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High-Reliability PCBs in Mil/Aero Applications
March 19, 2013 |Estimated reading time: 9 minutes
Editor's Note: This article originally appeared in the January 2013 issue of The PCB Magazines.
PCB design, fabrication, and assembly for mil/aero, medical, and other high-reliability applications demand 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.
At Layout
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.
Figure 1: Datum points must be properly noted in fabrication and assembly drawings.Blind or buried vias should be avoided wherever possible in aerospace PCB applications, because they limit the number of manufacturers that can fabricate these bare boards with utmost accuracy. On the other hand, through-hole vias with a lot of plating wall are considerably more reliable than the smaller blind or buried via.
In some cases, it’s well worth going the extra mile during layout. For example, the electrical engineer’s function is to create the schematic and select the right components. However, at layout, the PCB designer can spend a little extra time verifying that a particular component does in fact comply with its associated temperature, electrical and power ratings. This added step makes sure the component operates correctly and effectively in extreme temperature conditions in rugged aerospace applications.
Safe and continuous operating temperatures must be maintained for components and PCB material, alike. This means keeping a sharp eye on coefficient of thermal expansion while the layout is being performed. In this regard, the aerospace PCB designer must constantly be aware of thermal matching. When through-hole, glass-based components are used together with surface mount, ceramic-based components, there will be different coefficients of thermal expansions. The differences or mismatches in thermal expansion can create a fractured joint in extreme thermal conditions. Care should be taken to review and verify these mismatches and efforts made to reduce them, when and if it happens. Failing to do so can result in fracturing components.
Figure 2: Fractured component due to overheating.
Also, a major requirement for maintaining a coefficient of thermal expansion is to ensure thermal relief is properly designed in. The goal is to greatly reduce thermal stresses to improve component and product reliability. One heat dissipation technique is to have a gap between the PCB and component. That can be implemented by using a clamp or thermal mounting plate, which increases conductivity area for dissipating the heat.
At times, depending on the aerospace application, an experienced EMS provider can provide indirect cooling such as air or liquid-based cooling. Also, for the more traditional PCB design, applying heat sinks on external planes is vital to dissipate heat generated by circuitry to ambient atmosphere. Special heat sink fixtures play a part in thermal relief, as well. At times, they are connected to the metal frames. Another associated design consideration deals with heat sink mounting and specific board areas from where heat is to be dissipated. This step involves hardware and ensuring heat sink soldering at proper locations.
In some cases, residues of processing agents or solutions can remain under a heat sink and components such as BGAs or CSPs. Therefore, it is important to include special instructions in assembly notes to make sure cleaning is performed under devices such as vertical heat sinks (Figure 3). Sometimes it’s a good idea to dissipate the heat using a thermal transfer medium such as thermal grease or boron nitride (Figure 4). These solutions can be used to make the heat flow from one segment of the circuitry generating more heat towards the cooler part of the circuitry and eventually into the atmosphere.
Figure 3: Assembly notes must include instructions to clean residue from under vertical heat sinks and associated devices.
Figure 4: Thermal grease is used to dissipate heat.
At Fabrication
Well documented fabrication notes are essential for detailing all the specifications and ensuring that a PCB is correctly fabricated to comply with aerospace specifications. This includes PCB surface finish considerations. Normally, HASL or gold are recommended for long-term reliability and not necessarily silver or OSP. Silver is to be avoided due to short shelf-life and the possibility of corrosion.
Avoiding warpage is an important factor in some aerospace applications. When temperatures are extreme, and copper distribution is not balanced on the board’s surface, the board can warp. So the limit for warpage can be made even tighter, say by 20%, especially for boards less than 62 mils thick. If the board warps, the integrity of components’ connection solderability is at risk. Poorly soldered joints cause mechanical stress, and eventually, the joint cracks, compromising the joint assembly.
At Assembly
Aerospace applications demand consideration of multiple factors at the assembly stage. Up front, the PCB application has to be clearly defined either as a J Standard or IPC Class III. There may be slight differences, and depending on the application within aerospace circles, some applications would suffice as Class III versus others as J Standard. So it is important to define the PCB based on the application.
At times, in aerospace applications, non-standard parts and components are used. Availability can be an issue. But an even more important issue is the testability of that part after assembly has been performed. If non-standard parts are used, it is important there exists some historical data available for testing that part and assuring it will have longevity. Gathering that historical data can be challenging at times.
Always check to see if the assembly drawing requires components to be fastened or tied down. At this point, assurances are made for properly defining a complete bill of materials, including hardware fastening material, thermal grease, epoxies, spacers, and other ancillary items that are part of the complete product build. It’s important to review these details at the beginning of the assembly stage in order to ensure acceptable and repeatable assembly on a consistent basis.
Tall components, fixtures, complex packages, and conformal coatings are special factors, and they require extra attention. Tall components will likely require tie-downs so that they remain sturdy and withstand mechanical stresses. Moreover, they will need special handling during flying probe or ICT testing, since tester probe movement could be restricted due to component placements. The use of fixtures is a good idea for aerospace applications because they support consistency, speed, and reliability. Fixtures to be considered should be used for wave solder, pick and place, and AOI.
Proper and reliable inspection of such complex packages as BGA, QFN, and flip-chip commands the use of advanced X-ray machines with acceptable tolerance limits defined (Figure 5). For example, when a void is detected, what kinds of void calculations are specified? What are the acceptable limits? When BGAs are inspected, void calculations must be tightened to acceptable limits for the aerospace applications.
Figure 5: Inspection of complex BGA, QFN, and flip chip packaging demands the use of sophisticated X-ray machines.
Conformal coatings, also known as potting, are used for protecting the boards from agents such as moisture, dust, chemicals, and extreme temperatures. There are different kinds of conformal coating techniques, including brush, spray, and dipping. These techniques are based on cure time, substrate type, and type of drying agents to be used. The type of coating to be used depends on the particular aerospace application. And, there are acceptable criteria for conformal coating from different agencies like IPC, UL, and ASTM. These agencies provide the definitions and criteria for usage, which depends on the type of material used for conformal coating, shelf life, viscosity, appearance, flammability requirements, moisture, and insulation resistance.
Testing
Here, the focus is on implementing design-for-test (DFT) techniques, which originates with layout of the PCB for an aerospace application. At this stage, different nets are being probed with test coverage being as close to 100% as possible. Temperature cycling for an aerospace PCB is considerably different than that for a commercial one, which goes through a normal temperature cycle.
However, for aerospace PCBs, extreme temperature cycling is essential. Thus, procedures for temperature cycling must be defined. The same is true for power cycling; an aerospace PCB has to be exposed to different power levels for optimal performance under extreme conditions.
As for aging testing, different test labs can age boards by running them through various kinds of stress testing, such as mechanical vibration, and temperature power cycling. It is a good idea to expose an aerospace PCB to ensure all critical steps and techniques have been implemented to attain high-quality and reliability goals prior to production.
Zulki Khan is president and founder of NexLogic Technologies, Inc., in San Jose, California. He may be reached at zulki@nexlogic.com.