The Challenges of IoT for Electronic Assembly
- Raluca
- Dec 1, 2017
- 6 min read
Updated: Oct 12
An old saying goes, “Too much of anything is bad,” but even in those times, a remedy was found: “est modus in rebus” — there is a measure in all things. But who can decide where the limit lies? Take, for instance, the Internet of (all) Things. Some are already wondering whether “the changes brought by the Internet of Things might do more harm than good” (Frank Palermo, InformationWeek, July 7, 2014).
By 2020, when there will be more objects than people connected to the internet (the parity point was reached, according to Cisco, sometime between 2008 and 2009), and when over 50 billion objects will exist compared to 7.6 billion people, will we still be able to send a New Year’s greeting email?
From this vast number of internet-connected objects, the largest share consists of sensors.
As is well known, every sensor has an element 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 rare-earth elements), materials with piezoelectric or pyroelectric properties, etc.
It is natural for a technologist to ask whether the soldering process — which, with the transition to lead-free technology, subjects components to higher thermal stress (along with more aggressive fluxes and stronger cleaning agents) — might affect the sensitive element, potentially compromising the correct and long-term operation of sensors on electronic modules. The first step, of course, is to consult the sensor datasheets.
Figure 1: Gas Sensor HS-133
According to datasheets for gas sensors such as HS-129, HS-131, and HS-133 (Figure 1), which are based on the property of tin dioxide layers to exhibit lower conductivity in clean, oxygen-rich air and higher conductivity in the presence of target gases, one might conclude that the soldering process poses no problems. However, the datasheets accessed online lack manufacturer information (perhaps only an indication such as “.com.tw”), although the TME website lists Sencera as the producer. These documents contain no assembly recommendations.
This is not the case with other gas sensors based on similar metal oxide properties — for example, SGX Metal Oxide Gas Sensors.
Figure 2: SGX Sensortech Assembly Recommendations
On the very first page, the datasheet identifies the manufacturer, SGX Sensortech (IS) Ltd, registered in England. Document AN-0172, Issue 1 (July 14, 2014), includes a section titled “How to connect sensors”, stating that the best soldering method on printed circuit boards is reflow soldering.
It recommends using solder paste with sufficient flux content (typically 11%) to ensure proper joints, but warns against excessive exposure to flux. Therefore, the process should occur in a neutral atmosphere, ideally nitrogen. The manufacturer even specifies the solder paste — Heraeus F640, SAC405, Type 3 — and provides a thermal profile (Figure 2).
Figure 3: NDIR Gas Sensors
Another family of sensors based on the NDIR (non-dispersive infrared) principle from the same manufacturer includes IR31SE (CO₂ detection) and IR32BC (methane and hydrocarbon detection) (Figure 3).
CO₂ detection relies on infrared spectroscopy, as CO₂ molecules absorb light in the infrared spectrum at characteristic wavelengths, distinct from other gases like methane, water vapor, or carbon monoxide.
The datasheet includes a section titled “Handling Precautions”, recommending avoidance of shocks, blockage of gas inlets, immersion in fluids, and exposure to dust or sprayed cleaning agents after assembly.
Figure 4: Light Sensor APDS-9006 (Avago Technologies)
In the datasheet for the APDS-9006 miniaturized light sensor (Figure 4), Avago dedicates over a page to assembly issues, including stencil design details (thickness, aperture dimensions) and recommended thermal profiles for soldering (Figure 5), which are more detailed than previous examples.
Figure 5: Recommended Soldering Profile for the APDS-9006 Sensor
In the datasheet for the APDS-9006 miniaturized light sensor (Figure 4), Avago Technologies dedicates more than a full page to assembly-related issues, including information for stencil design (thickness, apertures) as well as detailed thermal profile specifications for the reflow oven, much more detailed than those presented earlier (Figure 5).
NXP dedicates an entire chapter, “Soldering SMD packages” (a summarized version of Application Note AN10365 – Surface Mount Reflow Soldering Description), to its KMZ60 magnetic sensor. The document briefly outlines the main characteristics of wave soldering and surface-mount technology, and presents the requirements of Standard J-STD-020D for lead-free soldering, specifying the maximum allowable temperature during the reflow phase, depending on PCB thickness.
Also, NXP, in adopting the MAG3110 digital 3-axis magnetometer from Freescale (Figure 6), includes in the PCB Guidelines section several recommendations — such as stencil thickness (100 μm or 125 μm, left to the technologist’s discretion), aperture size reduction (by 0.05 mm relative to pad dimensions), and a maximum soldering temperature of 260°C. Manual soldering is not recommended, as the component’s planarity is critical and could be compromised by uneven joints. The thermal profile follows the standard for lead-free soldering technology.
Figure 6: MAG3110 Magnetic Sensor (NXP)
In the documentation for the L3G4200D three-axis motion sensor, STMicroelectronics includes just four lines of soldering information, referring to the JEDEC J-STD-020 standard — only for pad layout models and the soldering process — as well as to the website www.st.com. However, the information cannot be found directly on the site; you need to type the word “soldering” into the search bar to access a large number of documents, including application notes containing soldering recommendations for ST components.
VTI Technologies Oy provides an entire technical note dedicated to its series of MEMS-based inclination sensors, TN71 – Assembly Instructions for SCA6x0 and SCA10x0 series. The document covers all stages — from design (package types, pads, dimensions) to assembly, inspection, and even repair.
For assembly, both material and process aspects are discussed in detail for each operation:
Materials: Use “no-clean” type 3 solder paste, as cleaning is not recommended — the metal lid of the preformed package is not fully sealed, and cleaning fluids could enter the capsule. Ultrasonic agitation is strictly prohibited for VTI MEMS components, as ultrasonic vibrations could destroy their internal structures.
Stencil: Recommended thickness — 150 μm, minimum 125 μm; aperture size should match the pads (1:1) or be reduced by at most 5–10% (especially when the pad finish is HASL alloy).
Printing: Attention must be paid to the squeegee movement speed.
Component placement: These components are relatively heavy, especially in double-reflow processes; therefore, it is recommended to pre-fix the inclinometer packages with an adhesive paste (glue).
Soldering: A typical thermal profile is specified, with a maximum temperature of 250°C in the reflow zone.
Inspection: The document also includes a microsection of a solder joint for a DIL terminal package.
Figure 7: Desoldering an SCA6x0 Package Using a Soldering Iron
The document also provides text and images with recommendations for manual soldering of these components, including repair (rework) instructions — both for desoldering and resoldering operations.
The use of a hot-air rework station is recommended; however, a dual-tip soldering iron with two parallel jaws that can heat all solder joints of the DIL circuit simultaneously (Figure 7) may also be used.
The datasheet for the MiCS-6814 gas sensor (SGX Sensortech) contains only one sentence regarding soldering:
“The sensor must be assembled in a neutral atmosphere, free from solder paste flux vapors.”
The MiCS-6814 is a MEMS-type sensor designed to detect gases such as carbon monoxide, nitrogen dioxide, ethanol, hydrogen, ammonia, methane, propane, and isobutane. As shown in Figure 8, the sensor’s package includes several openings that allow gas molecules from the atmosphere to reach the sensitive elements.
If assembled in an infrared/convection reflow oven using lead-free solder alloys (SAC or SN), the process must take place in a nitrogen atmosphere, considered neutral.
However, it appears that SGX Sensortech and other manufacturers have not yet taken into account that vapour-phase reflow soldering has become a fairly common technology following the transition to lead-free processes.
As is known, UPB-CETTI provides electronic production services for companies within the ELINCLUS cluster — including Syswin Solutions — a cluster managed by the Association for the Promotion of Electronic Technology (APTE). For prototype and small-series production, UPB-CETTI uses a vapour-phase soldering oven.
Figure 8: MiCS-6814 Gas Sensor
Due to its operating principle, heat transfer occurs through a thin film of liquid that covers the entire mass of the electronic module during assembly. As a result, all unsealed cavities of the sensor package become filled with liquid. Theoretically, the polyfluoropolyether (PFPE)-based liquid used in the process is inert; however, gases originating from the solder paste may be present in the vapour chamber.
Another concern, highlighted by Terry Brown, Senior Product Manager at SGX Sensortech, is that if vapours enter the interior of the sensor and surround the gold bonding wires, they could damage the sensor connections by exerting additional mechanical stress on the wires. This issue could be mitigated by covering the upper part of the sensor with a temperature-resistant adhesive film during vapour-phase soldering. The company representative recommends this measure, although it cannot be guaranteed to work 100%, since the method has not been tested for this technology.
Mountain Switch specifies in the one-page datasheet for its Rolling Ball Tilt Switch (107-2006-EV) — a low-cost inclinometer — that this through-hole component must be assembled only by manual soldering, as the maximum allowable temperature of 250°C may not be applied to the component body for more than 3 seconds.
In conclusion, sensors — as part of the IoT ecosystem — pose challenges for both designers and technologists, which must be properly addressed, as the accuracy of the information they provide can be affected. Consulting datasheets, including their technological guidelines, is essential. And if the expected answers are not found, contact the manufacturers’ specialists — you might even challenge them.
If such information is missing — as is often the case in datasheets for low-cost sensors — this is a clear sign that extra caution must be exercised during the soldering and assembly process.
Author: Gaudențiu Vărzaru, Syswin Solutions
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