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Soldermask: it's not just green anymore whether it's to identify revision changes or to look trendy in a see-through box, soldermask is taking on a rainbow of colors. But your processes will have to change with the hues.

For a seemingly endless time, soldermask has been green and only green. But all that has started to change.

If you're wondering why anyone would even bother using a soldermask of a different color, here's one example of a reason for using a variety of colors. It was 1997 in the lobby of the headquarters of Apple Computer. Jack Evans of Marcel Electronics and one of Apple's commodity managers were comparing soldermask samples with the colored banners hanging from the rafters. The challenge was to produce printed circuit boards that would match the colors of the new Apple iMac whose new look put a dent in the design world from that day forward.

Since then we have seen a trend towards clear or colorful electronics, in everything from toasters to blenders and from phones to computers. With OEMs trending toward clear appliances and electronics, boards are taking on much more of a personality. Any marketing professional will tell you if you're going to see the guts of something, it better look interesting and it had better be cool.

Green has been the color of choice for soldermask for as long as most of us can remember. There are a variety of theories as to why green was chosen (See "Color your world.").

Today there are a variety of commercially available colors from which to choose. Alternative colors certainly aren't new. But the past several years have seen a growing use of colors other than green, primarily driven by assemblers and OEMs.

This article will explore the reasons for choosing alternative colors, provide guidance on choosing the appropriate color for a given application and suggest some ways to minimize the impact of running this specialty product in your shop.

Design and appearance are a few of the reasons why circuit boards of different colors are demanded by end-users today. The more common and very practical reason many companies are using different colors is to provide a highly visible indicator of revision changes, or prototype runs.

There are also cases where the light reflectance, absorbance or transmission properties have an impact on assembly equipment or the end use of the board itself. Clear soldermask is sometimes used in cases where the fiducials used for the optical alignment of components must be covered with soldermask. The contrast between the soldermask and the laminate can also play a role in the ability to read laser-ablated bar codes. Applications in which the board is integral to some type of display can also generate a requirement for a color other than green in order to produce the desired contrast or brightness.

With the oncoming storm of lead-free assembly processes, alternative colors could play a big role in identifying lead-free assemblies, helping to avoid mistakes in a mixed-assembly environment.

The Right Color

The most commonly used colors are clear (technically not a color), red, blue, yellow, black and white. Given that the preceding list includes the three primary colors, a variety of others are possible simply by mixing them at the correct ratio.

Unfortunately, not all colors are created equal. It is important for the end-user and PCB designer to recognize that there are tradeoffs to be considered with certain color options. Resolution is typically the sacrifice in question. Black, yellow and white are notorious for poorer resolution than other colors, since it is difficult for UV light in the appropriate wavelength to penetrate the entire coating. Red and blue will usually provide resolution equivalent to that of green, while clear is often superior. Changes in color can also occur during the numerous thermal excursions and chemical exposures a board sees. Clear and white are the most susceptible to color shifts.

The performance of a given color is also driven by the quality of the soldermask used and the care with which it is processed. Top suppliers provide a wide variety of colors and the process support to make them work on a day-to-day basis.

By now the collective groans of production supervisors can be heard around the world. Introducing a variety of soldermask colors will impact the production environment in the same manner that any other custom process does. Integrating small production runs with a different color into the existing soldermask process on automated coating equipment can cause tremendous disruption.

The most common way to deal with this is to coat the soldermask using a manual screen table or semiautomatic screen printer that can be quickly cleaned and set up. Duplicate troughs and screens can speed up changeover on vertical, double-sided screen coaters and, in the case of spray equipment, a separate set of hoses can go a long way toward minimizing the disruption. Some fabricators will attempt to schedule jobs so that several of one color can be run at the same time, allowing for use of automated equipment with a minimum of downtime. While this is certainly effective, in today's world of time-sensitive products, it is not very practical in most cases.

Hand-screening LPI soldermasks has always required special attention. Thickness uniformity problems can easily occur, resulting in resolution issues, excess mask in holes, and air entrapment just to name a few. When hand-screening colors other than green, it is extremely important to control thickness, especially when using resolution-sensitive colors such as black, yellow and white.

Any single-sided screen coating process will inherently deposit more material in through-holes than a double-sided process. Excessive mask in holes can have a tremendous impact on the obtainable resolution of any soldermask, due to the long developer dwell times required to remove the mask from small vias. Combine this with a material that has inherent resolution limitations and disaster is just around the corner. Minimizing the number of print strokes and the pressure used will help. Use of a dimple plate for coating both sides prior to tack dry will allow the fabricator to optimize the tack dry cycle and avoid overdrying the material. A last resort for colors such as black is to create a dot pattern stencil that will eliminate mask deposit in the holes, assuming the registration of the screen to the board is accurate.

The exposure process will likely be affected as well. TABLE 1 shows some general exposure guidelines with green as the baseline.

Obviously, the above information is very generic. The particular soldermask used and the process employed will have significant effects on performance on the shop floor.

The Business Perspective

At this point you may be asking, given the hassles described above, why even bother with alternative soldermask colors? Frankly, by offering a rainbow of colors, the fabricator will endear himself to his customers. He will demonstrate quite literally that he is very flexible and willing to do whatever it takes to please his customers. He should take advantage of this trend and treat it as an opportunity to differentiate his products and services from the other guys.

Let's face it, the company that embraces the concept of giving the customer what he wants will entrench itself with that customer for years to come. Providing special services to your customers is one of the most effective methods of differentiating your company from the plethora of competitors. Most likely you are already doing this in a variety of ways. Providing a choice of colors in soldermask is just one more advantage for your sales force to tout.

Adding additional processes or disruptions always increase cost. It is certainly reasonable for a supplier to expect fair compensation. If putting red soldermask on a board prevents the scrapping of even one assembled part, the savings are many times the premium added by any fabricator. In the same vein, if a color other than green makes the end-product stand out at the consumer level, the value can be immeasurable.

Designers and OEMs have discovered that it is possible to call out an assortment of colors when designing their boards, and most importantly, their end-products. When it comes to using different colors of soldermasks on a PCB, we've only just begun. As mentioned earlier, there are some companies such as Apple who do it just for pure aesthetics and design, though it also helps in the marketplace.. We predict that we will see a great deal more of this in the very near future.

The majority of companies currently using colors other than green for soldermask seem to be using the color as a unique and obvious identifier during assembly and beyond. They use red, blue and yellow in the prototype stages of the design, and then settle on green once the design has stabilized and they go into production.

Bringing a variety of colors into your soldermask offering is not without some pain, but it isn't rocket science either. With some planning, a little engineering and a flexible supplier it can be relatively painless and enhance your business.

Flexibility is a hallmark of North American business. In our industry it has become a way of life and, ultimately, of survival. Offering alternative colors may not be as sexy as some of the other unique things you do, but it is one more item you can easily add to your portfolio of services.
TABLE 1. Exposure guidelines (compared to green)

SOLDERMASK EXPOSURE REQUIREMENT RESOLUTION
COLOR (COMPARED TO GREEN) CAPABILITY

Clear 20% less Equivalent or better
Blue Equivalent Equivalent
Red Equivalent Equivalent
Yellow 2-3X Significantly less
White 2-3X Significantly less
Black 2-3X Significantly less


ACKNOWLEDGMENT

The author would like to thank Pete Binzel and Dan Blattman of Taiyo America for their contributions to this article.

COLOR YOUR WORLD

Have you ever wondered why soldermask is green to begin with? Why didn't we start out with soldermask that was blue, yellow or red? A number of years ago a there was a discussion about this issue on IPC's Technet forum. Here are some of the most interesting quotes from that discussion. Which one--if any--is correct? You decide.

"... it goes back to the old military requirements which said that you could have any color soldermask as long as it was green."

"Soldermask is green because when it was originally produced, the base resin was brownish yellow in color and the hardener was very muddy brown and when you mixed them together you got green. Also, laminates at that time were mostly green so it was easy to accept the idea of green soldermask as well"

"The green color of soldermask was chosen after extensive testing by the US military at the National Materials and Procurement Center in Cedar Bluffs Virginia in 1954 ... That particular shade of green produced the right contrast with the white legend ink while being tested under all types of adverse conditions. Every other color at that time failed to produce the same contrast under the same extensive testing"

"Soldermask is green because green has been proven to be the color most visible to the human eye. Individual colors have specific wavelengths, but combinations of wavelengths produce differences in hues and intensities. Yellow and green colors are the easiest to see in normal light. Thus green is the easiest color to see, the color easiest on the eyes and so the color best suited for board inspectors and assemblers"

There you have it: just a few theories about why green soldermask was the only choice for so many years. Do you agree or disagree with these theories? Do you have a different theory? If so I would love to hear from you.

JODY WILLIAMS is the new business development manager at Taiyo America. He is a 20-year industry veteran and has held a variety of technical and management positions for suppliers and fabricators. Williams can be reached at jodyw@taiyo-america.com.
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Title Annotation:Coatings
Author:Williams, Jody
Publication:Printed Circuit Design & Manufacture
Geographic Code:1USA
Date:Mar 1, 2005
Words:1945
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