Flat Packages Mounting Adhesives
Introduction
Both consumers and industry are demanding that electronic equipment integrated with IC or
LSI be made smaller and lighter, and have enhanced features. Products such as personal computers, video players, video cameras, cellular phones, and audio equipment, as well as automotive electronic products, are becoming smaller while retaining the same features, or are
adding features if they remain the same size.
To make a piece of electronic equipment “lighter, thinner, and smaller,” the LSI package
mounted on it also needs to be smaller. As a result, the LSI package has evolved from a DIP(Dual In-line Package) with pins for insertion, to a flat package that can be installed on theboard. The package has also become smaller, to match the narrow pitch of the terminals.
This issue introduces different types of flat packages, and the adhesive that Three Bond has
developed to fix or connect the flat package to the board.
Contents
Introduction
1. Flat packages and connection methods ·················································· 2
2. Instant adhesive and curing accelerator
for temporary fixing of the flat package ··················································· 3
3. BGA/CSP mounting adhesive·································································· 6
4. Flip-chip mounting adhesive ···································································· 7

11. Flat packages and connection methods1.2)
The LSI package is a metallic lead frame mountedwith LSI chips, and molded with epoxy or ceramics.
Epoxy-based plastic packages are prevailing in themarket, because they cost less. Figure. 1-1 shows theinternal structure of a flat package.
The package has changed from DIP (pin insertiontype) to SOP and QFP (surface-mounting flat package), and further to thinner TSOP and TQFP. Table
1-1 shows the features of each package.
To support the increased number of pins, a newconnection method that uses ball solder called“bump” instead of lead is used to connect flat pack-Fig. 1-1. Internal structure of a flat package

ages such as BGA and CSP. Figure. 1-2 shows how
packages have evolved.
Recently, a new connection method called “flipchip mounting” (or bare chip mounting) has beendeveloped, by which bare chips are mounted without
being packaged (that is, they are unmolded). Thisnew method seems likely to prevail in the future.
Figure. 1-3 shows flat packages, the different connection methods, and the comparison of mounted IC
size.
The following sections describe the Three Bond’s
adhesives designed for flat package installation andthose designed for flip chip mounting.

Fig. 1-3. Comparison of the areas for IC chip
mounting

Fig. 1-2 Evolution of the outside of packages

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Table 1-1. Features of each LSI package

Package

Appearance

Features

DIP

Dual In-line Package
Package with leads that protrude from its two longer sides

SOP

Small Outline Package
Package with leads that protrude from its two longer sides
and are formed into gull-wing shape (L shape)

TSOP

Thin Small Outline Package
SOP package, 1.0 mm thick

QFP

Quad Flat Package
Package with leads that protrude from its four sides andare formed into gull-wing shape (L shape)

TQFP

Thin Quad Flat Package
QFP package, 1.0 mm thick

BGA

Ball Grid Array
Package with an array of bumps (ball-shaped solder) on
its top or bottom surface

CSP

Chip Size Package / Chip Scale Package
BGA package miniaturized to the size of a chip on a little
larger, and having narrow-pitched bumps

2. Instant adhesive and curing accelerator for
temporary fixing of the flat package
Flat packages, such as SOP and QFP, are used for
double-sided printed circuit boards in audio andvideo equipment, such as audio and video cassette
players, and in cooking equipment such as
microwave ovens, electromagnetic cooking devices,and electronic pots.
The flat package must be securely fixed to theboard at the correct position by the solder dip
process.
ThreeBond 1738 and 1739 are instant adhesives,
developed for temporary fix of the flat package.
ThreeBond 1796B, 1796E, and 1796G are curing
accelerators that speed up the curing of the adhesive.
Conventionally, rubber-base adhesives, doublesided adhesive tapes, or epoxy-based adhesives havebeen used. Rubber based adhesives, however, have
low heat resistance, and often drop or move the temporarily fixed packages during the solder dip process.
Double-sided tape is not practicable, because it isdifficult to correct the position of a package after it isfixed to the board. Epoxy-based adhesive has to be
cured in an oven, and the packages tend to move dur-ing heating. Further, the adhesive might melt andflow into the terminals, causing failures. The packages can be soldered manually with a soldering
knife, using no adhesives, but this method requires
skill and is especially difficult for narrow pitch terminals.
With Three Bond adhesives and curing accelerator,
a flat package can be securely bonded and fixed, andthe adhesive takes only a few seconds to cure. No
special skill is required, and the process can be automated.
2-1. How to use ThreeBond 1738 and 1739
(1) Apply an appropriate amount of the instant adhesive, usually about 0.01 g, to the surface to bebonded.
(2) Place the flat package in position.
(3) Apply the curing accelerator from between theflat package and the panel. Because of the capillary effect, the curing accelerator disperses to theentire surface of the flat package, and curesinstantaneously.

32-4. Insulation resistance to moisture
Table 2-2 shows the insulation resistance measured
on comb-shaped electrodes onto which ThreeBond
1738 and 1739 were applied and cured withThreeBond 1796E.
Whether voltage is applied or not, the insulation
resistance does not deteriorate even after long exposure to high temperature and humidity.
2-5. Influence of the curing accelerator on theflux
Two types of flux have been tested. ThreeBond
1796E was added to each of them by 10% in weight,
and the flux impedance of them was measured after
one day and again after five days at room temperature, using comb-shaped electrodes.
The color of the flux might change when the curing accelerator is added, but its insulation property
remains unchanged.

The solder dip process

2-6. Notes to remember when using the adhesive
To reduce failures, read the following before using
the adhesive to fix the flat package.
Make holes in the panel on which the flat package
is to be installed (see the photos in section 2-1).
These holes serve two purposes: They let the excess
adhesive flow to the back of the board instead of
spreading over the terminals. Also they release thegas generated during the solder dipping process andprevent the flat package from floating, moving, or
falling.
When you shift from a manual soldering processthat does not use adhesive to the method proposed
here, check the location of flat packages and peripheral parts to prevent the solder from bridging
between leads during the solder dipping process.

2-2. Properties (Table 2-1)
The adhesive comes in the form of gel, and will not
run down even on an inclined workbench. It has arelatively slow setting time (30 to 40 seconds), so
positions can be adjusted after the adherends are
positioned.
2-3. Curing speed (Fig. 2-1)
The use of the curing accelerator makes it possible
to fix a flat package in 2 to 10 seconds.

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Fig. 2-1. Curing rate of ThreeBond 1738

Table 2-1. Properties and characteristics of the instant adhesive and its curing accelerator for
temporary fixing of the flat packages

Main component

Instantaneous adhesives
TB1738
TB1739Hardening accelerators
TB1796B
TB1796E
TB1796G

Ethyl-2-cyanoacrylate

Amine compound
Light yellow toLight yellow,
Light yellow,
Light brown,transparent
transparent
transparent
0.9
0.9
0.9
{0.9}
{0.9}
{0.9}
---------------------0.82
0.86
0.82

Appearance

Light blue

Colorless,transparent

Viscosity
mPa·s {cP}
Thixo ratio
Specific gravity

5000{5000}
5.5
1.04

23000
{23000}4.5
1.03

Features• Gel-type instanta• Gel-type instantaneous adhesive
neous adhesive• High-speed cur• No flow after• No flow aftering hardening
application onto aapplication onto a accelerator
vertical surface
vertical surface• Easy to handle

Productshape

20 g plastic bottle

20 g aluminum tube
45 g plastic bottle
500 g plastic bottle• High-speed curing hardening
accelerator• Super-high-speed• Not subject to thecuring hardening
Organic Solvent
accelerator
Intoxication
Prevention
Regulations
50 g glass bottle

500 g glass bottle

Table 2-2. Insulation resistance of the cured adhesive when exposed to high humidity environment
Setting time at 60°C, 95% RH

TB1738/
TB1796E
TB1739/
TB1796E

Initial24-hours

100-hours

500-hours

1.7 × 1012 Ω
6.4 × 1013 Ω
1.0 × 1012 Ω
1.3 × 1012 Ω

1.0 × 1013 Ω
1.0 × 1013 Ω
5.9 × 1012 Ω
9.1 × 1012 Ω

6.2 × 1012 Ω
7.3 × 1012 Ω
8.5 × 1012 Ω
3.8 × 1012 Ω

1.0 × 1012 Ω
2.3 × 1012 Ω
1.2 × 1013 Ω
7.1 × 1011 Ω

Applied voltage
None
DC 100 V
None
DC 100 V

Test sample : A comb-shaped electrode specified in JIS C 6480, type 2

Fig. 2-2. Influence of ThreeBond 1796E on flux

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3. Mounting adhesive for BGA and CSP
The continuing need for ever smaller, more densely integrated electronic parts makes high density
mounting necessary. The conventional QFP imposes
limits on the possible number of pins because of its
line arrangement. Also, it requires circuit boards
whose surface areas may range from a few times aslarge, to a few tens of times as large, as that of theinternal chips.
At present, the packages most widely used are
BGA, and its variant, CSP, also called FPBGA (FinePitch Ball Grid Array). CSP is a narrow-pitch version of BGA; the packages are about the same size as
the chips, or slightly larger.
BGA and CSP have several advantages over QFP:* They require less surface area.
* They offer a larger tolerance for height variations,because the solder balls are fused for connection.
* They avoid lead deformation, which can occur in
QFP mounting* They have an effect of self-alignment, because of
the surface tension of the solder balls when they
are fused. By this effect, any mounting deviation
can automatically be corrected up to 0.6 mm fora 1.5-mm-pitch package (Fig. 3-13).
We will not go into details here, but the mounting
yield has improved and is now better than thatobtained with QFP.
3-1. Sealing resin for the BGA/CSP mounting
The solder joint for the BGA/CSP mounting issmall, only 0.2 to 0.5 mm in diameter. To improve
the reliability of the connection against external
stress, a sealant called underfill is placed between the

package and the board. Epoxy resin is most commonly used for this sealant.
An underfill must have the following properties:
1. It must maintain connection reliability in repeated cycles of low to high temperature shift.
2. It must resist solder reflow.
3. It must be repairable.
For productivity, the underfill must cure at low
temperature in a short time. Our recommended onepart, low-temperature-curing epoxy resin is
ThreeBond 2206; its properties are shown in Table 31.
The conventional one-part epoxy resin had to beheated at least to 80°C for curing. ThreeBond 2206
needs to be heated only to 60°C, but still offers excellent bonding strength, for a one-part epoxy resin.
The glass transition point of ThreeBond 2206 is only
107°C, but it offers sufficient reliability.
The repairability and the reliability of a bond are
mutually contradictory; priority has to be carefully
given to one or another. A package bonded withThreeBond 2206 can be peeled off the board with ascraper after the package is heated to 180°C andabove.
3-2. Conclusion for the BGA/CSP mounting
Because of the advantage, that BGA and CSP offer
in surface area and yield, their use is sure to increase.
Accordingly, the demand for improvement in the
productivity and reliability will increase. Resin manufacturers must continue to develop resins for the
purpose.

Fig. 3-1. Self-alignment effects (1.5-mm pitch)

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Table 3-1. General properties of ThreeBond 2206

Unit

ThreeBond2206

Test method

Black liquid

3TS-201-02

100 {1,000}

3TS-210-05

Specific gravity

1.20

3TS-213-02

Standard hardening
conditions

60°C × 180 min. or
70°C × 50 min. or
80°C × 20 min.

Appearance
Viscosity

Pa·s {P}Storage stability

Month

5

Stored in a refrigerator

Peeling adhesive strength

N/m {kgf/25mm}

785 {2.0}

3TS-304-21

Glass transition point

°C

107

3TS-501-05

Dielectric breakdown voltage

kV/mm

20

3TS-406-01

Water absorption rate (%, 1h)

%

+2.9

Conforms to JIS K 6911
Hardening condition 70°C × 50 min

4. Flip chip mounting adhesive
One of the technologies that support
making mobile information equipment
thinner and lighter while adding newfeatures is the density in which semiconductors can be mounted. The flip
chip mounting method is especially
noteworthy. It uses adhesives resulting in short connection passes andexcellent frequency characteristics.
To fix a bare chip (an IC chip that is
not packaged) by flip chip connection(mounting the semiconductor circuit
face down) using an adhesive, equipment manufacturers have developed
and reported their own unique methods, described in Fig. 4-1.
Three Bond has developed and marketed some underfill agents of thepressure-bonding type, used as
described in Figure 4-1, b and e, as
well as electrically anisotropic paste
used as shown in Figure 4-1, d. These
applications are described below.

Fig. 4-1. Different methods of bonding bare chipsby flip chip connection

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4-1. Pressure-bonding underfill agent
A few different methods are available to makeelectric connection for the flip chip mounting. As
was explained in the section about the underfill agent
for the BGA/SCP mounting, underfill agents are also
indispensable for flip chip connection, to maintainthe reliability after mounting. Today, Three Bondoffers three products--ThreeBond 2271, 2271B, and
2271C--for pressure-bonding an underfill agent.
Each of these has a different viscosity and filling

agents, for different applications. Table 4-1 shows
the general properties and characteristics of thesethree products.
Common characteristics:
1) Long potting life:
8 hours at 25°C
2) High reliability:-40°C to 125°C for 1,000 cycles or over3) Low concentration of chlorine ion:
10 ppm or below

Table 4-2. General properties of pressure-bonding underfill agent

Item

Unit

ThreeBond2271 ThreeBond2271B ThreeBond2271C Test method

Appearance--------

Light yellow liquid Light yellow liquid Light yellow liquid 3TS-201-02

Type of filler

SilicaAlumina

Alumina--------

17 {17000}

12 {12000}

4 {4000}

3TS-210-05

Thixotropy

1.0

1.0

1.0

Specific gravity ofhardened substance

1.47

1.65

1.47

3TS-213-03

Viscosity

Pa·s {cP}

Glass transition point

°C

120

125

125

3TS-501-05

Thermal expansion
coefficient
α1α2ppm/°C

50
130

52
130

55
150

3TS-501-05

Impurity ion density
Cl
Na
K

ppm

< 10
< 10
< 10

< 10
< 10
< 10

< 10
< 10
< 10

> 9.8 {100}> 9.8 {100}> 9.8 {100}Shearing adhesiveMPa{kgf/cm2}
strength83TS-511-01

3TS-301-11

Standard hardening conditions
temperaturetime
post-cure150 ~ 180°C
30 ~ 120 sec
150°C 2 hours

4-2. Microencapsulated anisotropically conductive adhesives
Microencapsulated (MC) anisotropically conductive adhesives are designed for fine-pitch connection,
low connection resistance, and simple mounting. A
conductive Ag filler dispersed in the epoxy-based
adhesive gives it anisotropically conductive features.
The Ag filler is covered beforehand with a layer of
insulation resin about 1,000 Å thick. Only the MC

fillers that are pressed between the bump and theelectrode become conductive, because the pressure
breaks the insulation layer. The MC fillers between
the electrodes receive no pressure, and they maintaininsulation even if they are in contact with one another. After pressure is applied, the adhesive is heateduntil it is cured. As it cures, it shrinks, securing the
IC chip on the board and maintaining electric conductivity.

Characteristics
1) Fine pitch connection supported
Over 1010 Ω has been recorded in an insulation test
by use of a comb-shaped electrode with gaps of 10µm.
2) Low resistance
The electrical resistance of anisotropically conductive adhesive with conductive resin particles coatedwith gold (Au/resin) was compared with that of theanisotropically conductive adhesive with MC fillers
in Fig. 4-2. As you see, the resistance recorded forthe adhesive with MC fillers was 2 to 3 mΩ, 1/10 to
1/50 that of the Au/resin adhesive. Also, critical current, which is the maximum electric current that canflow through the connection point without changing
the resistance, is 4,000 mA, 8 times the value for
Au/resin.
3) Simple mounting
The process of electric connection and resin sealing is completed by pressurizing and heating the chip
after it is aligned.
An anisotropically conductive adhesive of this typecan be adapted to users’ needs by altering the resin
sealant (the base), the diameter of the MC fillers, andthe amount of filler to be added. Table 4-2 shows thegeneral properties of prototype 33A-469B, an adhesive of this type.

Fig. 4-2. Electrical properties

Table 4-2. General properties of 33A-496B

Item

33A-496B

Test method

Appearance

Gray liquid

3TS-201-02

Viscosity

24 Pa·s {24000cP}

3TS-210-05

FillerSilicaConductive filler

5 ~ 10 µm Ag 5 vol%Coating with thermosetting resin

Electric resistance< 10 mΩDielectric resistance> 1010 Ω

DC 10 V

Shearing adhesive strength> 6.9 MPa {70 kgf/cm2}

3TS-301-11

Standard hardening conditions
temperaturetime
post-cure150 ~ 180°C
30 ~ 120 sec
150°C 2 hours

94-3. Future issues
The flip chip mounting with adhesive depends
heavily on the method of each equipment manufacturer. The chip, the boards, and the mounting conditions differ and are customizable. This method isstill in development, and needs a series of technicalbreakthroughs. Both the underfill agent and theanisotropically conductive adhesive need further
improvement, to meet various users’ requirements,
such as other mounting methods.
A problem not yet solved is how to repair failures
found after the mounting. This problem must be
approached by developing adhesive chemistry as
well as controlling mounting conditions.

Three Bond Co., Ltd.
Research and Development Division

H. Mikuni
T. Haijima
K. Kuboyama

References
1) Collection of Precision Bonding Techniques (Latest Version), Comprehensive Technology Center Inc.
2) Road Map for Semiconductor Packages, The Electronic Industries Association of Japan, Electronic Device
Department.
3) E. Hagimoto: “All about the CSP Techniques,” p. 26
4) T. Satoh: “Bonding Technique for the Anisotropically Conductive Adhesive,” The Journal of the Institute of
Circuit Mounting Technology Vol. 10, No 6, (1995).
5) R. Estes: “Polymer Flip Chip PFC: A Technology Assessment of Solderless Bump Processes andReliability” Proceedings of the Adhesives in Electronics 94, Berlin, November2-4,1994.
6) Y. Matsumoto, A. Fujii, K. Kakui, Y. Kamiya, Y. Sasaki, H. Kira: “Solderless Micro Bonding,” Japan
Welding Society, Micro-Bonding Research Paper MJ-230-94 (1994-9)
7) Y. Tomura, Y. Bessho, M. Ono, K. Tsukamoto, T. Ishida, K. Omoya, T. Kobayashi: “Flip-Chip Mounting
Using Anisotropically Conductive Adhesive,” Electronic Materials, No.9 (1994), 22
8) T. Tsukakoshi, A. Nakajima, H. Kobayashi, K. Ueno: “Current Status of High Density Mounting Using
Ansotropically Conductive Film,” MES ’95 reports, pp.139-142(1995-4)
9) H. Date, Y. Hozumi, H. Tokuhira, M. Usui, E. Horikoshi and T. Satoh: “Anisotropic Conductive Adhesive
for Fine Pitch Interconnections”, Proc. of ISHM Fall Meeting 1994(1994-11)
10) K. Hatada: “Bump Direct Connection,” Circuit Mounting Institute, Study Group, Fluxless Soldering
Technique and Trend of the Solderless Bonding Technique, pp 29-33, 1995.3.
11) T. Nukui: “One Method of Low-Temperature Solderless Bonding: Elastic Method,” Low-Temperature
Bonding and Solderless Bonding-Advanced Mounting Research Report, Print Circuit Institute, Advanced
Mounting Research Department, pp 68-75, (1993-7)

1456 Hazama-cho, Hachioji-shi, Tokyo 193-8533, Japan
Tel: 81-426-61-1333

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