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Considerations in Mounting, Soldering, and Handling Sensors



There are many aspects of handling and mounting of sensors, critical to achieving optimal sensor performance. These include such elements as moisture sensitivity, ESD considerations, and the impact of mechanical shock, as well as physical mounting, placement and soldering.

This article is a non-exhaustive overview of “dos and don’ts” with regard to soldering, handling, protection and mounting of sensors. It includes helpful hints, as well as information on where additional data can be found to make informed decisions and to assist in avoiding unnecessary problems.

Moisture sensitivity levels (MSL)

Moisture sensitivity levels are established and described in IPC/JEDEC J-STD-020D.1 “Joint Industry Standard: Moisture Reflow Sensitivity Classification for Non-Hermetic Solid State Surface Mount Devices.”¹

These levels are important, as the vapor pressure of moisture inside a non-hermetic package increases dramatically when the package is exposed to the high temperature of solder reflow. Delamination of packaging materials from the die, internal cracks, bond damage, wire necking, bond lifting, die lifting, thin-film cracking, and cratering beneath the bonds are some of the issues that may occur.

In severe cases, stress resulting from moisture levels can cause a “popcorn” phenomenon, whereby the internal stress causes the package to bulge and crack, producing an audible pop sound. This typically happens with SMD devices.

Also important is IPC/JEDEC J-STD-033A “Joint Industry Standard: Handling, Packing, Shipping and Use of Moisture/Reflow Sensitive Surface Mount Devices,”² which establishes proper handling techniques. For example, typical factory floor life is one week at ambient temperature ≤ 30°C/60% RH.

Electrostatic discharge (ESD)

Many sensors today have ESD protection circuitry built in that can withstand 2,000 V of human-body model (HBM) electrostatic discharge. Still, proper ESD handling, established in JESD625A “Requirements for Handling Electrostatic-Discharge Sensitive Devices”³, should be observed.

Mechanical shock

It is not out of the ordinary for sensor packages or components to be dropped on a hard surface. When components are dropped from a height of more than 5 cm or if a hard object directly impacts them during assembly, they should be discarded. Chip shooters and IC placers may also generate repetitive shocks that can exceed the survivability of some sensors. Plastic or compliant-tip pick-and-place nozzles are recommended over metal nozzles. Sensors should be the last components placed onto a PCB, and should be placed with minimal direct force. Chip shooters and IC placers should feature placement force control. The use of ultrasonic PCB assembly cleaning should be avoided, as it is damaging to MEMS sensors.

Physical mounting

The physical location of the sensor on a PCB is important and incorrect placement can affect performance. Avoid the following placement locations:
  • Near PCB anchor points or bends to avoid excessive mechanical stress
  • Where epoxy resin partially cover the sensor causing asymmetric mechanical stress
  • Near hot spots that elevate the sensor temperature, changing performance
  • Directly under buttons to avoid excessive mechanical stress and surface contact
  • Near excessive vibration areas to avoid noise degradation
  • At a distance of less than 2 mm from the package
Figure 1 is a comparison of correct and improper PCB placement of the Kionix KXTI9, a tri-axis ±2-, ±4-, or ±8-g silicon micromachined accelerometer.

Figure 1: Incorrect (top) and correct (bottom) PCB placement. (Courtesy of Kionix.)

As is typical for a MEMS part, the KXT19 carries the following warning: “Caution: ESD Sensitive and Mechanical Shock Sensitive Component, improper handling can cause permanent damage to the device.”

The Kionix KXTI9, shown in schematic form in Figure 2 below, is a low power, high performance tri-axis accelerometer with digital I²C output. The accelerometer includes the company’s Directional Tap/Double-Tap feature, a detection feature reporting the acceleration axis and direction of each tap detected and allows up to 12 user-defined function commands.

Figure 2: The KXTI9 accelerometer used in EVAL-KXTI9-1001.

Here are some considerations involved in the soldering of this type of sensor.

Soldering guidelines

While it is not practical to define a specific soldering profile for each sensor, using the time and temperature reflow profile customized for the particular manufacturing practice and application is recommended.

Standard tin-lead Sn63/Pb37 solder paste or a lead-free solder paste may be used. When testing the alternative solder compositions the user must consider such issues as:
  • Is the material selected going to be compatible with the plating on the sensor leads or the finish specified on the circuit board?
  • Will the material chosen compromise manufacturability?
  • What is the residual effect of the alternative reflow profile on the sensor package, the passive components, and the board itself?
When assembling or soldering pressure sensors, here are a few important considerations:
  • Components should be placed flat onto the PCB and they must be level.
  • Should sensors have pressure ports or vent holes, these should be blocked with Teflon tape during the soldering process.
  • Use dry, non-corrosive air rather than steam, water or cleaning agents as they may affect sensor performance.
  • Maximum temperatures should be approximately 250°C.
  • The maximum reflow time at which temperatures exceed 215°C is 75 seconds.
  • Total time that the sensor exceeds 125°C is less than 5 minutes.
  • Wave soldering is the best method to solder through-hole sensors.
SMT pressure sensor mounting

There are useful guidelines as to how to handle SMT pressure sensors during the mounting and soldering process. For example, one should pick up sensors by the sides of the lid or, in the case of a tube style, by the tube itself.

An example of the tube-style sensor is the Model 1451 SMD Sensor (Figure 3) by Measurement Specialties, Inc.

Figure 3: The 1451 Tube SMD Sensor by Measurement Specialties, Inc.

Following are some mounting tips for our example, the 1451 tube SMD sensor:
  • The gauge holes on the bottom of sensors should be open and unobstructed. The venting hole in the PCB should line up directly below the sensor gauge hole and must be open and unobstructed.
  • Seal the sensor pressure port when using a wave solder process. However, manual soldering can be done without sealing.
  • Tube versions can be sealed with a small plastic cap that will survive high temperatures.
  • If there is a pressure-port hole, it can be sealed with a piece of high-temperature Polyimide-film tape.
  • Gel filled versions should be hand soldered, as exposure to high temperatures involved in wave soldering can burn the gel.
  • Minimum solder connection height shall be 0.008 in. (0.2 mm) per MIL-STD-2000A.
  • When direct mounting on PCB, it is recommended to conduct tests with a temperature probe attached to the sensor.
  • Only solder the sensor one time.
Summary

This article has provided guidelines for the handling, mounting, and soldering of sensors, including some basic precautions and preventive measures to insure reliable operation. These guidelines are general in nature and are based on recommended industry practices. It should be noted, however, that the engineer also must apply his/her actual experiences and development efforts to optimize techniques and processes that best meet the needs of varying end-use applications.

References
  1. IPC/JEDEC J-STD-020D.1 “Joint Industry Standard: Moisture Reflow Sensitivity Classification for Non-hermetic Solid State surface Mount Devices.”
  2. IPC/JEDEC J-STD-033A "Joint Industry Standard: Handling, Packing, Shipping and Use of Moisture/Reflow Sensitive Surface Mount Devices."
  3. JESD625A "Requirements for Handling Electrostatic-Discharge Sensitive Devices."
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