Electronic component manufacturing is a miracle of modern technology. It’s a fascinating and highly complex process to extract the raw materials from the earth, purify them, and then produce the finished products. Millions of man hours have been expended to manufacture materials that are low in cost, easy for the end user to assemble, and electrically plus mechanically stable. This is true for all components from the “simple” resistor all the way to the most complex multicore processor.
Despite our most valiant efforts, the fact remains that many electronic components have a finite shelf life. In this engineering brief we will explore a few of the constraints such as environmental conditions suggested in Figure 1. Here the tarnished trumpet mouthpieces are an extreme example of the unseen deterioration of the electronic components.
Figure 1: A collection of darkened trumpet mouthpieces surrounded by resistors. Exposure to the elements has caused the two on the right to tarnish.
Tech Tip: Most solder and solder pastes contain an organic flux that removes the oxide layer. This increased the solder “wetting” allowing a solid reliable electrical and mechanical connection between the component and PCB. The power of the flux is limited in its ability to remove oxidation. Consequently, heavily oxidized, or tarnished components are to be avoided. This is a situation where the parts are perfect from an electrical perspective, but a troublesome from a manufacturing perspective.
Oxidation and solderability
For nearly all my life I have played the trumpet. Like me, these instruments are starting to show their age, as we see in Figure 1. The two mouthpieces on the right are deeply tarnished. While the one on the left is used but still in good shape. While I can’t give an exact reason, the bulk of the tarnish happened when I was living in Kodiak, Alaska. Perhaps it was the sea water or maybe it was because I lived a less than a thousand feet from an active airport taxiway. Either way, there was something in the air that attacked the metal.
The same corrosive elements are always acting on our electrical components. This is especially true for the component’s surfaces that will be soldered to the Printed Circuit Board (PCB). The shelf life of electronic components is dominated by the condition of these solderable surfaces. This statement is directly related to manufacturing. While your components may be electrically and mechanical stable for decades, tarnished surfaces can reduce your manufacturing yield, as old tarnished surfaces do not readily accept solder.
Tech Tip: Unsoldered components past the manufacturer recommended 2-year mark do not immediately expire like rotten produce. Since the components represent a sizable investment, you should conduct solderability tests to verify the integrity of the component’s finish.
Prolong shelf life
The integrity of a component’s solderable surfaces may be prolonged with proper storage. A typical storage specification may read like this:
-
50 to 90 degrees Fahrenheit or 10 to 32 degrees Celsius
-
25% to 50% relative humidity
-
no direct exposure to sunlight or other ultraviolet light
-
no exposure to corrosive elements in the air such as ozone or sulfur compounds
-
no exposure to radioactivity
As a rule, these conditions will result in a shelf life of two years for “simple” components such as resistors. We must consult the manufacturer’s data to determine the specific recommendation for more complex parts.
Tech Tip: There is nothing simple about a resistor especially when viewed from a materials stability perspective. This is especially true when we consider the physical stresses associated with high temperature and thermal cycling. At the same time, the resistor must retain its chemical, and electrical integrity for long service life.
Humidity
Humidity drives corrosion as implied in the previous section. However, many parts are hygroscopic and will readily absorb water. This internal moisture can produce a destructive popcorning defect so named by the popping sound made by components. Here, water trapped inside the component flashes to steam during the soldering process. The result is a destruction of the mechanical integrity of the component with lost or compromised electrical functionality. This damage may be immediate or delayed as the compromised component quickly suffers environmental degradation.
Humidity sensitive components must be protected in sealed waterproof packaging. Desiccant should be included for long term humidity control. A humidity indicator card such as the SCS card shown in Figure 2 should be included in the package.
The parts should be kept sealed in the protective package until they are ready to be used. Once removed, they should be immediately soldered, ideally that day.
Tech Tip: Moisture sensitive components can be rendered unsolderable if left unprotected over a long weekend. Plan your production runs accordingly.
Restoration through baking
Many moisture sensitive components may be restored using a baking process. This process uses heat to drive the moisture out of the components. Consult the manufacturer before baking high dollar value components to ensure the moisture is fully removed in a controlled process. They should be able to provide appropriate time and temperature requirements. For example, they may provide a recommendation of 230 degrees Fahrenheit (110 Celsius) for 24 hours.
Tech Tip: The baking process is a balancing act. Elevated temperature accelerates the corrosion process as described in the previous section. This can be especially problematic for larger parts that require extended baking times. Repeated baking may render components unusable.
Figure 2: A humidity indicator card showing the exposure level for the components while stored in their protective packaging. The card suggests baking the components if the central indicator turns pink.
Capacitors
Capacitors have their own unique shelf-life considerations. The most well know is the aluminum electrolytic. Under normal operating conditions, the critical oxide layer on the plate is preserved and maintained. However, the oxide layer degrades when the capacitor is unused. This reduces the capacitance, reduces the working voltage, and increases the leakage current. This can cause problems when voltage is forcefully applied after the capacitor is installed.
An electrolytic capacitor can often be restored to 100% operation status by applying a forming voltage. This may be done in two ways:
-
direct connect the capacitor to a DC power supply and slowly raise the voltage to the full working voltage of the capacitor.
-
connect the capacitor to the DC power supply via a series current limiting resistor.
In both cases, the capacitor voltage is slowly increased allowing time for the oxide layer to reform. Be sure to consult the manufacturer for reforming recommendations. Also, be cautious as high-capacity, high-voltage capacitors present a serious electrocution hazard. A safety enclosure with safe and effective discharge measures is a necessity.
This capacitor degradation is something that should be mitigated when designing electronic equipment. After all, it’s not unreasonable to have equipment or ready spares set on a shelf unpowered for many years. On the other hand, the vast majority of assembled electronic equipment can remain unpowered for years with no ill effects when powered up. While we cannot ignore this capacitor property, it may not be a significant issue if we capacitors with a reasonable safety margin. I will leave it to you to define the appropriate safety margin for your given application.
Tech Tip: There is an old repairman’s trick for reforming the capacitors in vacuum tube radios and stereo equipment that has been sitting for many decades. Rather than applying full line voltage, a variac is used to gradually apply the voltage. With slow application of voltage, many capacitors are restored to life. It’s not foolproof but it often works. The advantage of this technique is safety as the capacitors and the associated high voltage are confined within the equipment enclosure.
Other
Long life is an important design consideration. As a rule, the shelf life considerations described in this article are not a significant issue once the components has been soldered onto the PCB. For example, while a manufacturer may specific a 1 year shelf life for a given component, we can expect that component to operate for a decade or longer once installed.
With that said, we recognize that there are many unique materials used in producing an electronic assembly. Be vigilant for shelf life and be sure to consult the device datasheet.
Parting thoughts
Manufacturing is a complex undertaking. Shelf life of your electrical components is one of many considerations for you process. Unlike produce, electronic components don’t have a firm expiration date. We could argue that the component manufacturers are being conservative. At the same time, we recognize that these components represent a sizable investment. Protect your investment by storing all components in controlled environments. Avoid storing open reals in a box on the floor of a damp warehouse.
This article is a brief introduction to a few common components. Please leave a comment below if you have questions about a class of components that were not addressed. Also, be sure to test your knowledge by answering the questions at the end of this article.
Best wishes,
APDahlen
Return to the Industrial Control and Automation Index
About this author
Aaron Dahlen, LCDR USCG (Ret.), serves as an application engineer at DigiKey. He has a unique electronics and automation foundation built over a 27-year military career as a technician and engineer which was further enhanced by 12 years of teaching (partially interwoven with military experience). With an MSEE degree from Minnesota State University, Mankato, Dahlen has taught in an ABET-accredited EE program, served as the program coordinator for an EET program, and taught component-level repair to military electronics technicians. Dahlen has returned to his Northern Minnesota home and thoroughly enjoys researching and writing educational articles about electronics and automation.
Highlighted Experience
Dahlen is an active contributor to the DigiKey TechForum. At the time of this writing, he has created over 150 unique posts and provided answers for an additional 500 customer posts. Dahlen shares his insights on a wide variety of topics including microcontrollers, FPGA programming in Verilog, and a large body of work on industrial controls.
Connect with Aaron Dahlen on LinkedIn.
Questions:
The following questions will help reinforce the content of the article.
-
Describe the ideal environment for storing electronic components at your facility.
-
True / False: Corrosion of a component’s solderable surfaces is largely a manufacturing problem.
-
True / False: Shelf life is a consideration for the unsoldered PCB itself.
-
True / False: Popcorning is a problem for components years after they have been soldered to the PCB.
-
What is the purpose of solder flux?
-
Describe the process of reforming an electrolytic capacitor.
-
What is the downside of extended baking to remove moisture?
-
You have just removed a half used real of 0805 surface mount LEDs from your pick and place machine. Identify and describe the steps necessary to preserve the integrity of the components.
-
With regards to the previous question, how does your answer change if the real was inadvertently kept in place over the holiday break?
-
Research and then describe the shelf life limitations for a component not identified in this article.
-
Locate a humidity indicator card used in your facility. Describe the card and how to interpret the results.
-
What is thermal cycling and how does it impact a products life?
Critical thinking questions
These critical thinking questions expand the article’s content allowing you to develop a big picture understanding the material and its relationship to adjacent topics. They are often open ended, require research, and are best answered in essay form.
-
Shelf life may be extended if all components are treated as if they were moisture sensitive e.g., by storing them in sealed individual bags with desiccant. Is this a reasonable action when we consider the time and added expense?
-
How can component shelf life issues be mitigated by PCB design? Is this a desirable way forward?
-
Research and then contrast the vulnerability and impact of corrosion between surface mound and through-hole components.
-
Research and then compare the impact of corrosion on component shelf life for lead and RoHS components.