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Tech Tips

Akber Roy

Are your components in balance?

Warping of flat or planar printed circuit boards is a significant yet common problem when manufacturing and assembling PCBs.

Warpage can prevent pick-and-place machines from accurately placing components. Warpage can cause the lead-free wave solder machine to pick up solder and flood the board with solder. Even worse, a warped printed circuit may not fit in the case or cause problems with automated handling equipment (buffers, etc.).

A few items cause warpage, all known and preventable. The primary reason why a PCB will warp is uneven or imbalanced copper percentages in different layers. When a design is presented to a PCB manufacturer, the fabricator will run a check of the percentage of copper on each layer. This is to ensure the design is balanced; i.e., the copper plane percentages are even about the center. Consider, for instance, a typical 8-layer PCB. A copper power plane would be on layers 4 and 5 and signal layers on the remaining layers. The innerlayer 4-5 has an almost full copper percentage on both sides. The remaining layers are low-copper-percentage signal layers. The stresses locked in by lamination will even out or equilibrate.

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Akber Roy

An hour-by-hour look at the quickturn fabrication process.

We were presented with a challenge: Is it possible to build 10 prototype 12-layer boards in 72 hours? It wasn’t a rhetorical question; a customer really wanted just such an order. So, with time at a premium, our engineers put their heads together and created a “plan of attack” that optimized all resources. One key to success is performing a number of the steps in sequence as needed, so panels are ready when they are required. We’ll describe the procedure hour-by-hour as follows:

Hour 1: The CAM operator runs a DRC (design rule check) process and accepts or rejects the data files. If a problem exists, they contact the buyer to work out a solution; e.g., if two traces are too close, and one needs to be moved. Once the data file is accepted, the next action is to set up the innerlayers.

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Akber Roy

More lasers and improved and integrated software have factories humming.

The methods and equipment used to fabricate PCBs are becoming increasingly advanced and centralized. For example, computers, lasers, and AI are ever more common in all areas of PCB processing. In recent years, a considerable number of PCB manufacturers have invested heavily in the integration of the complete shop, with all equipment controlled by one central computer. The interconnection enables quicker file processing, higher accuracy, and improved yields.

One of the most expensive pieces of production equipment is the laser-direct imaging system (LDI), which has made significant improvements in accuracy, speed, quality, and in reducing overall manufacturing rejects. The newer models feature multiple cameras to locate lamination holes, compare them to the original Gerber file, then digitally scale the image to fit the panel. Newer laser imagers are capable of imaging down to 15µm line widths and spaces.

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Matt EvelineBuild-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.

 

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Akber RoyThe 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.

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Cleanliness is next to stickiness.

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