Build-in cleaning time during chemistry changeovers and after extended line shutdowns.
Much time and planning are invested in the choice of the ideal conformal coating material and process to adequately protect printed circuit boards. This often includes multiple qualification trials. There is also sometimes long and detailed testing in areas such as electrical performance, flame resistance, and thermal or mechanical cycling. Unfortunately, the qualification and testing process for conformal coatings is simply a snapshot of the process at the start. To maintain consistency, an often-overlooked activity remains: regular cleaning and flushing of the selective conformal coating equipment.
In general, the following comments and guidelines are designed for a discussion involving typical modern selective coating equipment (FIGURE 1). However, nearly all the principles are applicable to manual spraying operations as well.
The basics of bare board laminate selection.
Selecting the right materials for a printed circuit board assembly is essential to ensuring the robust reliability of the product and circuitry throughout the entire life of the equipment. In other words, it’s not just how you build it, but what you construct it with. A wide range of substrate materials is available, with different specific properties, and these properties are tied to the expected performance and environment for the circuit being built. Will the PCB assembly be a high-power circuit? Harsh environment capable? High-density?
Engineers specify the material for PCBs with great care and detail. TABLE 11 lists laminates by their industrial names and material types, and specifies material characteristics related to each type. However, to reach that level, it is necessary to understand the various characteristics such as thermal decomposition temperature (Td), glass transition temperature (Tg), dielectric constant (Dk) and dissipation factor (Df) related to PCB materials. Although such tables are important, it is easier to start with a higher-level perspective of selecting materials depending broadly on their actual use, based on a comparison of PCB materials.
Cleanliness is next to stickiness.
Surface roughness can increase electric field strength and capacitance.
Conductor surface roughness directly interferes with conduction in high-frequency circuits. How is this? Surface conductivity (or RF resistivity) of a metal film is a function of frequency, as conduction decreases exponentially from the surface into the film. But it’s a Catch-22; the “roughness rule” states the rougher the interface between metal and substrate, the better the adhesion, but the higher the attenuation.
Scientists have long studied the effect of grooves present on the surface of a conductor, having noted the additional losses through the conductors caused by them. In worst-case scenarios, the grooves cause losses that sometimes reach a factor of two. The explanation proposed was electromagnetic (EM) waves travel mostly along the surface of a conductor; e.g., the copper signal trace. The grooves effectively cause the signal paths to become longer, as the EM waves, while traveling along the surface, enter in and then exit from the grooved shapes.
Could vendor collaboration get to the root cause of an intermittent soldering problem?
Simply put, the ultimate function of a PCB assembly line is to create millions of solder joints without error. This task is complicated by the myriad materials that come together during assembly, and relies on the quality of each lead, pad and sphere to be soldered. When a soldering defect is discovered, it is common practice to presume the soldering materials are the cause, which seems logical, considering it is a solder defect. This assumption is often misplaced. This scenario plays out regularly, as illustrated in a recent case submitted to our failure analysis team for diagnosis.
In this case, the assembler had an intermittent solderability issue with a component. It brought the problem to its local representative’s attention on several occasions. The issue was isolated to a single component and was repairable at the rework station. It was a nuisance but didn’t interfere with production schedules. The situation was difficult to address; it was intermittent and subtle, but persistent. And each time the representative brought its solder supplier’s field engineer for site visits, the solderability issue was not present. Solder companies detest these types of issues because they have a negative effect on customer satisfaction and product perception.