DPM Barcode Basics: Cell Modulation
The increasing use of DPM barcodes in a variety of applications is also giving rise to questions and concerns about why they do not always work as expected. Isn’t the whole point of direct part marking to enable these symbols to be imaged onto virtually any surface and still be scanable? The simple answer is—no.
Think about how a scanner works (this is not rocket science). The scanner captures a digital image of the entire symbol. In DPM usage this is usually a Data Matrix Code. The collected data is analyzed by a decode algorithm that is looking for reflectance differences between the symbol and the background. This is no different than linear barcodes and laser scanners of 3+ decades ago: it’s all about reflective difference.
Now think about some of those DPM substrates: iron castings, engine blocks, aluminum sub-assemblies, plastic housings, circuit boards—almost anything you can imagine. An important key consideration of those substrates is whether the reflectivity is uniform. For example, a cast iron part is a lunar surface of bumps and crevasses; light reflects on all directions off the top surface of the bumps and reflects variably off the crevasses that encircle the bumps. DPM barcodes are imaged on a machined area, not onto the unimproved cast areas, to create a substrate with uniform reflectivity.
Printed circuit boards also present unique challenges. By the time the 2D symbol is ready to be scanned, the surface has been coated with a laminate—often a bright green, the PCB has been populated with its various components, sent through a high heat wave soldering operation which may have changed some of the surface coloration, the components themselves create a sort of canyon landscape at the bottom of which the symbol is positioned. Whether that symbol was imaged onto the bare, pre-assembled circuit board or as a final operation after the board is completed, the substrate should be uniformly reflective.
Ideally what the scanner captures has sufficient reflective difference between the symbol and the substrate to quickly and accurately distinguish one from the other. When the reflectivity of either is non-uniform, there is trouble. Even with DPM technology, there is a range of acceptability or tolerance for the high reflectance or light value. In other words, the light reflectance value—often the substrate or background beneath the symbol—should be uniform within a defined range. Defined by whom? In the case of DPM, the AIM-DPM (ISO 29158). Likewise there is a tolerance for the low reflectance reflective or dark value: the returning reflectivity signal to the scanner should be uniform within a defined range.
What causes these values to exceed these defined ranges and fail to scan? A visual inspection of the symbol with low power magnification will usually tell the tale. Sometimes the cause is more subtle. Recently we received a heavy casting with a dot peen Data Matrix code in a machined area. Although the substrate had significant machining marks, proper lighting negated those reflective differences: the symbol failed simply because one corner was located too close to the edge of the machined area, violating the quiet zone.