Moisture/Reflow Sensitivity Classification for Relays

Relays were so far not considered to be moisture sensitive components. The introduction of lead-free solder processes result in an increase of the maximum temperatures from typically 235°C today, to approx. 260°C in the future.

In opposite to most other electromechanical components, relays have an enclosed and sealed gas volume.

Impact of lead-free SMD soldering processes

Major differences between a standard and leadfree solder process are the increase of the peak temperature to 260°C on one hand and a longer process duration - up to 8 minutes - on the other hand. Longer duration and higher temperature result in an increase of the internal pressure inside the relay. At the same time the mechanical characteristics of all plastics are dropping to values less than 10 percent of the initial values at room temperature. This is even valid for all high performance engineering plastics like LCP, which is most commonly used for Signal and Telecom relays.

• possible outgassing of the plastics
• moisture adopted in the plastics

While the pressure increase caused by the outgassing of the plastics, can be prevented by careful selection of the materials and the manufacturing processes, the impact of humidity is much more difficult to control, until the SMD relays are soldered to the boards.

Uncontrolled humidity is the most critical aspect when introducing lead-free soldering processes with increased temperatures.

The definitions and requirements for moisture sensitive components are given in the following standards

• IPC/JEDEC J-STD-020B-Classification for Nonhermetic Solid State Surface Mount Devices
• IPC JEDEC J-STD-033 -Standard for Handling, Packing and Shipping and Use of Moisture/Reflow Sensitive Surface Mount Devices

The classification procedure applies to all plastic Surface Mount Devices (SMDs) in packages that, because of absorbed moisture, could be sensitive to damage during solder reflow. The classification levels are intended to be used by SMD producers to inform users of the level of moisture sensitivity of the relays. The table (copy out of standard J-STD-20B, page 5) below gives an overview about the levels:

Classification of SMD relays

In order to classify the moisture sensitive level of SMD relays the following tests were performed:

• Measure of the initial functional value
• Exposure at damp heat see table 1
• SMD reflow process, 3 times (255°C, lead-free)
  Measuring of the pressure rise inside the relays
• Measuring of the final functional values
• Visual inspection. Criteria: cracks, bulging

Moisture sensitive level MSL = 3.

For more detailed information contact AXICOM Application Engineering