Assuring PCB reliability by building in additional tolerances -

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.

Marshalling resources at assembly
At assembly, a number of steps must be deployed — both small and large to assure nothing has been left to chance. For example, when it comes to printing, fully automated machines with advanced vision systems are critical to properly align the fiducials.

Also, at this step (Figure 2, below ), it’s important that either the printer efficiently measures the proper amount of dispensed paste on an empty pad or a standalone paste height inspection machine is used for that purpose,

Figure 2: Paste height machine efficiently measures the proper amount of dispensed paste on an empty pad.
After paste height inspection is completed, placement is performed followed by automated optical inspection (AOI). Right before reflow, AOI ensures all placements are proper with an absence of angles, skews, misplacements, and/or polarity issues.

But even if there are problems, they are relatively easy to adjust the SMT placement program without making any further chances. Enough cannot be said about the importance of AOI after placement, but before reflow. It is a critical step and plays a key role in regulating quality and reliability.

Thermal profiling and the correct one are at the top of the list as well. Normally, for commercial standard products, one to two iterations of the first article are executed to verify the correct thermal profile. But for a hi rel PCB, three to four iterations of the first article are mandated to assure that a perfect thermal profile is created and subsequently verified.

Other key steps to increase board reliability include applying thermal compound to insulate heat sinks, soldering swages to the pad and PCB, pre-tinning stranded wires, extra spacing between component and PCB, correct top side board temperature to assure sufficient solder, and conformal coating.

As shown in Figure 3 below , thermal compound properly insulates heat sinks from each other, But more than that, it reduces vibration, which can create cracking on the lead solder formations. Plus, thermal compound acts to effectively dissipate heat from adjoining components. Applying thermal compound to insulate heat sinks make this assembly step an estimated 10 percent more reliable.

Figure 3: Thermal compound properly insulates heat sinks from each other. It also reduces vibrations leading to possible lead solder cracking.

Flange mounts are normally press fit on the board for commercial applications. However, with a hi rel or mil/aero PCB, once flange mounts are press fit, each swage is hand soldered to the pad and the PCB. This assembly step minimizes vibration and in doing so, the reliability at these points on the PCB is increased by an estimated 10 percent.

Pre-tinning is a process that involves dipping stranded or braided wire into hot melted solder, removing it, and drying it so that the solder formation solidifies. In effect, soldering transforms stranded wire into one solid wire to eliminate air gaps and to allow current flow to be more stable. Thus, electromagnetic interference (EMI) effects and attenuation are reduced and reliability at this point is increased by about five to 10 percent.

Next, if certain components dissipate more than one watt of heat, one of two remedies may be applied to minimize heat. Such components can use a spacer between the heat sink and the component, itself. This produces the desired spacing, allowing component heat to be thermally dissipated, rather than permitting it to adversely affect all the circuits on that PCB.

This spacing also acts as stress relief so that if the PCB and its end product are smashed or dropped in extreme conditions, there will still be sufficient clearance to protect components and keep the product operating. By applying this spacing, an estimated 20 percent reliability increase is realized.

Further quality and reliability assurance is applied when PCBs with through-hole components undergo wave soldering. But here, it’s important to perform a first article profile.

Running a first article through the PCB shop prior to running the actual job ensures that thermal profiles are properly studied and correctly dialed in and all the kinks of the assembly have been worked out, as well.

The objective is to get the correct top side temperature of the PCB to make sure there is more than sufficient solder wicking up to the board’s top side. This ensures solid and reliable connections and provides an estimated five-percent increased reliability.

Some hi rel PCBs are frequently subjected to long and continuous exposure to water, moisture, and extreme humidity and temperature. They can be made more reliable by an estimated 10 to 15 percent by ensuring they have sufficient conformal coating applied with high quality acrylic-based sprays to protect them from these harsh elements.

Take for example a motherboard for an underwater mine detection device. This particular military product remains underwater for most of its life. To assure reliable operation, sufficient PCB design and assembly precautions must be taken to protect its PCBs and circuitry from even the smallest amount of moisture.

Another assembly step to increase reliability by an estimated 10 to 15 percent is to increase solder paste volume by 25 percent more per square inch of area unit compared to commercial PCBs.

Also, for acceptable quality levels, (AQL), mil/aero boards must undergo 100 percent inspection to assure 100 percent reliability compared to the commercial AQL standard of 0.65. To meet MIL standards, PCBs must have zero defects.

Zulki Khan is the Founder and President of NexLogic Technologies, Inc ., San Jose, CA, an ISO 9001:2008 Certified Company, ISO 13485 certified for medical electronics, and a RoHS compliant EMS provider. Prior to NexLogic, he was General Manager for Imagineering, Inc., Schaumburg, IL. He has also worked on high-speed PCB designs with signal integrity analysis. He holds B.S.E.E from N.E.D University and M.B.A from University of Iowa and is a frequent author of contributed articles to EMS industry publications.

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