COTS oscillators for Hi-Rel mil/aero programmes
02 July 2019
COTS components are not new to the aerospace, defence or space industries, but it is true to say that the momentum behind the use of such components, rather than expensive, bepoke, highly specialised military parts, has intensified in recent years.
This article was originally featured in the July 2019 issue of EPDT magazine [read the digital issue]. Sign up to receive your own copy each month.
As Andy Treble, Sales & Marketing Director at manufacturer of quartz crystals, oscillators, filters & frequency-related products to the electronics manufacturing industry, Euroquartz explains, this is partly due to the improved production quality of the industrial grade products that are now available.
For engineers, the use of commercial off-the-shelf (COTS) components is a matter of risk assessment and risk mitigation in their specific system design, and the correct choice of supplier is vital when procuring frequency components for this type of application. A good supplier of crystals and oscillators should implement a four-point mitigation strategy to ensure reliability and quality of the product.
1) Reliability testing: routine reliability data should be available for COTS parts and a typical cycle of reliability testing is shown below in Figure 1.
2) Traceability: a date code alone is insufficient to provide production batch traceability, since it merely indicates the date the product was last tested prior to shipment. The inclusion of a lot-code will identify the exact production batch, operator and production line the parts were produced on. An example can be seen below in Figure 2.
3) QA access: free access to the production facility should be available, even if the option is not taken up by the customer.
4) Stability of supply: can the supplier demonstrate stability of the supply chain? This is vital, especially as many COTS applications do not require huge volumes of product, but could have a production life extended over many years – and sometimes even decades.
Obviously the prime motivation is to reduce the build cost of systems by down-rating some of the military specification components, but without risking overall reliability. However, care should be exercised when purchasing industrial products for these applications, as there are a multitude of frequency product suppliers to choose from, all of which will have ISO9001 quality systems.
Suppliers will often state that full traceability is offered, but in many cases, the customer will not know where the factory is actually located, other than maybe a general statement of China or Taiwan, which then means that they are reliant on statements from the supplier. That is not to say that suppliers are not offering full traceability, or just providing vague information, but in ISO9000 jargon, it should be demonstrable.
In some cases, customers may require specific tests to be performed relating to a particular aspect of the application. For instance, a restart test to be made at a specific temperature, and a good supplier will be able to offer this facility.
Another important aspect to consider is gold embrittlement. Gold embrittlement and voiding can appear in SMT solder joints when using gold-finished components. This means that, over time, such components may become less reliable in terms of their mechanical bond strength to the PCB.
J STD-001 Revision F states that gold shall be removed:
a. From at least 95% of the surfaces to be soldered of the through-hole component leads with >2.54µm [100µin] gold thickness and all through-hole leads that will be hand soldered, regardless of gold thickness.
b. From 95% of all surfaces to be soldered of surface mount components, regardless of gold thickness.
c. From the surfaces to be soldered of solder terminals plated with >2.54µm [100µin] gold thickness and from all solder cup terminals, regardless of gold thickness.
A double tinning process, or dynamic solder wave, may be used for gold removal prior to mounting the component on the assembly. Many customers will consider automotive approved parts manufactured to AEC-Q200 as suitable. Once again, care needs to be taken in this area, as many of these components are produced for high volume usage and obtaining small quantities may present a challenge.
COTS programmes will continue to grow in the years to come and it is up to both the customer and supplier to work closely together to ensure that risk mitigation is maintained at a high level.
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