suffer from eye fatigue or chronic neck ailments from tilting the head to inspect the underside of boards.

That’s where the magical combination of simple mirrors and markers comes in to play. They enable inspectors to quickly check and mark components without eye stress and neck strain. Fluorescent markers are available in a range of very bright colors that are very easy and fast to spot, reducing eye fatigue. They may be used to mark the location of an error, to mark a part as error-free or to mark it as having been inspected by a particular individual. Tiny mirrors that fit into extremely tight spaces allow the inspection of the underside of PCBs without tilting one’s neck and head and without tilting the board. Particularly today, as the real estate on boards becomes tighter and tighter, mirrors make it easy to inspect even the tiniest section of the boards. Since the mirrors have very bright, sharp reflections, inspecting and spotting errors can be accomplished without eyestrain. Mirrors and markers can easily be used together. The inspector uses one hand to hold the mirror and the other to hold and squeeze the marker to deposit the desired type of liquid.

Markers for Every Use

Markers are available in an assortment of different options including a wide variety of colors (including custom color), inks and needle tips (see Figure 1). They may be obtained already filled with ink, or ink may be purchased in pint or gallon sizes. Air-dispensed syringes are also available, as are empty pens for manufacturers who prefer to use their own marking liquids.

Liquid formulas include permanent ink, removable ink, solvent-wash off ink, ink that washes off in hot water, solvent-resistant ink, solder fluxes, solder mask, adhesives and lubricants. In addition to a wide variety of fluorescent and neutral colors (red, green, orange, blue, yellow, brown, violet, grey, black, white, gold, and silver, invisible ink is also available for making discreet marks that can only be viewed by the original manufacturer. While invisible ink is not detectable under normal light, it appears as a bright, fluorescent blue when placed under a UV light. Invisible ink is popular for quality control marks or to mark the manufacturer as the part’s origin. For instance, when a customer returns a part, it can then be easily identified as having originated with the company or being a competitor’s product. The various colors of visible inks may be used to identify different inspectors or to distinguish between types of errors or locations on boards, for example.

Types of needle tips include a standard tip for general use, a micro-tip for very small markings and a long tip for hard-to-reach places. The standard needle produces about 12,500 dots per pen, the micro tip, about 25,000 dots per pen. The micro-tip pens are particularly handy to use on cell phone boards and other tiny components. Markers that allow upside-down writing are also available.

Invensys Controls, for example, uses markers with regular tips and orange-colored ink to mark the solder joints on smoke detector boards. “The inspector makes a mark next to the solder joint as evidence that it has been inspected and has been found to be good,” says Bernardino Morales of Invensys’ Manufacturing Engineering (see Figure 2).

Permanent ink can only be removed with a chisel. However, removable ink is easily removable with any kind of friction; for instance, it can be brushed off, flicked off or rinsed off. Solvent wash-off inks have excellent adhesion until they go through a solvent wash. Solvent-resistant ink resists all solvents, but comes off in hot water.

Mirrors for Easy Access

Mirrors are available in a whole assortment of options and may be custom-designed as well (see Figure 3). Mirror options include the size, shape, angle and thickness. The thinnest mirrors are as thin as two human hairs. The size of mirrors ranges from .372 square inches to less than one millimeter for boards with tight real estate. The tiniest mirrors are used in conjunction with a microscope. Lighted mirrors are also available for hard-to-see places.

Mirror handle options include plastic, stainless steel pin vise (non-magnetic for applications in clean rooms, the medical industry and similar environments) and black steel. The pin vise handle also makes the mirror telescopic for hard-to-reach places. All mirrors are designed for use by both right-handed and left-handed individuals.

The best mirrors have their coating on the front surface for a very bright, clear reflection. Rear-surface coated mirrors may produce double images because one looks through the glass first and the image reflects off the coating in the back. Other options are compound mirrors that go around more than one corner, fiber optic lighting, scratch-resistant diamond coating and autoclaveble mirrors.

Mirrors are much more cost-effective than automated inspection equipment. They can be used to see under components, under J-leads, soldering, and in between BGA rows without having to tilt the board. In the inspection of BGAs, the I84BGA mirror replaces the slower and more costly X-ray machines, making it possible to inspect in between the rows.

A Winning Combination

Markers and mirrors are much more cost-effective for inspecting PCBs than expensive automated equipment. While it is expensive to maintain and train personnel to operate inspection machines, training someone to use the markers and mirrors can be accomplished in a matter of minutes. Markers and mirrors are, of course, much less expensive to purchase and replace than the latest electronic devices and software. Marking pens are also much faster and cost-effective than the adhesive dots and arrows used in the past. Where necessary, markers and mirrors can be used in conjunction with automation. When a human inspector detects an error, production equipment can be quickly stopped to prevent the costly production of many more defective boards.

Back to Basics

In the past, the human eye was the only inspection tool. Then automation became ubiquitous, proceeding to replace human labor in every field of endeavor. Although the benefits of automation in general are undeniable, in some fields they are more of an illusion encouraged by the makers of automated devices. In PCB inspection, for example, automated equipment has its place in high-volume runs--if quality is not absolutely critical. When boards must be 100 percent defect-free, no machine can beat the human eye or a human’s ability to make judgment calls and reach decisions. Humans are still the best, most reliable and often most cost-effective way to make an inspection. Now that ergonomic, precision hand-held instruments are available to reduce the stress and fatigue associated with human inspection, there is really no reason to use expensive automation unnecessarily.

Charles Kempf is President of Metron Optics, Inc., a company that has been manufacturing products for the quality assurance market since 1971. Metron Optics won Frost & Sullivan’s 1998 Market Engineering Entrepreneurial Company Award “for developing a manual process to inspect BGA packages that is far less expensive than the more common X-ray equipment.” For further information, visit www.metronusa.com or call (877) 523-0923.


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