PCB design, fabrication, and assembly for mil/aero, medical, and other high-reliability applications are demanded considerably more than the routine processes and procedures being used for commercial applications. That’s because these special breeds of PCBs require the highest available quality, reliability, and consistency to comply with stringent and rugged environmental requirements.
Experienced EMS providers and contract manufacturers (CMs) know to apply special steps and techniques at layout, fab, and assembly to build in product reliability, and these steps and techniques must be implemented at various design and manufacturing stages to ensure high quality and reliability.
Experienced EMS providers and contract manufacturers (CMs) know to apply special steps and techniques at layout, fab, and assembly to build in product reliability, and these steps and techniques must be implemented at various design and manufacturing stages to ensure high quality and reliability.
During PCB design layout, a number of different techniques can be implemented--too many to describe here. However, the following are some of the more salient ones. Foremost is the use of mil-spec components with tolerances of 1 to 2% versus commercial ones, which have 10% tolerances. While they are more expensive, they are considerably more reliable with gold finish terminations rather than tin lead finishes on the leads.
Also worth noting is this next technique, which may sound amateurish and unnecessary to some industry observers. But having an extra set of eyes review the accuracy of the parts library after it has been created is a great idea. The importance of applying two or more sets of eyes (e.g., a designer’s and a checker’s) on the parts library, footprints, and pad stacks helps to prevent mistakes. One designer can make the pad stacks, for instance, and another can perform verification to ensure accuracy. If pad stacks are not made correctly, internal shorts on power and ground planes may result.
Here, it’s important to pay close attention to tolerances, as multiple suppliers may manufacture the same component. Those different manufacturers may have varying mechanical, electrical, footprint and temperature tolerances, some of which can adversely affect a placement of the component if they are not taken into consideration during the design layout phase.
This is especially true for BGA, CSP, QFN, and flip-chip packages because room for error is considerably smaller and limited. These components cannot be seen via the naked eye; X-rays or other devices must be used to clearly view those components. But if the wrong footprint or pad stack is created, there is no recourse but to perform costly rework, which in some extreme cases may not even be possible.
Pay special attention to detail when creating fabrication and assembly notes. An EMS provider should be required to comprehensively detail IPC or mil-spec standards along with their respective classes, (e.g., adding a note specifying that the PCB must comply with MIL-PRF-31032). There should not be any question or ambiguity, and notes should clearly state the required standard and its class. Also, as shown in Figure 1, datum points and definitions must be properly noted in fabrication and assembly drawings to avoid confusion. A datum point is the center from which all dimensions are calculated; it is important for it to be precise and clear
