Moisture Sensitivity Level (MSL) misconceptions can increase cost or delivery time due to unnecessary packaging restrictions
Posted: 8th June 2016
Some plastic packages used in the manufacture of quartz crystals and oscillators can absorb moisture from the atmosphere while in storage. When these parts are subjected to the high temperatures seen during reflow soldering, the moisture in the package expands into steam and can cause micro-fractures within the package or worse. These can result in destruction of the wire bonds within the package and thus product failure. The wire bonds form the electrical connection between the IC die and the metal lead-frame and as such are often encased in plastic during the moulding process.
How likely this phenomenon is to occur is down to a number of factors such as the type of plastic used and the finish on this plastic, as well as the humidity, temperature and time of storage. This issue became of increasing importance after the introduction of lead-free solder and the consequential rise in the maximum peak temperature seen during reflow soldering.
To manage this issue, it is necessary to quantify how susceptible specific crystal and oscillator designs are to moisture ingress and following on from that to recommend minimum storage requirements to minimise any potential damage. There are two international standards used in electronics to do this. The first is JDEC-STD-020 Moisture/Reflow Sensitivity Classification for Non-hermetic Solid State Surface Mount Devices which defines moisture exposure levels, reflow profiles and pass criteria to grade components from levels 1 to 6, thus giving a standardised way to classify a component. The second, JDEC-STD-033 Handling, Packing Shipping and Use of Moisture/Reflow Sensitive Surface Mount Devices, follows on from the first standard and recommends specific storage conditions depending upon the classification number attributed in JDEC-STD-020.
The misconception is that all crystal and oscillator packages fall under one of the levels ascribed in JDEC-STD-020. In practice, the vast majority of frequency products manufactured by IQD are not covered by these standards because they are hermetically sealed, meaning moisture cannot ingress. Hermetic seals are tested using helium bombing to be less than 10-8mbar l/s which doesn’t permit water molecules to pass into the package.
Unnecessarily applying an MSL grade to a hermetically sealed product is significant because even for MSL level 1, (the least onerous of the 6 levels and significantly less demanding than for levels 2 through to 6), the packaging, storage and handling restrictions detailed in JDEC-STD-033 are onerous. For example, MSL level 1 product must be labelled with a Caution Label as defined in JDEC-STD-033. JDEC-STD-33 also defines a floor life that is a maximum time allowed between unpacking and exposure to reflow. For MSL level 1 components this floor life is unlimited, but only if the ambient atmosphere is below 30°C/85RH levels. But perhaps most significantly, in order to classify a component as MSL level 1, sample parts must pass a test prescribed in JDEC-STD-020. In fact this is the most onerous test prescribed in JDEC-STD0-20 involving soaking at 85°C/85RH for 168hrs before exposure to reflow profile and subsequent inspection for damage.
This test is written to assess the susceptibility of the devices under test to moisture ingress. So on a hermetically sealed part where it is not possible for moisture to ingress there is no benefit to be gained from performing this test.
The JDEC standards provide a framework for the manufacturer of the component to both assess the appropriate MSL level and consequently specify the packaging, handling and labelling requirements that each subsequent company in the supply chain must follow. Each Goods-In department can then clearly identify the requirements upon receipt of the goods and pass this on down the chain. It is not uncommon for electronics components to pass through many different companies between manufacturing and assembly into the finished end product. Is it important that no misconceptions are inserted into this chain which result in increased cost or delivery time due to unnecessary packaging restrictions.
Therefore when IQD are asked to complete QA questionnaires rating our crystals and oscillators from MSL level 1 through to 6 you can understand why, for hermetically sealed devices, we answer as ‘Not Applicable’ as it would be incorrect for us to suggest even MSL level 1 and add unnecessary handling costs into our customer’s processes.
IQD do manufacture a small number of crystals and oscillators that are non-hermetically sealed and therefore correctly classified with the appropriate MSL level. Where this is the case, the MSL level will be clearly shown on our product data sheet and appropriate storage, handling and packaging precautions will have been observed throughout the production process and at our finished goods storage facilities. In addition, the products will be shipped with appropriate packaging and labelling to allow your Goods-In team to clearly identify the product as such and take appropriate action.
In the time and cost critical world of crystal and oscillator manufacturing, unnecessary costs and processes need to be avoided wherever possible!