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Design, Fabrication, Assembly What Changes for Harsh-environment Electronics?
December 31, 1969 |Estimated reading time: 6 minutes
By Zulki Khan, NexLogic Technologies Inc.
Manufacturing harsh environment electronics takes into consideration a number of key aspects dealing with PCB design layout, fabrication, and assembly. With your OEM's approval, a reputable EMS provider should actually increase board specifications for this sector. The EMS provider that goes beyond an OEM's minimum engineering requirements and implements additional safety ensures that the board and end product maintain high performance and optimum reliability, regardless of environment, terrain, or temperature conditions.
The three areas for injecting additional reliability and safety are at board layout design, fabrication, and assembly, with various key steps at these stages getting an extra 520% reliability factor (Chart 1). In all cases, it is important for the EMS provider to consult the OEM customer to explain the importance of increasing a board specification and to obtain permission to proceed with a revised specification.
Design PhaseLet's say a board may be rated to operate at 5 A at design. However, during pre PCB layout simulation, it's best to add a 2030% buffer or cushion as a precaution in the operational current. Thus, operational amperage is increased to 6 to 7 A 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 pair of two layers should be considered to provide extra ground planes, if there is a chance of crosstalk occurring between signal paths of different layers. The reason for adding these layers is to ensure clear signals with no crosstalk or mixed signals, and to keep its signal to noise ratio (SNR) at acceptable levels.The more solid the ground planes, the better the signals are separated from each other. Conversely, if there are multiple power planes or split planes on a signal layer, for example, they don't provide a solid ground for traces to suppress noise and crosstalk. By designing in additional amperage and adding more ground planes, you can increase reliability by an estimated 1520%.
Extra shielding on critical traces, especially on clock traces, also boosts reliability by about 1520%. In the case of RF circuitry, an aluminum shield is used to separate the sensitive signals from each other (Figure 1), thereby making transmissions clean.
This shielding protects a sensitive digital signal going from point A to point B, ensuring that it doesn't get mixed with an analog signal of the board, thereby jeopardizing the signal integrity. This provides extra protection by helping to create clean signals and minimizing crosstalk.
Other key design aspects include multiple modules for redundancies and high availability, component selection, feedback loops, monitoring circuitry, and then extra precaution and protection of analog and digital signals.
A good design with built-in redundancies can be exposed to harsh environments and still perform efficiently and reliably. In cases like this, multiple or redundant modules perform the same function, as cost is not as much a concern as is high quality and high reliability of the product.
One module acts as a main function generator of the circuitry. If the original module fails, an identical function generator or backup function module kicks in to maintain steady and uninterrupted operation. There are a variety of harsh environment applications requiring multiple modules. Two examples are mil/aero applications and deep oil drilling equipment. These and multiple other applications demand redundancy and precise accuracy.
Components normally are classified as military (mil) spec and commercial specs, and selecting the right ones is critical to meet rigorous harsh-environment demands. Mil-spec components are more expensive because they are built with considerably tighter tolerances and surface/material finishes that are more conducive to work in harsh conditions and climates. For high reliability, it is best to use a mil-spec-classified component with only 12% tolerance for higher reliability. A closely associated issue is component packaging material. Components with silver- or gold-plated leads are used for harsh-environment designs, whereas inexpensive lead- and copper-based alloys are used for commercial applications.
Here, it's vital to study spec sheets closely, because component makers usually don't call out mil versus commercial grades. EMS providers and contract manufacturers (CMs) should have expertise in electronics circuitry to decipher thick datasheets, understand the differences between mil and commercial specs and operational ratings and tolerances, and be able to comply with the demands of harsh environments.
When designing harsh-environment circuitry, factor in feedback loops. They provide critical data when signals are being sensed and monitored continuously and remotely. If a system is highly sophisticated, feedback loops can provide real-time information, fueling immediate corrective actions when necessary.
If current is leaking, it is not generating the desired output, adversely affecting the quality, reliability, and repeatability in harsh environments. Extra care must be applied to SNRs as well as tightly controlled current leakage, and crosstalk. This can be done by carefully studying and controlling critical factors that create noise.
At FabricationMaterials selection goes with fabrication, as well as layout. When you are specifying material, chose a high-temp material for board fabrication, such as FR-408 or polyimide-based materials, which have been specified to withstand higher temperature and hostile environments without delamination or peeling off soldermask layers.
These products should be built at mil-spec-certified PCB houses to ensure the highest quality of fabrication. Due to excessive heat and/or temperature fluctuations in the harsh environment, board warpage should be given special consideration. Warpage breaks circuit connections and solder joints, creating field failures.A technique called "copper thieving" evenly spreads the copper density on the board. Non-performing patches of copper pads are added to the unused empty areas on the board. This helps in uniform copper distribution as well as uniform plating, a factor for long-term board reliability.
During AssemblyAssembly processes must be more repeatable and reliable than those applied to conventional PCBs for commercial applications. For repeatability, reliability, and process control, component placement by pick-and-place machines is more effective than hand placement, due to human judgment and interaction.
The same holds true for other automation. The use of automated optical inspection (AOI) systems, paste height inspection machines, thermal profilers, and other assembly equipment virtually prevents humans from making inadvertent errors leading to lower quality and reliability. With well-calibrated machines and equipment, assembly processes are considerably more repeatable, more reliable, and better quality controlled.
Process sampling size is another area requiring special attention. For example, with a paste height inspection machine running a PCB for commercial application, sampling size could be at 20%. For rugged environment electronics, that percentage should be reduced to 10%. What this provides is more process control and monitoring capabilities within the manufacturing systems. By keeping the small sample size you increase reliability by monitoring the manufacturing processes and procedures. This small sampling size also allows inspection to provide real-time feedback to the manufacturing floor, reducing rework time and cost.
Stringent guidelines call for verifying various assembly steps at post-simulation. That's performed in a well-equipped and professional environmental testing lab. Here, the first article is used to perform various testing, including temperature cycle tests, drop and vibration tests, mechanical sturdiness test, and others. All these tests are performed in the first stages of product development cycle, eliminating issues early from all different stages of product manufacturing.
ConclusionOEMs should be prepared to incur added expense for manufacturing harsh-environment electronics. Select a CM or EMS provider that can successfully perform design, fabrication, and assembly requirements to comply with extra demands of harsh-environment electronics so these expenditures are rewarded.
Further, EMS providers and CMs with special certifications are better trained to produce demanding PCBs. Mil/aero, medical, and other harsh-environment electronics applications require Class III or J-standard certification. It is vital for manufacturing technicians and supervisors to be Class-III certified, as well as having J-standards certification required for mil/aero PCB assembly.
Zulki Khan, founder and president, NexLogic Technologies Inc., 2075 Zanker Road, San Jose, Calif. 95131, may be contacted at (408) 436-8150 ext 102; zk@nexlogic.com; www.nexlogic.com.