Capillary Underfill Adhesives
This document provides an overview of capillary underfill adhesives (CUF) used in board level and semiconductor packaging applications (IKB097).
Electronic assemblies continue to shrink in size while becoming more complex and operating under increasingly demanding conditions. As a result, the performance requirements placed on underfill materials have increased.
Underfill materials play a key role in protecting solder joints and improving mechanical integrity and long-term reliability.
At the same time, semiconductor packaging is evolving from single die designs to chiplet based, heterogeneous integration. This increases underfill flow distances while interconnect pitch continues to decrease. These trends place greater demands on material flow behaviour and void free filling.
Applications such as data centres and high-speed electronics also require improved electrical performance, including lower dielectric properties.
What is Capillary Underfill?
Capillary underfill adhesives are used to fill the gap between a semiconductor device and its substrate after solder reflow.
The material is dispensed along the edge of the component and flows underneath the device by capillary action, without the need for external pressure. Once cured, it encapsulates the solder joints and provides mechanical support.
Key functions include:
- Compensation for coefficient of thermal expansion mismatch
- Protection of solder joints during thermal cycling
- Resistance to moisture ingress and delamination
- Improved resistance to shock, vibration, and mechanical stress
- Increased long term reliability
This document focuses on board level capillary underfill systems. Alternative technologies such as no flow, moulded, and wafer level underfills are not covered in detail.
First Level vs Second Level Underfill Requirements
Underfill applications are typically divided into two levels within the packaging structure, based on where they are used in the assembly.
First level underfill refers to die to substrate or chip to interposer connections, as found in flip chip or CSP assemblies. Second level underfill applies to the attachment of packaged devices to the PCB, for example in BGA or interposer to PCB. Each level presents different geometries and therefore places distinct demands on both material properties and processing.
First level underfill is generally more demanding due to smaller gaps and finer pitch, which increase flow distance and make void free filling more challenging.
By comparison, second level underfill involves larger geometries and shorter flow paths, with a greater emphasis on mechanical reinforcement and overall assembly reliability.
Lower dielectric properties are particularly important in high frequency and data centre applications.
Material Selection Considerations
Device geometry, reliability targets, and performance requirements all influence the choice of underfill material. Selection must balance processability with long term reliability, with key factors including:
- Gap size and standoff height
- Die size and associated flow distance
- CTE compatibility between device and substrate
- Balance between thermal performance and electrical requirements
- Cure temperature constraints of components and substrates
Material selection should align with both the assembly process and the intended operating environment.
Processing Guidance
Consistent process control is as important as material selection when working with capillary underfill. The dispensing method, typically needle or jetting, should match the component geometry and production requirements.
Flow behaviour must ensure complete gap filling within an acceptable time, particularly for larger die or longer flow paths. Substrate preheating is often used to improve flow by reducing viscosity and aiding wetting.
Cure conditions are typically defined within a range, often between 120 and 160 °C depending on the material, to achieve full cure while minimising stress and warpage.
Void control remains critical to reliability. This depends on appropriate material choice, optimised dispense patterns, controlled thermal conditions, and clean substrate surfaces.
Mechanical and Reliability Benefits of Underfill
The mechanical world inside an assembly is highly complex. Different materials expand and bend at different rates (CTE), thereby stressing each other. Underfill redistributes this stress and improves overall mechanical stability.
Modelling and simulation are commonly used to predict stress behaviour and guide material selection. This enables targeted material design, fewer test loops, and faster, more reliable test development.
Recent developments by DELO have resulted in industry-leading underfill adhesives, with demonstrated void-free performance under test conditions. The smaller particle sizes and lower curing temperatures of these materials reduces stress and warpage, whilst extended working life enables larger cartridge sizes, lowering costs, minimising waste, and supporting energy‑efficient processing.
No-Bleed Underfills
In underfill applications, bleed refers to the migration or separation of low molecular weight components, such as resins or additives, from the main material body during or after dispensing. This can result in visible residue, contamination of adjacent areas, or inconsistent material properties, potentially affecting reliability.
Tests on a 35mm x 35mm PBGA, and an FR4 substrate with solder resist, using DELO MONOPOX CU2955, showed no evidence of bleeding. The parts were preheated to 70ºC for 15 minutes, then, after dispensing, allowed to “sit” at room temperature for 3 hours in 30% RH, with curing then taking place at 140ºC for 60 minutes: The absence of bleed under typical processing conditions supports consistent material behaviour during assembly and curing.
Conclusion
Underfill adhesives are critical to the reliability of modern semiconductor assemblies.
Interposer sizes in next generation semiconductor devices are increasing, potentially approaching 200 mm x 200 mm in advanced applications, placing further demands on thermal management and mechanical performance.
As technologies evolve, developments in capillary underfill materials are enabling devices with finer geometries, longer flow paths, and more demanding operating conditions.
Author
Date
Version
Author
Eamonn Redmond
Date
23 May 2026
Version
IKB097 Rev. 1
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Author Biography
Eamonn Redmond is a Senior Technical Sales Engineer at Inseto (UK) Ltd, leading the Materials and Adhesives divisions. With nearly four decades of experience in microelectronics, he combines strong technical knowledge with practical commercial insight.
After completing a B.Eng. in Electronics at the University of Limerick, Eamonn began his career at Analog Devices, working as an In‑Line Test Engineer and Process Engineer. During this time, he developed hands‑on expertise in die attach, wire bonding and environmental testing, including burn‑in, temperature cycling and thermal shock. He later progressed to IC Production Manager, gaining experience in wafer‑level and package‑level testing and modern semiconductor production operations.
In the early 1990s, Eamonn moved into technical sales, specialising in die attach adhesives, eutectic solders and materials for wire and ribbon bonding. This role established a strong foundation in adhesive technologies for high‑reliability microelectronic applications.
Since joining Inseto in 1996, Eamonn has supported customers with a wide range of microelectronic materials, including die attach adhesives and encapsulation compounds, alongside substrate materials such as thick‑film inks and thin‑film substrates. He is widely regarded as a key technical resource for customers seeking reliable bonding and encapsulation solutions.
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