Assuring PCB reliability by building in additional tolerances
High reliability or HiRel is the nomenclature or class given to printed circuit boards (PCBs) designed and manufactured for mil/aero or medical electronics systems and equipment, and on occasion, for special OEM gear.
However, in some cases, vital steps in high reliability PCB design, fabrication, and assembly are overlooked for a variety of reasons -- inadvertently, to save time to get the project out the door, or just to save money.
Therefore, embedded systems developers must be vigilant and cast a wary eye on the processes and procedures that contract manufacturers (CMs) and EMS providers institute at those three product phases -- design, fabrication, and assembly -- to ensure his product is indeed highly reliable. A majority of these techniques and steps are based on a CM’s experience and proven track record since virtually none of them are found in textbooks.
As shown in Table 1 below, making sure there are added tolerances built into processes and procedures is at the foundation of high reliability PCBs.
Put another way, it’s going above and beyond the regular ones applied to commercial PCBs. The savvy CM or EMS provider injects additional reliability at those three stages with various key steps at each stage getting an extra five to 20 percent reliability factor.
Table 1: Increased PCB reliability can be added at key points in the layout design, fab, and assembly stages.
For hi rel PCBs, those extra steps are critical to ensure devices are safe and secure under harsh and rugged environments, as well as highly operational under mission critical conditions. At all times, it’s advisable to consult with the OEM to explain the significance of increasing PCB specifications and get permission to proceed with revised specs.
Hi rel PCBs such as mil/aero ones must meet the MIL-P-55110 fabrication standard and assembly standard IPC 610 Class 3 Rev. E. Beyond complying with these standards, it’s important to “beef up” those boards. In short, a beefed up PCB, either through hole or surface mount, means the CM or EMS Provider literally increases board specifications beyond the OEM’s minimum engineering specifications so that the board and its end product maintain high performance and optimum reliability, regardless of environmental, terrain, or temperature conditions.
Steps to take at the design stage
At design, for example, let’s say a board may be rated at five amperes. However, during pre PCB layout simulation, it’s best to add a 30 to 40 percent buffer. Thus, operational amperage is increased to six and a half to seven amps just in case the board is exposed to extraordinarily hot, rugged, and/or hostile environments as it reaches the limits of its original specification.
Also, if such a board is specified at six layers, an additional two layers should be considered to provide extra ground planes, if there is a chance of crosstalk occurring between different signals. The reason is to ensure clear signals with no crosstalk or mixed signals.
The more solid the ground planes, the better the traces are separated. Conversely, if there are 10 different ground planes or split planes on one layer, for example, they don’t provide a solid ground for traces to suppress signals. By designing in additional amperage and adding more ground planes, reliability increases by an estimated 20 percent.
When designing traces on the board, initial specs may call for them to carry 0.5 milliamps (mA) of current, for example, and 10 percent coverage or 0.6 to 0.65 mA is fine for commercial boards. However, for hi rel PCBs you want at least 25 percent coverage assuring at least 0.75 mA current capability for those traces.
Extra grounding and shielding on critical traces is also important to boost reliability by about 15-20 percent. Figure 1 below shows how a group of devices are protected with an aluminum shield.
Figure 1: An aluminum shield provides additional protection for a group of circuits.
Particular traces demand extra grounding and shielding to protect a digital signal going from point A to point B, for example, and to make sure it doesn’t get mixed with an analog portion of the board.
Countermeasures at fabrication
The military spec 55110 for fabrication has tighter requirements for tolerance towards acceptability of the board. Drill wander on board drilling, for instance, is considerably more tightly controlled for mil spec boards. One can apply these specifications to commercial boards at slightly higher cost. But this ensures the boards will be tested and will give better yields during in-circuit and functional testing.
The tolerances are tight in the fabrication process, which results in improved yields. By deploying the military standard as a typical environment for board layout, fabrication, and assembly, greater results occur for both reliability and yields. For example, clean layout design will achieve virtually perfect fabricated boards and with tighter military standard requirements and during fabrication will assist in providing better yields.