Tech Note: Quartz Based Frequency Products Shelf Life
On occasion IQD receive notification from customers that they have problems receiving goods with a date code beyond a specific time period.
The purpose of this brief is to detail what can happen to a quartz based timing device while stored on the shelf and when this could cause a problem.
In almost all quartz based frequency control products suffer no detrimental effect from a long shelf life.
Of the remaining few, IQD are managing this in house, so there is no cause for concern to the customer.
Shelf life: Time between component manufacture and use in production.
Product data code
Common to most products in electronics, frequency control products are marked with a date code.
There are two possible moments when a data code can be marked on a product. It can be marked as part of the final stage of production process. Here it is used to identify the date the product was manufactured. Alternatively the product can be marked moments before packaging and shipment to the customer.
If the product in question is a Made To Order product and has been produced with a particular sales order in mind then it is likely that these two dates will be the same.
However if the product is of a standard specification, or is of a specification that can later be tested and sorted into a range of different specifications, then there is likely to be a time lag between the production date and the shipping date.
At IQD Frequency Products we feel it is useful to us to understand the date a product was manufactured. Hence our products are marked with a date code that represents the date of manufacture not the date of shipping.
Some of our competitors may choose to mark there products with a date code at the point of shipping.
Management of products with old shelf life
IQD Frequency Products stores area is located in our Crewkerne facility in the UK. The stores area is a dedicated, air-conditioned room and product is stored packaged in sealed bags.
At IQD Frequency Products we run a tight control of our stock levels so it is rare that we have stock older than a few weeks. However on occasion we are able to help a customer by making use of products manufactured some time ago.
On these occasions we do have an internal policy which dictates that the product is tested before shipment. The testing includes a solderability test to ensure that the terminations of the product are not suffering any form of corrosion which would cause them to fail during our customers soldering process.
In nearly 40 years of shipping frequency control products we can think of no occasion when products have failed this testing.
Products which are subject to MSL level 2-6 (ref JDEC-STD-020)
There is one exception to the standard practice regarding shelf life of quartz crystal based frequency control products.
Non hermetically sealed solid state surface mount devices may be sensitive to moisture ingress over time. If so this is established though testing in accordance with JDEC-STD-020. If necessary the packaging must then comply with that laid out in JDEC-STD-033. Within this standard certain shelf life periods are prescribed. If the product is stored for longer, then the product must be baked in a dry oven to remove the moisture before repacking and restarting the shelf life clock.
Rest assured IQD Frequency Products has internal standard in place to handle products of this nature and will take the necessary steps before shipping to the customer.
Sources of quartz crystal frequency product ageing
Quartz crystals frequency control products contain a quartz crystal wafer. This wafer is plated in silver or gold to create the electrodes. The wafer is then mounted and bonded to the package by the use of an epoxy glue.
During operation the crystal wafer is physically moving. This movement is directly proportional to the electrical frequency of the product. Any physical disturbance to the vibration, or change in the mass of the moving parts of the wafer will directly affect the frequency of the product.
To reduce the possibility of a physical disturbance to the vibration of the wafer, it is processed in a clean room. Processes include washing in very high resistance water, centrifugal drying and baking in vacuum oven for several hours.
To reduce the possibility of the mass changing, the wafer is hermetically sealed within the package, in an inert atmosphere of either pure nitrogen or under a vacuum. The inert atmosphere means the internal components of the crystal cannot oxidise and subsequently change in mass.
One other factor that can affect the long term stability of a quartz crystal frequency control product is the stresses induced on the surface of the wafer. Causes of stress can include the jigging used when plating the wafer or the mixing and curing of the epoxy glue used to mount the wafer. As the wafer is physically vibrating any stresses or changes in the stresses across the wafer will affect the frequencies and stability of the vibrations.
Techniques are used to reduce the stresses induced on the wafer such as controlled curing of the epoxy and high temperature baking of the wafers before assembly.
The end result is a product which is hermetically sealed with the frequency source held in an inert atmosphere. The levels of cleanness and the purity of the internal atmosphere affect the long term frequency stability of the product. This can be assessed by looking at the ageing parameter in the product specification. Tighter control of these processes gives better ageing performance, and higher cost.
For low specifications products the rate of ageing is well within the products tolerance so ageing is of no significance.
For higher specification products there may appear to be a close margin between ageing rates and tolerance. However the effects of ageing are well researched in the quartz crystal industry and are most pronounced in the initial period while the internal stresses relax and any contaminations within the package take their effect. Over longer time periods these effects tend towards zero.
Therefore, for a quartz based frequency control product it could be said that a longer shelf life product is in a better state than a more recently manufactured product.