Concerns Surrounding PCB Assembly Perhaps the two most essential design issues for PCB assembly, after schedule and performance, are size and price. In quite a few cases, the PCB footprint is decided at the start of the project, as is the per unit assembly cost. Some lucky designers are only provided rough targets and told to “minimize" size and expense. The design issues are the same in any case. Here are a handful of ideas on how to approach the design. External Connections Should Be the First Step The size of the connections with regards to other boards or to the housing will determine the size of the boards on many occasions. There are constraints on connector positioning or orientation more often than not. Even in the best cases, the periphery of the board won't be smaller than the total of the connector widths. If at all possible, this is when you should be diligently challenging connector choices. For example, can an USB mini be utilized rather than a USB-B? Or can a BNC be replaced by an SMA? After these choices are made, designers can massage an initial connector replacement. Often, the overriding concern is to keep the power connectors away from the sensitive signal inputs. The associated sensitive components can be isolated by this strategy at the same time. Proceed to Component Choice The next largest contributor to both size and expense is the choice of components. The largest components will often be leaded discretes requiring through-hole assembly. These parts are very good choices for the early part of a product lifecycle since they make the boards easiest to debug. In terms of assembly, they are fairly costly since they require hand placement. Components which are Surface Mount To take advantage of the savings of automated placement and wave soldering during assembly, engineers can choose surface mount components. These also minimize the size of a PCB drastically. As long as components are not chosen too small (for instance, a 0805 resistor/capacitor package), the boards are still relatively easy to debug and to replace components manually, if necessary. Designers can also use mixed-mode designs when some components don't have a surface mount version. All the surface mount components must be added first, followed by a manual assembly step for the leaded components, if this is the case. In four-layer board design, the inner layers are usually reserved for power planes, leaving the bottom portion allocated solely to surface mount components. High-Density Components To attain further size reductions, engineers can select high-density components. One example of this strategy is the selection of integrated circuits. The utilization of leaded DIP packages for ICs is afforded with through-hole design. A surface mount version that can shrink sizes by over 50% is included in most modern ICs. One high-density version of an integrated circuit is a ball-grid array (BGA). In this case, the connections come from underneath the part instead of the side. ICs with lots of pins can achieve a size reduction to 10% of the DIP package or more. However, these parts XPtronics Inc.
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Concerns Surrounding PCB Assembly can't be soldered manually nor are they easy to debug. When a design is proven and mature, these are suitable choices. The smallest footprint is afforded with high-density components. However, typically a design loses the efficiency of small components if the designer is unable to manage the increase in the number of traces. Minimizing Cost and Size By employing an iterative design, beginning with the connectors and completing with the component choice and trace placement, generally, designers can reduce cost and size of PCB assembly. It's great strategy to evolve the design philosophy through the entire product lifetime, starting with leaded components, and then optimizing with surface mount and high-density parts when dealing with high volume products. Perhaps the two most essential design issues for PCB assembly, after schedule and performance, are size and price. In qu...
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