ThreeBond Technical News
Issued July 1, 2014

84

Light-curing Adhesive for Image Sensors
and Optical Pickups
Introduction
Light-curing adhesives are used in a wide range of fields thanks to fast curing which shortens
manufacturing process time. At ThreeBond we have developed, manufactured and releaseda variety of light-curing adhesives for hard disk drives (HDD), liquid crystal panels and otherelectrical/electronic devices that meet customer application needs.
Recent years have seen electrical/electronic devices grow smaller and lighter. With this progression,
optical devices such as image sensors used for digital cameras and cameras on smartphones as well
as optical pickups used for computers and other light-using devices have also grown smaller. As
the designs for such devices diversify, the adhesives used must exhibit high bonding strength ina small area for various materials while also suppressing misalignment of the optical axis both
before and after curing, and overcoming hurdles such as resisting heat caused by high densities ofelectronic parts.
As a solution to the above problems we are proud to present ThreeBond 3036G, ThreeBond 3038
and ThreeBond 3038B, light-curing adhesives for image sensors and optical pickups that canhandle both conventional mercury lamps as well as UV-LED light sources.
Hereafter, ThreeBond is abbreviated as TB.

Table of Contents

Introduction.................................................1
1. Required Properties................................2
2. Parts Used..............................................3
3. Product Introduction(TB3036G, TB3038, TB3038B)...............4

4. Property Changes Due to Changes inEnvironment............................................5
5. Reaction Speed, Outgassing,Environmental Performance...................6
6. UV Light Sources....................................7
Closing........................................................81

1.

Required Properties

The common feature of image sensors and optical pickups istheir use of light. These devices perform their functions by
accurately capturing light through lenses, mirrors, prisms
and other optical components.
For this reason, rather than simply requiring adhesion tospecific adherends, light-curing adhesives for affixing partsof image sensors and optical pickups require the following
properties.
(1) Bonding to various adherends
Depending on the parts used, adherends range from glass
to metals, plastics, and this range of different materials
continues to increase. The adhesive must be able to bondwith this variety of materials.
(2) Suppressing optical axis misalignment
Image sensors and optical pickups are affixed using anadhesive after the optical axis (optical path) is adjusted to bewithin a very narrow target range of only a few μm. Adhesive

contracts as it cures, and a large contraction ratio can cause
misalignment in the optical axis (Fig.1). Therefore, adhesivewith a low shrinkage ratio is required. The three adhesives
introduced in this article boast a low contraction ratio ofaround 3%.
(3) Low amount of change due to environment
Misalignment of the optical axis occurs during curing, andcan also occur due to environmental conditions such as
high temperature and humidity or due to sudden changes intemperature. In order to ensure intended function, it is vitalthat parts alignment in image sensors and optical pickups
not be disrupted while ensuring that adhesive strength is
not reduced.
We developed and released our light-curing adhesives withthe characteristics required to solve the above issues while
also being able to handle UV-LED light sources. In thefollowing sections, we will introduce TB3036G, TB3038
and TB3038B alongside the materials they are used with.

Optical pickup(1) Properly aligned

PD (photo detector)
Optical axis
Target range
Several µm

LD (laser diode)(2) Misaligned
Misaligned optical axis

Aligned optical axis

AdhesiveFig. 1 Misalignment2

2.

Parts Used Optical pickups

 Image sensors

Currently, the CMOS sensor is the main image pickup
device equipped in digital and smartphone cameras (Fig. 2).
The camera is vital to image quality performance because itis responsible for recording the image by exposing the image
sensor to light entering through the lens. This is the heart ofthe camera, and TB3036G, with its low curing contraction
ratio, and TB3038B, which possesses high bonding strength
to metal, are used for affixing these part.
Image sensors tend to be larger the higher the image
quality required, but this also means an increase in weightand stronger adhesion is then required to handle this.
Additionally, any misalignment in the image sensor will
negatively impact images shot, so this must be suppressed.

Optical pickups are devices used in the laser light sources,
receivers, lenses, mirrors and other optical parts for audio
and video playback and recording with optical drives forCDs, DVDs, BDs, etc. These lasers are output by an LD(laser diode), then reflected and transmitted by a mirror
or lens and captured by a PD (photo detector) to perform
recording or playback. The LD that outputs the laser and thePD that captures it must have high resistance to the heat thatso easily builds up and must suppress misalignment in orderto accurately record and play back data.
Light-weight and highly heat-resistant materials are required
for these parts, so PPS, aluminum, epoxy glass and othermaterials that are difficult to bond to are used. This is why
TB3038, which exhibits high adhesive strength for a variety
of materials, is used.

Pentaprism

Magnified view of the base area

PD, LD

Image sensor

Optical parts area
PDLDLens
AF sensor

Fig. 2 Digital Camera and Optical Pickup Structure33.

Product Introduction(TB3036G, TB3038, TB3038B)• TB3038B
Like TB3038, exhibits high adhesive strength for PPS,
polycarbonate plastic, glass and metal. Additionally, theadhesive changes from blue to pale yellow when exposed toUV, allowing the level of exposure and Degree of curing to
be visually confirmed.

The features and properties of each of these adhesives are as
follows (Table 1, 2; Fig. 3).
• TB3036G
Flexible with a low rate of contraction during curing. Bonds
well with a variety of adherends such as metal and plastic.

Hardness, adhesive strength and visual confirmation
performance vary per product, so choose the adhesive thatbest suits your application.

These traits make it most suitable for parts where highadhesive strength with impact resistance is required.
• TB3038
Exhibits high adhesive strength for PPS, polycarbonate
plastic, glass, metal and other materials that are difficult tobond to. Suitable for parts requiring high adhesive strength,a low curing contraction ratio and high hardness.

Table 1 Physical Characteristics
Unit

TB3036G

TB3038

TB3038B

Testing method

Remark(s)Appearance-WhiteWhiteBlue

3TS-2100-001-Viscosity

Pa·s

29.0

13.0

12.5

3TS-2F00-007

Shear rate 20.0s-1

Specific gravity

-1.53

1.56

1.56

3TS-2500-002

25°C

Table 2 Cured Material Characteristics

Hardness
Rate of contraction
when cured
Thick film curingperformance
Storage modulus (E’)

Unit

TB3036G

TB3038

TB3038B

Testing method

Remark(s)

-D40

D70

D75

3TS-2B00-004

-%

2.0

2.2

3.0

3TS-2600-001

-mm

2.5

2.9

2.8

3TS-3160-001-Pa

2.4×108

3.3×109

3.3×1093TS-4730-001

25°C

Curing conditions: UV-LED (365 nm) 400 mW × 17.5 sec

Tensile Shear Bond Strength (MPa)

10TB3036G
TB3038
TB3038B

864

Curing conditions: UV-LED (365 nm)
400 mW × 17.5 sec
Diagonal lines in the bar graphindicate material failure

20

Glass/GlassPPS/Glass

PC/Glass Glass epoxy/Glass AL/Glass

SUS430/Glass

Fig. 3 Tensile Shear Bond Strength4

Property Changes Due to Changes inEnvironment

As mentioned above, adhesives for image sensors andoptical pickups must change very little when subjected tovarious environmental conditions. Generally, applying
heat makes a substance softer and applying cold makes it
harder. Repeated stretching and contraction due to heating
and cooling generates stress in the adhesive interface whichmakes misalignment more likely.
As a result, adhesives for image sensors and optical
pickups must change very little when subjected to various
environmental conditions.
In this section we explain the results obtained whenmeasuring property changes.

These measurements indicate that, compared to typicallight-curing adhesives, the elastic modulus of our products
is affected very little in temperature changes and dramaticchanges are unlikely to occur.
 Reliability testing
Digital cameras and computers are used in a variety ofenvironments, so the adhesives used with them must also
maintain adhesive strength, even under harsh conditions.
TB3036G, TB3038 and TB3038B show very little changeof strength in high heat and humidity environments as wellas environments with changes in heat, making them highly
reliable light-curing adhesives (Fig. 5, 6).
8070 Dynamic viscoelasticity measurements
The typical allowable temperature for digital cameras andcomputers is from 40 to 80°C. Our design was created toreduce changes in elastic modulus within this temperaturerange.
The following results compare measurements of generallight-curing adhesives and the products described in thisarticle (Fig. 4).

Adhesive strength (N)

4.

60

BLANK
100h200h

5040302010• TB3036G0This is the softest of the three products introduced in thisarticle, and the shallow line in the graph indicates very
little change. Therefore, misalignment is suppressed while
maintaining flexibility.

TB3036G

TB3038B

Environmental conditions: 60°C 95% RH
Curing conditions:
UV-LED (365 nm) 400 mW × 17.5 sec
Adherend:
PPSFig. 5 High Heat and Humidity Test Results• TB3038, TB3038B
These have a higher Tg and elastic modulus than TB3036G.
This is indicated by a high line on the graph. Because Tg ishigh, elastic modulus changes gradually.

80
70

Adhesive strength (N)1.0E+12

Elastic modulus (E’) Pa

TB303860

BLANK
100 cycles
200 cycles

50
40
30
20
100

TB3036G

Typical light-curing adhesive (large change in elastic modulus)
TB3038
TB3038B(Maintains high elastic modulus with gradual change)

1.0E+05-40-200204060

TB3038

TB3038B

Environmental conditions: -40°C to 80°C, 30 min. eachCuring conditions:
UV-LED (365 nm) 400 mW × 17.5 sec
Adherend:
PPS

TB3036G (Relatively low elastic modulus with gradual change)

80

Fig. 6 Heat Cycle Test Results

Temp (˚C)

Fig. 4 DMA Measurements

5

We measured adhesive strength through the methodology
described below (Fig. 7).
M3 washer
Inner diameter 3 mm
Thickness 0.5 mm

AdhesivePPS
1. Washer is placed on the PPS test piece.
2. Inside of the washer is filled with adhesive and cured.
3. Pressure is applied horizontally to the side of the washer
using the measurement equipment. Adhesive strength
determined as the strength required to cause peeling.

Table 3 Outgassing Volume
Unit TB3036G TB3038 TB3038B Testing method
Outgassing
VolumeFig. 7 Adhesive Strength Measurement Method

Heating
residue

Reaction Speed, Outgassing,Environmental Performance Curing speed and reaction rate
Fast curing is required to reduce the time required for
manufacturing processes. Additionally, a high reaction
rate is required to keep large amounts of non-reactive
components from causing misalignment or outgassing. The
high reactivity of this group of products means that theywill cure in well under a minute, even when cured with UVLED (Fig. 8).
To measure curing speed, an FT-IR method was used inwhich results are checked from the peak change according
to the particular functional group.
120Reaction rate (%)

1008060
40
TB3038
TB3038B
TB3038G

0010203040
Time (sec.)

50

Light source: UV-LED (365 nm) 400 mW/cm2Fig. 8 Curing Speed and Reaction Rate

6%

0.7

0.5

0.5

99.3

99.5

99.5

3TS-4720-001

Curing conditions:UV-LED (365 nm) 400 mW × 17.5 sec
Measuring conditions: 80°C for 2 hours after curing

The three products introduced here cure quickly, have a highrate of reaction, demonstrate little outgassing and containvery little halogen and no organotin compounds.

20

As current electrical/electronic products become thinner,
the distance each of their parts is reduced, raising thelikelihood of contamination of lenses and mirrors whenadhesives generate gas. Even highly airtight designs aregreatly affected by gas. This group of products was designed
for low levels of outgassing.
Changes in weight in an 80°C environment indicate thatoutgassing volume is very low (Table 3).

Measuring device5.

 Outgassing Volume60

 Environmental performance
The values below indicate the amount of chlorine, bromine
and organotin (not contained) contained in TB3036G,
TB3038 and TB3038B (Table 4).
Table 4 Environmental Performance
Measurement
methodTB3036G

TB3038

TB3038B

Chlorine (ppm)

N.D.

N.D.

N.D.

Bromine (ppm)

N.D.

N.D.

N.D.

Combustion
methodOrganotin

N.D.

N.D.

N.D.

GC/MS

N.D. = Not Detected* Measured lower limit

6.

UV Light Sources

UV-LED light sources can be used to cure these products.
Differences in light sources are further explained below.
 Wavelength distribution
Compared to mercury lamps, metal halide lamps and otherdevices that generate ultraviolet rays, UV-LED utilized
here has a very narrow distribution of wavelengths, thewavelengths released are quite specific (Fig. 9).

As mentioned above, because UV-LED is a low energy
light source and there are issues regarding reactivity withconventional UV-curing adhesive, it is difficult to obtainhigh adhesive strength and suppress misalignment, so it was
not used in areas requiring positional accuracy.
However, TB3036G, TB3038 and TB3038B are light-curing
adhesives for image sensors and optic pickups that deliver
high adhesive strength and suppress misalignment when
cured with UV-LED.

1.1
1UV-LED
Mercury lamp
Metal halogen lamp

Emitted intensity

0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0250

350

450

550

650750

Wavelength (nm)Fig. 9 Wavelength Distribution by Light Source

UV-LED is an ecological source because it generates little
heat and is energy efficient. Another advantage is that highlevels of brightness can be output with compact equipment.
However, in order to use it as a light source for light-curing
adhesive, the adhesive must be designed in accordance tothe light source. This is because energy varies depending on
the wavelength.
 Light energy
In light energy, the shorter the wavelength, the higher theenergy produced, as indicated by the formula below (Table5). Additionally, when considering wavelength distributionof the light source in total, UV-LED demonstrates lower
energy than other light sources.
: Energy: Planck constant: Frequency of vibration: Light velocity: Wavelength

Table 5 Energy Differences by Wavelength
200 nm 300 nm 400 nm 500 nm
Energy (kJ/mol)

598

398

301

239

7

Closing

We are proud to present TB3036G, TB3038 and TB3038B, our adhesives for image sensors and optical pickups. We are certainthat image sensors and optical pickups will continue to become smaller and thinner, reducing the area for bonding and increasing
the influence of curing contraction and susceptibility to heat. This means that a higher and higher level will be expected oflight-curing adhesives.
Moving forward, we at ThreeBond will make every effort to meet these customer needs, continuing our development of lightcuring adhesives that meet the next generation head on.
<References>

Technical News No. 45, Visible light-curing Resin

Technical News No. 76, Light-curing adhesives compatible with UV-LED Light Sources
ThreeBond Fine Chemical Co., Ltd. R&D Headquarters
Development Division 2, Electric Development Division 2
Yasunori Sakamoto
Sei Saito

ThreeBond Co., Ltd.
84-3-3 Minamiosawa, Hachioji,Tokyo 192-0398
Tel: +81-42-670-5333