Next Generation Adhesive Innovation for Camera Module Active Alignment Assembly Process

The number of camera modules used in consumer electronic devices continues to grow aggressively as electronic devices become both more mobile and, increasingly, wearable.  Market estimates predict that total revenue for CCM (Compact Camera Modules) will exceed $35 billion (US) in 2021, rising to nearly $50 Billion (US) by 2025. In addition, the number of Cameras present on or in automotive vehicles will also increase significantly with Automotive camera module volumes predicted to be broadly equivalent to the number used in cell phones by 2025. 

KRYLEX research scientists have developed a full range of adhesives for the assembly of CCMs, leveraging corporate expertise and historical core competency around UV cure and dual-cure adhesive technologies. Depending on the type of CCM device, either fixed or autofocus, these CCM devices can use between 5 – 10 different adhesives in the construction of the final device. As such selection of the adhesive is critical to the final device performance and reliability. Depending on the specific bonding application each adhesive will have very specific performance targets. 

Active Alignment  (AA) Process in Camera Module Assembly:

A general increasing trend in the assembly of camera modules is a growth in the number of lenses being incorporated into the modules. As the number of lenses increases the importance of accurately fixing the correct focus is getting ever more critical. Dual Cure, UV + Heat, and Adhesive solutions are used to fix the correct focus during a production assembly process called Active Alignment (AA).

The adhesive AA assembly process involves a series of production steps. The AA adhesive is dispensed onto the image sensor housing. The lens holder is placed into contact with the dispensed adhesives. The next step is for the image sensor to be powered up and the lens aligned to the correct position for the best image quality capture. Once the perfect image focus is found the assembly equipment applies UV/ Visible light energy to instantly cure the adhesive and fix the correct lens alignment. The UV step provides sufficient strength to fix the camera focus in place correctly and allow the units to be moved and safely transported to the secondary adhesive cure step, box oven curing. The box oven cure step ensures that any remaining uncured portion of the adhesive is cured and all-polymer cross-linking is complete. This heat cure sets the final cured adhesive properties vital for a robust bond that will enable effective and reliable operation of the camera module throughout the device lifetime.  

Epoxy Adhesives for Active Alignment – The Benefits and Challenges

The vast majority of AA adhesives available in the market today are based on epoxy resin chemistry. Epoxy is an ideal choice for a production process that requires both a UV  and heat cure process. Epoxy curing agents are widely available for both UV and heat curing. In addition epoxies, resin adhesives are known for their ability to process easily and also have excellent adhesion to a wide array of different substrates, e.g. plastics, metals, glass while offering excellent environmental stability and reliability. 

Aside from the generic benefits of Epoxy resin, an AA adhesive must often meet specific and varying critical performance criteria on a case-by-case basis. Depending on the specific OEM or camera module device manufactures there are a number of factors that can create specific challenges. 

The use of lightweight, thermoplastic plastic components for the assembly of camera module devices is common. Plastics such as Polycarbonate (PC), Liquid Crystal Polymer (LCP), etc… are widely used because they are light, strong, stiff, have low moisture absorption, and have good chemical resistance. These plastics often have a variety of surface finishes depending on the manufacturer and are often available in a variety of different product grades. This can cause challenges for adhesives because often surfaces will be different enough between manufacturer to manufacturer or product grade to product grade, that adhesion and/ or reliability performance can differ significantly. 

Another consideration is the sensitivity of the plastics components to elevated temperatures, like that seen during a heat curing step. Any subtle change caused by elevated temperature to the plastic could fundamentally change the performance of the device, as such AA adhesive thermal cure steps need to be tightly controlled and typically restricted to no higher than 80◦C. 

Another important consideration to consider for a successful AA process is adhesive volumetric shrinkage. It is common for epoxy adhesives to have relatively high shrinkage e.g. 3-5%, however, if the adhesive shrinks a lot as a function of the adhesive cross-linking process then it is likely that the optimised alignment for perfect image quality will be affected. Shrinkage as a property can also be correlated with cure condition or temperature. When higher temperatures are used to cure the adhesive there is a potential for more shrinkage, whereas at lower temperatures the reverse is true. Therefore being able to effectively cure lower temperatures is beneficial when using heat-sensitive substrates or as a way to help limit negative impacts associated with adhesive shrinkage.  

Another product variable that can disrupt the alignment process is adhesive water absorption. If the cured adhesive absorbs too much moisture during device usage at ambient and elevated temperatures then the adhesive can swell and result in a shift of alignment and reduced image quality.

In addition, Epoxy adhesives used in Active Alignment are typically 1-part or pre-mixed. These pre-mixed formulas are typically highly reactive at Room Temperature (RT) and often require frozen storage at temperatures as low as -40◦C, requiring expensive, high energy usage, industrial freezers. Commonly, with most pre-mixed epoxy adhesives, once thawed the cure reaction begins and means work-life (time the product is stable at RT) on the production floor is often short and dispense performance can become variable over time. Most AA adhesives available today are designed to cure at elevated temperatures of around 80◦C because making the system react more quickly at an even lower temperature than 80◦C, typically results in product worklife being prohibitively short. KRYLEX KURA-LOW Technology addresses the traditional problems associated with 1-part, low-temperature cure epoxy adhesives.

KRYLEX Dual Cure Active Alignment Adhesives Incorporating KURA-LOW Technology

KRYLEX KURA-LOW Technology was developed to enable high product stability at RT but have good product reactivity at temperatures between 60-80◦C. The high product stability at RT enables extended product work-life e.g several weeks. The high product stability at ambient conditions has further benefits effectively eliminating the need for dry ice during shipment and use of high energy consumption -40◦C industrial freezers, potential reducing costs and lowering carbon footprint due to reduced energy consumption.

KRYLEX KD7009 is a AA adhesive that is based on KURA-LOW Technology. The product is able to be cured at temperatures as low as 60◦C but has a very long work life at RT. The product can be stored in refrigerator at -5◦C or in a standard commercial freezer at -20◦C. The UV cure is fast and the depth of UV penetration is high. The product has an optimised, thixotropic rheology, to enable easy, robust dispensing but a high enough thixotropic nature to ensure ‘shape stability’ and minimal unwanted movement post alignment, prior to UV cure. The product has excellent adhesion to both metal (SUS, Al) and plastics (PC, LCP) and excellent reliability performance (Heat & Humidity).

Graphical user interface, application

Description automatically generated

A full summary of KD7009 performance benefits can be seen below: –

  • No requirement for dry ice shipment.
  • No requirement for -40◦C frozen storage.
  • Long product work-life at ambient temperature
  • Fast UV Cure
  • Oven cure at 60 – 80◦C
  • Low shrinkage
  • Low moisture uptake.
  • Low weight loss
  • High adhesion to metals and plastics
  • High Impact Performance
  • Excellent hot wet stability 85◦C:85% or 60◦C:90%

For further product data and information about KD7009 or other recent camera product innovations (IR Filter and LCP Lens to Barrell bonding) please contact pgleeson@chemence.com or your local KRYLEX representative.

NextGen Camera Module adhesives: IR Filter bonding

Our electronic devices and automobiles have seen a dramatic increase in the number of Compact Camera Modules (CCMs) integrated into their assembly. For our mobile devices, the continuous integration of CCMs has resulted in exceptional clarity over a wide range of distances. The emergence of Advanced Driver Assistance Systems (ADAS) in the automotive industry has also increased demand for CCMs worldwide.


Depending on the type of CCM device, either fixed or autofocus, these CCMs can use between 5 to 10 different adhesives in their construction. The specification of the suitable adhesive is critical to the device’s performance and reliability. Depending on the specific bonding application, each bond will have very different performance requirements.
IR filters are one portion of the construction of CCM devices. Unfiltered or poorly filtered digital cameras can detect infrared waves reflected from an object that it is focusing on. This reflection can then negatively impact the quality of the captured image. The utilization of the IF filter ensures more vibrant imaging and display of more natural colors. The IR filter is attached to the VCM (Voice Coil Motor) module in auto-focus CCMs or the lens holder in fixed-focus CCMs.

   Fig 1: A typical IR filter and lens assembly


The IR filter component is usually manufactured from specialized optical glass and contains an additive coating to filter target wavelengths in the infrared range of light. This range is typically 700-1000nm. Ensuring reliable bonding of the IR filter is critical to providing a robust CCM. There are several challenging requirements that any adhesive solution must meet to ensure reliable adhesion.


A Reflective Index (RI) determines how much the light path is bent or refracted when entering a material. To ensure that the photos taken are not of poor quality or distorted, the refractive index or material used in CCMs must not be too high or too low. Cameras are frequently exposed to an extensive range of harsh environments, and they will experience excess humidity, moisture, and elevated temperatures. A camera is also expected to last several years. Its adhesive component, alongside the other CCM components, must be developed to withstand such harsh conditions to ensure the camera’s lifespan.

Fig 2: Cross section of a IR Filter and lens assembly with a typical adhesive bond line


Accelerated environmental aging will be performed on the CCM assembly to ensure optical stability. Transmittance with yellow and blue colors is measured at different pre-determined intervals to ensure the CCM assembly is optically stable.
Aside from the harsh environmental and optical performance requirements demanded of the adhesive, CCM design must be optimized for high-volume manufacturing. Other requirements also include good adhesion to difficult-to-bond plastic compounds such as LCP. This material is sometimes the other portion of the assembly alongside the IR filter.

Krylex’s KU5146 has been developed explicitly for bonding IR filters to ensure excellent optical performance and environmental reliability. KU5146 offers the following features to a CCM manufacturer while maintaining excellent optical clarity:


• Exceptional depth of cure
• Good adhesion to a variety of plastics and glass
• Outstanding strength retention after harsh environmental testing
• Excellent ability to withstand thermal shock

Why is a low MVTR critical for effective sealing?

If you think of the evolution of electronic devices, you will most likely envisage ever-increasing functionality along with a continuous reduction in size. This functionality can be attributed to certain advancements in technology, which include but are not limited to, advancements in materials science, sensor technology, and CPU processing power.

A device can only be functional if it can effectively operate within a range of different environments. These environments have become increasingly similar to the environments that their human user inhabits. Wearables, particularly medical device wearables, allow for the monitoring of the advanced ecosystem that is the human body. Such devices have the capability of monitoring vital signs including blood pressure, measuring blood glucose levels, or helping in the diagnosis of conditions such as sleep apnea.

 

Environment & Reliability:

Living on the skin of the user exposes the device to a surprising number of harsh environmental conditions. These conditions if not accounted for in the development stage will result in a loss of critical functionality. An average user may shower or bathe once a day. Some will engage in physical activity several times a week. Many will walk up continuous flights of stairs or run for a bus. Users quite often apply lotions, fragrances, or cosmetics to their skin. Perfume and sunscreen are examples.

Resistance to these chemicals can be challenging. Hazardous chemicals can often cause damage to the bond. However, the most common environmental hazard that a device can endure is excess moisture.

Bond Line Exposure:

Moisture can pose different types of hazards to such devices. Immersion in water can apply excess pressure on the adhesive bond line, especially at an above ambient temperature. This is where thermal stresses are induced onto the assembly. If the materials in combination do not have adequate adhesive strength, adhesive failure may propagate along this bond line. This can also be viewed as the path of least resistance.

Such risks regarding areas of possible stress concentration can be mitigated by ensuring the adhesive and material combination will have adequate bond strength, usually quantified by OLS (Overlap Shear) testing along with design considerations for an adequate bond line thickness. The repeatability of this thickness can be assured with correctly calibrated automated dispensing equipment.

Moisture Vapor Transmission:

With adequate development and testing to validate such assumptions an engineer can conclude that their bond line will be impenetrable to such hydrostatic pressure with very low failure rates. However, moisture does not only pose a threat to the functionality of the device when in fluid form. Users can live in a variety of climates. For example, relative humidity in parts of southeast Asia is often in excess of 80%. When exercising, a user will often sweat. Because these devices can be very close or even fixed to the skin, it is vulnerable to such perspiration. Water in its gaseous form, known as vapor, is another threat to the reliability of the device.

If we turn to the example of a user exercising, we can practically relate to the principle of Moisture Vapor Transmission. Modern sportswear is frequently advertised as “breathable”. There has been a divergence from more traditional fabrics such as cotton which tends to absorb moisture to materials such as polyester which is water repellent. This allows the user to perspire without the inconvenience of their clothes becoming soaked in sweat.

Comparing cotton with polyester, we can say that the breathable polyester has a high Moisture Vapor Transmission Rate, and the absorbable cotton has a low Moisture Vapor Transmission Rate. This metric can be described as the amount of moisture that will pass through a material in a given period. It is usually expressed by the following equation:

                                                                   MVTR = g/m^2/day

The following schematic shows how a basic Moisture Vapor Transmission Rate test is carried out. A known geometry of material is positioned as a divider between two chambers. One chamber contains a known temperature and percentage of relative humidity. As the moisture vapor is transmitted through the material, it is measured on the opposite end.

Image

                                         This schematic is representative of ASTME 96/E96M-16.

Material Selection & Risk:

When an electronic device is assembled, it will have a minimum of two materials exposed to the external environment. For example, a phone or wearable device could contain an outer casing of two components manufactured from a polymer and metal. In this case, an adhesive would be used to bond these dissimilar materials. This outer assembly’s primary function is to safeguard the complex and sensitive components housed inside.

Metals & Plastics:

When integrating metal components into a housing, moisture vapor will not be an issue. Oxides may form on the surface of these materials but the process of surface treatment such as the anodizing of aluminium prevents this. Phones or high-value wearables such as watches are normally manufactured from such aluminium or stainless-steel housings.

The construction of medical device wearables differs. Such devices are often only designed for short-term usage that may be limited to several days or weeks. The short lifespan of these devices is a massive driver when it comes to the selection of materials. Materials such as Polycarbonate or Nylon are often chosen due to their robustness. They will also require chemical resistance and mechanical strength. However, these materials are very cost-effective which ensure manufacturers maintain a low unit cost. Polymers, unlike metals, absorb moisture. MVTR must be considered when specifying these materials. However, the greatest material risk involves the adhesive selection.

Oxidation & Corrosion:

Oxidation of metals causes a surface change on the material. The level of change will depend on the type of metal present. Electronic assemblies often include Printed Circuit Boards, traditional PCBs are manufactured using an array of metal components such as aluminium, copper, and plated nickel. Oxidation can in turn cause the corrosion of their surface. This type of oxidation will propagate through the surface of the component and deteriorate the mechanical properties. Fortunately, metals used on PCBs are not likely to suffer this type of damage.

However, when oxidation is present, highly conductive materials used in these assemblies, such as copper, can develop a lower retention of its electrical conductivity. This is a major issue for PCBs, the reliability and in turn the functionality of the whole device.

Often electronic components are plated to ensure corrosion resistance, particularly in connector type applications. Noble metals such as gold and silver provide excellent corrosion resistance. This process, as robust as the results may be, is very costly to a manufacturer and cannot be implemented on board level.

There are many different forms of corrosion; galvanic corrosion or bimetallic corrosion can occur when dissimilar materials are in electrical contact and are exposed to a corrosive electrolyte. Water, although one of the weaker substances, contains electrolytes.

The most common type of oxidation is atmospheric corrosion. Atmospheric corrosion occurs when moisture from the air meets metallic components. The moisture vapor condenses onto the component which in turns forms the oxidative layer. This is one of the most challenging aspects of developing a reliable electronic device.

Polymers as a material are inherently permeable. If they are to be utilized in the design of electronic devices, there must be an acceptance that some moisture will pass through the protective casing onto the electronic components. As electronic assemblies become more complex in their design and functionality, researchers may discover other ways that moisture can damage these assemblies apart from the more well-known types of corrosion.

MVTR:

Engineers will not stop in their efforts to continuously try and reduce the risk of board level failures. To ensure the greatest possible reliability of the device, engineers must ensure that their material selection has the best possible barrier properties. The Moisture Vapor Transmission Rate or MVTR is widely regarded as the best measure of a materials ability to resist the absorption and transmission of moisture through its wall thickness.

Adhesives are widely regarded as the weak point of an electronic housing. Whether applied to adhere the components of a housing or utilized to encapsulate an assembly, a low MTVR is critical to safeguard against the permeation of the adhesive bond line or encapsulation area.

Adhesives are generally differentiated from sealants by their ability to offer a high level of structural adhesion while sealants are characterized by their ability to seal against such hazards as chemicals and moisture.

A Novel Solution:

Krylex’s KH9005 is a Polyurethane Reactive (PUR) adhesive that offers the lowest possible MVTR while ensuring good chemical resistance and excellent adhesion to common electronic device materials. KH9005 is unique as it offers the performance of a structural bonder alongside the properties of a robust sealant.

Krylex’s KH9005 also offers the following features that are important in the areas of electronic device and wearables:

• Excellent biocompatibility, ISO-10993-5 certification

• Excellent impact performance

• Low, stable dielectric constant

• Excellent green strength/ fixture time for high volume production