The Challenges of IoT for Electronic Assembly

An old saying goes, “too much of anything is bad,” but even back then, people found a remedy: “est modus in rebus”—there is a measure in all things. But who can truly decide where the limit lies? Take, for example, the Internet of (all) Things. Some are already asking: “Will the changes brought by the Internet of Things do more harm than good?” (Frank Palermo, InformationWeek, July 7, 2014).

By around 2020, when there will be more objects than people connected to the Internet (the balance point, according to Cisco, was reached somewhere between 2008 and 2009), and when there will be over 50 billion objects compared to 7.6 billion people, will we still be able to send a simple New Year’s greeting email?

Among this immense number of connected objects, the vast majority are sensors.

As is well known, every sensor has an element that is sensitive to a physical quantity—temperature, humidity, magnetic field, gases, gravitational acceleration, light, and so on. These elements may be semiconductor junctions, chemical compounds (including those containing rare earths), or materials with piezoelectric or pyroelectric properties, among others.

It is natural for a technologist to wonder whether the contacting process, which with the transition to lead-free technology subjects components to higher thermal stress (along with more aggressive fluxes and stronger cleaning agents), could affect the sensitive element in such a way that the proper and long-term functioning of the sensors on electronic modules might be compromised. And the first thing one can do in this situation is to consult the sensor datasheets.

Figure 1: HS-133 Gas Sensor

What do these documents tell us?

From the datasheets of several gas sensors such as HS-129, HS-131, HS-133 (Figure 1), and others — based on the property of the tin dioxide layer to exhibit lower conductivity in clean, oxygen-rich air and higher conductivity corresponding to gas concentration — one might conclude that the contacting process does not pose any particular issues.

The datasheets available online contain no information about the manufacturer (perhaps only a faint indication in the background, www...com.tw), although on the TME website, they are listed under Sencera. However, these documents include no assembly recommendations.

The situation is different with the datasheet of other gas sensors, also based on metal oxide properties — namely the SGX Metal Oxide Gas Sensors.

Figure 2: Thermal Profile for Heraeus F640 SAC405 Solder Paste

Right from the first page, we can read that the manufacturer, SGX Sensortech (IS) Ltd, is registered in England. Document AN-0172, Issue 1, dated July 14, 2014, includes a chapter titled “How to Connect the Sensors”, which states that the best method for mounting the sensors on the printed circuit board is by using a reflow oven.

It is recommended that the solder paste contain sufficient flux (typically 11%) to ensure good solder joints, but that the sensor should not be exposed to excessive flux. Therefore, the process should take place in a neutral environment, meaning practically in a nitrogen atmosphere.

The recommendation goes even further, specifying the exact type of paste to be used: Heraeus F640, SAC405, Type 3. The thermal profile of the process is also provided (see Figure 2).

Figure 3: Gas Sensors (SGX Sensortech)

Another family of sensors, based on the NDIR (Non-Dispersive Infrared) principle and produced by the same manufacturer, SGX Sensortech, includes the IR31SE — sensitive to carbon dioxide — and the IR32BC, a detector for methane and hydrocarbons (Figure 3).

Carbon dioxide can be detected using infrared (IR) spectroscopy, since its molecules absorb light in the infrared spectrum in a characteristic manner and at specific wavelengths, differing from other gases such as methane, water vapor, or carbon monoxide.

Figure 4: APDS-9006 Light Sensor (Avago)

In principle, the sensor consists of an IR emitter and an IR detector placed inside a chamber through which the gas circulates.

In the datasheet, there is a section titled “Handling Precautions”, which recommends avoiding shocks and preventing blockage of the gas inlet slot in the chamber, as well as avoiding immersion in liquids and protecting the sensor from dust and splashing of other fluids (such as during spray cleaning after the electronic module has been assembled).

Figure 5: Recommended Reflow Profile for the APDS-9006 Sensor

In the datasheet for the miniature APDS-9006 light sensor (Figure 4), Avago Technologies dedicates more than an entire page to assembly-related aspects, including information for stencil design (thickness, apertures) and detailed guidelines for establishing the reflow oven thermal profile, which are far more comprehensive than those presented earlier (Figure 5).

NXP dedicates an entire chapter — “Soldering SMD Packages” (a summarized version of the application note AN10365: Surface Mount Reflow Soldering Description) — to its KMZ60 magnetic sensor. The document briefly highlights general characteristics of wave soldering and surface-mount technology (SMT), and specifies the J-STD-020D standard requirements for the lead-free soldering process, particularly the maximum allowable temperature during the reflow phase depending on PCB thickness.

Also, NXP, having taken over the 3-axis digital magnetometer MAG3110 from Freescale, includes in the PCB Guidelines section several recommendations — among them: stencil thickness (100 µm or 125 µm, at the technologist’s discretion), aperture reduction (by 0.05 mm relative to pad dimensions), and a maximum soldering temperature of 260°C.

Manual soldering is not recommended, since component planarity is essential and may be compromised by uneven solder joints. The thermal profile follows the standard lead-free soldering process.

Figure 6: MAG3110 Magnetic Sensor (NXP)

In the documentation for the L3G4200D three-axis motion sensor, STMicroelectronics includes just a few lines under “Soldering Information”, referring to the JEDEC J-STD-020 standard only for pad layout and soldering process models, as well as to the official website, www.st.com. However, this information cannot be found directly on the site — one must first enter the keyword “soldering” in the search field to access a wide range of documents, including Application Notes providing guidance on soldering ST components.

VTI Technologies Oy (now part of Murata) has a comprehensive Technical Note TN71 titled “Assembly Instructions for SCA6x0 and SCA10x0 Series MEMS Inclinometer Sensors.” The document covers all stages — from design (package types, pad dimensions, layouts) to assembly, inspection, and even repair.

For the assembly process, both material and technological aspects are addressed for each operation:

• Materials: Recommended solder paste type — Type 3, no-clean, since cleaning processes are not advised. The metal cap of the molded package is not fully sealed, so cleaning fluids could penetrate the enclosure. Ultrasonic agitation is strictly prohibited for VTI MEMS components, as ultrasound could damage their internal structures.

• Stencil: Recommended thickness — 150 μm, with a minimum acceptable thickness of 125 μm; aperture ratio 1:1, or up to 5–10% reduction relative to pad size (if pad finishing is HASL).

• Printing: Particular attention should be paid to the squeegee movement speed.

• Component placement: Since these components are relatively heavy, when double reflow is used, it is recommended to pre-fix the inclinometer packages with adhesive paste (glue).

• Soldering: A typical thermal profile is specified, with a maximum reflow temperature of 250°C.

• Inspection: The document also includes a microsection image of a solder joint on a DIL-terminal package..

Figure 7: Desoldering an SCA6x0 Package Using a Soldering Iron

The document also provides both textual and visual recommendations for the manual soldering of these components, including repair (rework) instructions such as desoldering and resoldering.

It is advisable to use a hot air rework station, but a parallel-jaw soldering iron, capable of simultaneously heating all the solder joints of the DIL package, can also be used (Figure 7).

The datasheet of the MiCS-6814 gas sensor (SGX Sensortech) contains only one sentence regarding soldering:“The sensor should be assembled in a neutral atmosphere, free from flux vapors originating from solder paste.”

The MiCS-6814 is a MEMS-type sensor designed for detecting gases such as carbon monoxide, nitrogen dioxide, ethanol, hydrogen, ammonia, methane, propane, and isobutane. As shown in Figure 8, the sensor’s package includes multiple openings through which gas molecules from the ambient air reach the sensitive elements.

If the assembly process uses infrared/convection reflow soldering with lead-free alloys such as SAC or Sn, then a nitrogen atmosphere—considered neutral—should be ensured.

However, it appears that SGX Sensortech and other manufacturers have not taken into account the widespread adoption of vapor phase reflow soldering as a standard process following the transition to lead-free technology.

As is well known, UPB-CETTI (Center for Technology Transfer in Electronic Product Engineering, Politehnica University of Bucharest) provides electronic manufacturing services for companies within the ELINCLUS cluster, including Syswin Solutions. The cluster is managed by the Association for the Promotion of Electronic Technology (APTE) and has vapor phase soldering equipment, used for prototype and small-series production.

Figure 8: MiCS-6814 Gas Sensor

Due to its operating principle, heat transfer in this process occurs through the thin liquid film that covers the entire mass of the electronic module being assembled. As a result, all unsealed cavities of the sensor package will become filled with liquid.

In theory, the polyfluoropolyether (PFPE)-based liquid used in vapor phase soldering is inert. However, the vapor chamber may also contain gases released from the solder paste during processing.

There is yet another concern noted by Terry Brown, Senior Product Manager at SGX Sensortech: if vapors enter the interior of the sensor and surround the gold bonding wires, they may cause sensor damage by exerting additional mechanical stress on the connections.

Such issues could be avoided by covering the top part of the sensor with an adhesive film resistant to the temperature inside the vapor phase chamber. The company representative recommends this precaution but does not guarantee its full effectiveness, as no tests have been conducted for this specific soldering technology.

Mountain Switch, in its one-page datasheet for the Rolling Ball Tilt Switch (107-2006-EV) — a low-cost inclinometer — specifies that this through-hole component must be assembled only by manual soldering, as the maximum permissible temperature (250°C) applied to the body must not exceed three seconds.

In conclusion, sensors — as an integral part of the IoT universe — pose challenges for both designers and process engineers that must be addressed correctly, since improper handling can affect the accuracy of the data provided. Consulting the technological sections of datasheets is therefore essential, and if the expected answers are missing, one should contact the manufacturer’s specialists — you may even put them in difficulty.

If such information is absent, as is often the case in low-cost sensor datasheets, this is a clear indication that extra care must be taken during the soldering process.

Author: Gaudenţiu Vărzaru, Syswin Solutions

Source: 

http://electronica-azi.ro/2017/12/01/provocarile-iot-pentru-asamblarea-electronica