Tracking with Bar Codes

By Joe Thunhorst

ave you considered using bar codes as a system for tracking printed circuit H boards (PCBs) as they

go through production and/then out into the field? Perhaps your man- agement would like to have better control of warranty claims. Maybe the goal is to secure a contract to build boards for a major manufac- turer and the company requires your product to have a unique identity. Or maybe you’re having problems with component failure and you buy from more than one vendor. How do you determine which board is caus-

in quality. However, the main con- sideration is still the label and the message on it. Aside from labeling red-hot steel ingots, surviving the surface-mount production line is the most challenging label environment found. But it is getting easier and cheaper to label boards.

Labeling Material Kapton, or polyimide film, has for some time been the only material available that could withstand the temperatures of an IR oven. Kap- ton’s main drawbacks are its amber color and lack of opacity. For years, this translucent film has been coated with a white, print-receptive materi-

When using the proper resin-based ribbons, good contrast grades (97%) can be achieved with bar code verifi- cation equipment.

The best news is that the cost is now roughly 30% of the original ma- terial. With millions of boards being manufactured each year, these savings should now make previously deferred projects cost-effective investments.

Imaging Considerations All is not rosy, however. Diligence and prudence must be exercised. Ribbons and printers can play a major role in the success or failure of a system.

A wax-based ribbon/image will not survive the heat generated or the

ing the headache? cleaning chemistry used in a Did you research bar cod- surface-mount line. Ribbons

ing and decide that the im- that use a resin as the carrier provement in productivity have a much better chance of was insufficient to warrant surviving the process. But a the cost of increased labor resin ribbon makes the trans- hours and/or hardware? Or fer difficult because high worse, did you implement a temperatures are required system and find that the to carry out the transfer. hidden costs and snafu fac- Another alternative is a tors were more than you combination of wax and had bargained for? I know of at least a1 for imaging, a coating that is both resin that transfers cleanly a t a low- one corporate buyer who purchased expensive and limiting. Since the la- er heat and will survive the assembly a system to generate the tracking bel surface is rough, it did not lend process. data and a printer to produce the la- itself to thermal transfer imaging New ribbon products are also be- bels only to be told, after the first methods, which meant that on-de- ing developed that use an extremely breakdown, that he “really should mand labels were impossible to use. thin Kapton film as a carrier. The have” purchased two printers to cov- Sensing the potential demand for an image and film are transferred to the er downtime! That buyer now has a opaque Kapton, Du Pont developed label. This combination holds great pretty bad attitude. a “filled” product that could be promise for both image quality and

If any of this sounds familiar, , imaged with existing thermal trans- durability throughout the manufac- there is hope. Software to track the turing process. boards is readily available. Hard- The pale yellow material that was The printer also plays an impor- ware for data capture has improved developed retains the dimensional tant role. The print heads must be in both price and quality. Thermal stability of the original product and durable because the combination of transfer printers are improving (al- a heat resistance of up to 370°C, but the Kapton material and the resin though a backup is still necessary) it also has the opacity to allow for ribbon requires a maximum heat and label media with the ability to good bar code imaging. The surface setting. The printer should be capa- survive surface-mount assembly is is smooth and can be imaged with ble of reaching 76°C for good im- coming down in price and going up existing thermal transfer printers. age transfer. The printer manufac-

fer printers.

AUGUST 1993 CIRCUITS ASSEMBLY 6

turer is t h e best source o f this type of information.

Spacing or registration (image to label variance from top to bottom) can also be a problem. The manu- facturer should provide samples of labels or demonstration programs that illustrate print quality. Make sure that you specify the label size in accordance with your circuit board requirements. Generally, the

PCB requires a label no higher than a quarter of an inch and often less.

A demonstration run of about 50 labels is usually typical. However, if labels are to be produced in pro- duction quantities, a valid test of 5,000 or more is recommended. Since these labels are going to be produced in the shop, you’ll want to ensure that the printer is easy to

Booth #628 CON C E PTR 0 N I C D 1992, Conceptronlc, Inc

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CIRCLE 48

63 CIRCUITS ASSEMBLY AUGUST 1993

operate a n d n2a in ta in . Stopping the production l i ne while the operator adjusts the printer will make label- ing a nightmare.

One final point: touching the ma- terial with naked fingers leaves an oily spot. The resin from printing will not anchor to the oil and, there- fore, a void will be left in the print. Dust can also leave pits or voids. Op- erators should wear gloves and be extremely careful while loading the printer. The label converter (the ma- chine that marries the Kapton to the adhesive and die-cuts the label) should have a clean, dust-free area to ensure that the material reaches you in usable form.

Is it Easier? All of this information sounds pretty involved, but it’s important to know the pitfalls of label production and how to avoid them. Follow the sug- gestions and it will become easier.

Bar codes are the logical tool to

drive the building of a database of

product ion in format ion.

Preprinted labels can be pur- chased, and there are three good rea- sons to do so: security, integrity and the negative snafu factor. If, howev- er, you require unique or on-demand serial numbers, refer to the above points and skip the rest of this article.

Security. Since you’re committed to going through all this trouble to put a unique piece of information on the printed circuit board, it is as- sumed that you’d like it to stay there. The thermal transfer image is tough but not indestructible.

The first pitfall can be the clean- ing method. An aqueous wash is designed to dissolve resins on the solder mask. Guess what’s holding down the heat-resistant image on the label? A resin! Semiaqueous washes, depending on their pH val- ue, can destroy enough of the im- age to make it unscannable. No- clean technology or high-pressure

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Securing the image to the Kapton can only be 100% effective if a Kap- ton laminating film is used to protect it. This type of security can be achieved with a preprinted label. Since the over-lamination is added at the same time the adhesive is laminated to the back and the label is die-cut, the cost is minimal and the bar code is sandwiched between two tough, heat-resistant films. Some print contrast is lost, but the numbers are well above the ANSI minimums.

Integrity Number integrity is crucial because most process track- ing projects are implemented to con- trol the quality of the product. Au- diting a system where duplicate or missing numbers are “designed out” or “designed for quality’’ is a much easier proposition. The system may be doing everything right, but some- thing as basic as a connection, an

Aside from labeling red- hot steel ingots, surviving the SMT

production line is the most c h a1 lenging

environment found.

out-of-adjustment printer, a wrin- kled ribbon or a greasy thumbprint may distort the data or produce un- scannable codes. Even when these errors are caught, reproducing the Exact data and slipping it into the stream magnifies the possibility of sequence errors.

Preprinted labels can go a long way toward certifying that there are no missing numbers, no duplicate numbers and that all labels scan. To :nsure this, a fixed-mount scanner md a program that compares the la- bels produced with the labels or- dered is necessary. This confirms that your vendor is as careful with your data (yes, it’s data, not just a label) as you would like to be if you had the time and the manpower to review each label.

The Snafu Factor. You know that anything that can go wrong will md that it will do so at the worst ti- me-when you’re on vacation in

Pango Pango and your boss has to fix it.

Conclusion The solutions 10 PCB tracking with bar codes have become infinitely easier over the past three years. With quality issues becoming so im- portant, building a database of pro- duction information is a critical first step. Bar codes are the logical tool to

drive the building of these files and now the labels are easy to use and more reliable. Perhaps preprinted la- bel are a bit more expensive, but they are still a fraction of the cost they were three years ago. IC3

Joe Thunhovst is a regional sales manager with Watson Label Prod- ucts, St. Louis, MO.

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CIRCLE 49

AUGUST 1993 CIRCUITS ASSEMBLY 63