Understanding Storage Life of Electronic Components: Definitions and Calculation Rules

Effective storage management is essential for ensuring the reliability of electronic components before they are assembled into final equipment. Defining and calculating storage life correctly helps maintain product quality and prevents unexpected failures during use.

1. Key Definitions

1. Storage Period (ts)

The time interval from when a component is manufactured and passes inspection to the moment it is installed into equipment, under specified storage conditions.

2. Valid Storage Period (tVS)

The time during which a batch of components remains within quality requirements when stored under specified environmental conditions.

3. Basic Valid Storage Period (tBVS)

The valid storage period without considering differences in quality grades.

4. Storage Quality Coefficient (CSQ)

A coefficient used to adjust the basic valid storage period according to the component’s quality grade.

5. Re-inspection After Expiry

A set of tests performed on components that have exceeded their valid storage period to determine whether they are still suitable for use.

6. Extended Valid Period

The additional storage period granted to components that have passed re-inspection after exceeding their valid storage period.

2. Determining the Start Date of the Storage Period

Accurate calculation begins with defining the start date. Common rules include:

1. For components that have undergone secondary (supplementary) screening
   If the screening items and conditions are no less stringent than non-destructive tests in an expiry re-inspection, and the screening or production date is within 12 months, the start date is the completion date of the supplementary screening.

2. Using the date code printed on the component

   • Week code examples: “9908” indicates production in Week 8 of 1999.

   • If only year and month are shown, the 15th of that month is used as the start date.

   • If only a week number is given, the date of Thursday of that week is used.

3. Using the inspection date on the certificate of conformity

4. Using the packing date on the container
   The start date is set as one month prior to the packing date.

5. Using the acceptance date
   If the production date cannot be confirmed, the start date is two months prior to the acceptance date. If the production date is known, use the production date.

After determining the start date, the storage period equals the time from this date to the planned installation date.

3. Factors Affecting the Valid Storage Period

The valid storage period depends on:

• Component materials

• Packaging and structure

• Storage temperature, humidity, and cleanliness

• Component quality grade

Different component categories naturally have different storage characteristics due to structural and material variations.

Some component specifications explicitly define storage limits. These limits can be used as the effective storage period when available. With modern manufacturing improvements, many components now have longer storage life compared to older generations.

4. Industry Practices for Setting Valid Storage Periods

Ideally, component manufacturers should provide the valid storage period based on long-term reliability tests. However, not all manufacturers supply complete data. In practice, organizations commonly:

• Combine reliability experience and historical usage data

• Refer to publicly available guidelines for similar component types

• Define their own valid storage periods based on category and grade

Since storage life depends heavily on environmental conditions, any storage environment that does not meet the assumed standard should result in a shorter valid storage period or additional re-inspection requirements.

5. Recommendations for Storage Management

To maintain component quality before assembly, consider implementing:

• A traceable storage control system to record production, inspection, and screening dates

• Appropriate storage conditions, including temperature/humidity control, dust prevention, ESD protection, and contamination control

• Category-based storage management, especially for sensitive components

• Re-inspection procedures for components nearing or exceeding their valid storage period

These measures help ensure that components maintain their performance and reliability up to the point of installation.