White Paper – Stuck on Precision: UV-Curable Adhesive Holds Thermocouples (TCs) Tight for Accurate Reflow Profiling

 

By David Dworak- Dymax Corporation, Material Scientist, Global.

 

 

 

 

Abstract
Producing high-quality printed circuit board (PCB) assemblies demands precise control of the time-temperature profile during reflow soldering. This profile must align with material and component specifications and remain consistent throughout the process.

Inadequate control can lead to defects such as cold solder joints, warpage, and delamination, making reliable, accurate, and repeatable time-temperature monitoring essential.

Time-temperature profiling typically involves attaching thermocouples to a sample PCB and running it through the reflow process to monitor temperatures at key points. However, the reliability, repeatability and accuracy of these measurements depend on the method used to attach the thermocouples.

This paper presents findings from testing at the Rochester Institute of Technology (RIT) on common thermocouple attachment methods. The objective was to identify techniques that deliver reliable, repeatable and accurate time-temperature measurements for high-density PCB applications.

Among the methods evaluated, including high-temperature solder, polyimide tape, aluminum tape, and adhesives, the study found that Dymax 9037-F, a UV-curable adhesive, delivered the most reliable, repeatable and accurate results. It outperformed the other methods tested in securing thermocouples, maintaining contact integrity, and preserving component stability.

Introduction

The trend toward miniaturization of electronics has accelerated the rise of high-density interconnect (HDI) PCBs, placing greater demands on process controls. Reflow soldering remains one of the most critical steps in HDI PCB assembly, with high yields depending on precise control of key parameters, such as time above liquidus, ramp rates, soak times, and peak temperatures.

Thermocouples are widely used in profiling the reflow soldering process. However, the accuracy, repeatability and reliability of their temperature measurements depends on how they are secured to the board. Poor attachment can lead to lifting, shifting or loosening during reflow, causing measurement errors that may go undetected until product testing or field failures. Optimizing TC attachment is therefore critical for accurate monitoring and control of temperatures in the reflow processes.

Common TC Attachment Methods

Several methods are commonly used to attach thermocouples to PCBs:

1.High-Temperature Solder
Long considered one of the most robust attachment methods, this technique involves soldering the thermocouple junction directly to a test pad using a high-temperature, lead-based solder alloy.

While this approach creates strong, thermally conductive bonds, it also presents several risks. The elevated temperature required to form the solder joint can introduce thermal stress, potentially damaging the PCB or nearby components. Preheating the board to 95oC before soldering can help mitigate thermal shock but adds complexity to the process and increases the risk of technician burns. Furthermore, the use of lead-based solder is
restricted by environmental regulations, such REACH and RoHS, which limit their use in many industries.

2.Polyimide Tape
Polyimide tape is a common attachment method due to its simplicity. It involves attaching thermocouples directly onto the PCB surface using heat- and chemical-resistant tape, such as Kapton®. Unlike soldered attachments, it is much easier to remove and reapply, making it convenient for quick adjustments or repositioning.

Despite its simplicity, the adhesive on polyimide tape can degrade and lose its grip during reflow, often lifting and creating air pockets underneath the thermocouple. If the tape is applied over components, it may unintentionally pull them off their pads as it lifts, resulting in unreliable temperature readings.

3.Aluminum Tape
Another commonly used method for thermocouple attachment is aluminum tape. Like polyimide tape, it offers simplicity, ease of removal, and repositioning. In addition, aluminum tape offers higher thermal conductivity than polyimide tape.

However, aluminum tape alone lacks sufficient adhesive strength to remain in place through multiple reflow cycles and typically requires reinforcement with polyimide tape on both sides, a technique known as “framing.” As with polyimide tape, if applied over components, it may unintentionally pull them off their pads, resulting in unreliable
temperature readings.

4.Adhesives

Adhesives are widely used in electronics assembly, and UV-curable formulations are emerging as a preferred method for thermocouple attachment. These adhesives harden within seconds when exposed to controlled UV light, forming strong, low-profile bonds that do not interfere with surrounding components. This makes them particularly well-suited for densely populated HDI PCBs where space is limited.

However, adhesive performance in reflow soldering depends heavily on its chemical formulation. It must bond effectively to the solder mask and possess the right viscosity to allow precise application at the desired location.

RIT Study: Methodology and Test Setup

Tests were conducted on identical SMTA test boards populated with a variety of surface-mount components, including 01005, 0201, 0402, 0603, 1206, Amkor Pop, Amkor 12mm QFNs, and Amkor 13mm BGAs. Six thermocouples were attached at key locations across each board

(Figure 1) using the following methods:

• High-temperature solder (Sn10/Pb88/Ag2 high-lead cored solder wire)
• Polyimide tape
• Aluminum tape
• Adhesive A (UV-curable)
• Adhesive B (UV-curable)
• Adhesive C (UV-curable)

Each test board and attachment method underwent 20 reflow cycles, with cooling intervals between runs. A high-precision thermal profiler recorded temperatures throughout each cycle, including ramp rates, time above liquidus, and peak temperatures. Post-reflow visual inspections were performed to assess thermocouple attachment integrity.

Test Equipment and Materials

The following equipment and materials were used:

• Heller 10-zone reflow oven
• SAC305 no-clean solder paste, 220oC (428oF) liquidus melting temperature
• Type-K thermocouples, 30 gauge (0.010” diameter wire), rated tolerance +/- 1oC
• SMTA Test board (8.0” x 5.5”, 0.062” thick, 4-layer PCB)
• KIC SPS Smart Profiler for temperature data acquisition

Thermocouple Attachment Details
To ensure consistent contact, all thermocouples were bent downward to create spring-like tension
at the attachment point. Wires were additionally secured to the PCB with polyimide tape for
added stability (Figure 2).

Test Results: High-Temperature, Lead-Based Solder

Inconsistent solder joint volume and quality affected the accuracy of temperature measurements. Larger solder joints, with greater thermal mass, slowed heat absorption and dissipation, resulting in delayed thermal response and distorted time-temperature profiles.

Achieving high-quality solder joints at elevated soldering temperatures proved challenging. Components had to be removed to prevent component damage. In several instances, joints cracked or failed after multiple reflow cycles, dislodging the thermocouple (Figure 3).

Test Results: Polyimide Tape

Polyimide tape was effective for attaching thermocouples when applied in large strips directly to bare sections of the PCB. However, during the liquidus phase, air pockets often formed beneath the tape, compromising temperature measurement accuracy. When applied over components, the combination of air pockets and tape lifting caused components to detach from their pads.

These air pockets were removed between reflow cycles by applying firm pressure to resecure the tape. This suggests that, to obtain accurate temperature readings with polyimide tape, the attachment must be monitored for lifting and reseated between runs.

Test Results: Aluminum Tape

Initial testing with aluminum tape alone (without polyimide tape framing) led to frequent tape lifting and thermocouple detachment. Temperature data from detached thermocouples was removed from the analysis. Once reattached with polyimide tape framing, consistent temperature readings were obtained.

A major challenge with aluminum tape was component lifting during reflow when thermocouples were taped directly to the top of components. During the liquidus phase, components like 1206 resistors detached, resulting in a complete loss of adhesion to the PCB

Test Results: UV-Curable Adhesive A

Adhesive A bonded well to metallized QFN components and test pads but showed poor compatibility with solder mask surfaces. Like tape, it caused components to lift off the board during the liquidus stage of reflow when applied over them — an issue attributed to inadequate
adhesion to the solder mask (Figure 5).

Test Results: UV-Curable Adhesive B

Adhesive B bonded well to solder mask surfaces. However, early test runs showed that insufficient coverage led to poor adhesion and thermocouple detachment. Its low viscosity made it difficult to achieve full coverage in a single application, requiring additional, time-consuming coating steps.

Test Results: UV-Curable Adhesive C
Adhesive C exhibited the most consistent and reliable performance among the attachment methods tested. It demonstrated strong adhesion to solder mask surfaces and remained intact through more than 20 lead-free reflow cycles without signs of detachment or degradation. Its favorable viscosity and application properties enabled precise placement, avoiding interference with adjacent components, making it ideal for precise thermocouple mounting on densely populated circuit boards. The resulting bond was thin and lightweight, contributing minimal
thermal mass, an important factor in maintaining accurate time-temperature measurements.

Figure 7 shows Adhesive C after 20 lead-free reflow cycles, applied between closely spaced 0603 resistors. While a slight color change is visible, likely due to thermal exposure, the adhesive bond remained structurally sound. It continued to hold both the thermocouple junctions and the surrounding components securely in place, demonstrating excellent mechanical stability and resistance to repeated high temperature exposure.

Adhesive C: Key Findings

Adhesive C outperformed all other attachment methods across multiple categories:

• Bond Integrity – Maintained a consistent hold throughout 20 reflow cycles without
  detachment, cracking, or degradation. In contrast, tape methods often required
  reattachment or replacement after only a few cycles.
• Measurement Accuracy – Temperature readings were repeatable and stayed within thermocouple’s specified tolerance. No thermal lag or measurement bias was observed, due to the bond’s low profile and minimal thermal mass.

Figure 8 shows peak temperature data collected over 20 reflow cycles conducted over a two-day period. Notably, the first cycle of each day (runs 1 and 15) showed a slight temperature deviation of approximately 1oC, likely due to the oven not yet reaching full thermal equilibrium. From the second or third cycle onward, readings stabilized.

• Ease of Use – Adhesive C enabled fast, precise thermocouple attachments. Its rapid, light-activated cure ensured consistent bonding without lengthy setup, drying times or the need to remove components. Unlike high-temperature solder, its ambient temperature cure posed no risk of overheating or damage to nearby components.

Why Adhesive C Outperformed Polyimide Tape

A key advantage of Adhesive C is its ability to prevent component lifting, a common issue with polyimide tape. During the liquidus phase of reflow, lead-free solder melts at approximately 220oC (428oF). At these temperatures, air pockets can form beneath the tape, increasing the likelihood of components being dislodged as the tape lifts from the board. This issue is especially pronounced on high-density PCBs, where space constraints make it difficult to apply tape without overlapping nearby components.

In contrast, Adhesive C can be precisely dispensed only where needed, forming a strong, localized bond that doesn’t change with reflow dynamics. Adhesive C prevents component lifting or shifting components and ensures consistent, reliable thermal profiling.

Adhesive C (Dymax 9307-F) Application Considerations

Dymax 9037-F is a multi-cure adhesive formulated for chip encapsulation and wire bonding applications. It cures rapidly under a broad spectrum of UV/Visible light and offers a secondary heat-cure option for shadow areas. When used with a UV light-curing spot lamp, it cures within seconds. Dymax 9037-F is highly resistant to moisture and elevated temperatures, making it well-suited for demanding environments, and it is fully compliant with RoHS Directive 2015/863/EU.

To ensure optimal performance, Dymax recommends following the application methods below to achieve optimal results:

• Surface Preparation – Ensure the boards are clean and free of oils, flux residue, or other
  contaminants to promote proper adhesion.
• Adhesive Application – Secure the thermocouple wire in place with tape, then apply a small amount of adhesive over the thermocouple tip.

• Curing Process – UV curing systems should deliver consistent intensity and exposure times, in accordance with the material’s specifications and recommended cure schedules.

Spot-curing tools with integrated timers and intensity controls are recommended for maintaining repeatability.

• Thermocouple Sleeve – Use a protective sleeve to shield the thermocouple wire from chemical, thermal and mechanical stresses. Once sleeved, the thermocouple wires can be further secured to each other and to the board with adhesive.

 

Precision Bonding for Reflow Success

Adhesive bonding has long played a vital role in electronics manufacturing, for staking components, tacking wires, and reinforcing assemblies against vibration and shock. Extending this technique to thermocouple attachment is a logical next step. As with other adhesive applications, compatibility with PCB materials and processes is essential, underscoring the importance of rigorous testing like the study described here.

This evaluation showed that Dymax 9037-F UV-curable adhesive is a highly effective solution for thermocouple attachment during solder reflow profiling. It outperformed traditional methods, such as lead-based solder, aluminum tape and polyimide tape, across all key metrics: durability, measurement accuracy, repeatability, and ease of use.

As component densities continue to increase, precise and reliable reflow profiling becomes more critical than ever. An adhesive like Dymax 9037-F offers a reliable, future-ready bonding solution for thermocouples in modern reflow processes.

For more information on Dymax 9037-F visit
https://dymax.com/content/download/7550/file/be107427f6d1e595dfc84b8f35cb1fd3.pdf