Three Bond Technical News
Issued July 1, 2000

55

Underfill Agent for BGA/CSP Mounting
Introduction ________________________________________________
With the rapid evolution of such mobile information equipment as cellular telephones, PHSs, laptop
PCs, and VTRs with built-in cameras, the market is demanding that electric and electronic equipment
be made smaller, lighter, and faster, even as more features are in demand. As part of this trend, ICs(LSIs) need to be made smaller. At the same time, ball grid arrays (BGAs) and chip size/scale
packages (CSPs) are becoming increasingly popular to make the packages smaller, to similar sizes ofbare chips, and to improve their properties. This miniaturization is being achieved by protecting thebare chips, and by making use of package functions, such as stress relaxation, dimension matching,and standardization (generalization).
On a circuit board, a BGA or a CSP is connected to electrodes by means of solder balls. Stresses
such as heat cycling, shock, or bending can cause the connection of the BGA or CSP to be broken. To
prevent this, a sealant (an underfill agent) is applied to the clearance space between the BGA or CSP
and the circuit board; this helps to relax the stress and to hold BGA or CSP in place.
This issue briefly discusses BGAs and CSPs and the need for the underfill agent. It also introduces
underfill agents that Three Bond has developed for mounting BGAs and CSPs to meet a variety ofcustomers' requirements.
Contents

Introduction .......................................................................................................... 1
1. What is BGA/CSP?.......................................................................................... 2
2. BGA/CSP post-mounting problems................................................................. 3
3. Underfill agent for BGA/CSP mounting ........................................................... 3
3-1. Effects of using underfill agent ................................................................. 4
3-2. Requirements of underfill agent ............................................................... 4
4. Underfill agent with repairability ...................................................................... 4
4-1. Problems with underfill agent ................................................................... 4
4-2. What is repairability? ................................................................................ 4
4-3. Underfill agent with repairability ............................................................... 4
4-4. Repair techniques .................................................................................... 4
5. Grades of ThreeBond underfill agent for BGA/CSP mounting ........................ 6
5-1. Characteristics of the underfill agent of each grade................................. 7
Conclusion ........................................................................................................... 8

1

What is BGA/CSP? 1),2)

With the rapid evolution of such mobile
information equipment as cellular phones, PHSs,
laptop PCs, and VTRs with built-in cameras, themarket is demanding that electric and electronicequipment be made smaller, lighter, and faster, evenas more features are in demand. As part of this trend,
ICs (LSIs) need to be made smaller. To achieve thisgoal, we must downsize, integrate, and densely
mount electronic components. In the field of IC(LSI) packaging, there is a limit on the maximum
number of pins with the conventional QFP (Quad
Flatpack Package), as it uses a linear pin array andthe substrate area therefore becomes several to a
few tens as large as the inner chip area. Thus, it
becomes increasingly difficult for the QFPtechnology to meet the packaging requirements.
In turn, the BGA/CSP technology has attracted
attention in the area of chip mounting (see Three
Bond Technical News, No. 50 Flat Package
Mounting Adhesives).
• What is BGA?
BGA stands for "Ball Grid Array"; it is an IC
package in which solder balls are arrayed on
grids; it is smaller than the conventional QFP.
• What is CSP?
CSP stands for "Chip Size (Scale)
Package"; it is a narrow-pitch BGA,
sometimes referred to as "FPBGA (Fine Pitch
BGA)." CSP is the general name given to
packages that are as large as or slightly
larger than the chip itself.
In BGA/CSP packages, the area occupied bypackaging becomes smaller than in the case of QFP,and solder balls are melted for connection. As aresult, their height tolerance is large, and the
problem of lead deformation often seen in QFP is
not occurred. Furthermore, when the solder balls
melt, a self-alignment effect due to surface tension
occurs, and mismatch during mounting can be
self-corrected by as much as up to 0.6 mm in thecase of a 1.5-mm pitch (Fig. 1-3). On the other hand,in packages such as QFP that use to solder leads forinterconnection, the area in which there is surface
tension is small, and the self-alignment effect does
not occur. Thus, if the chip has been moved fromthe predetermined position and left on the circuitboard, it is soldered in the incorrect position,
leading to lower mounting yields due to theformation of a solder bridge (Fig. 1-4).
Recently, a bonding method known as "flip-chip2

mounting (bare-chip mounting)," in which bare IC
chips are mounted with no package (mold), hasbeen developed, and it is expected to makepackages even more compact in size. Despite thatnew technology, BGA/CSP have quickly become
popular, as they are much easier to use for chip
mounting under normal conditions (no need for aclean room).
As mentioned above, the application range of
BGA/CSP is expected to expand drastically into themassive consumer market for cellular phones, PHSs,
personal computers, and other digital devices, due
to its advantages of having a small packaging areaand high mounting yield.
DIP

Smaller

SOP

Thinner

TSOP
More
pins

QFP
More pins

BGA

CSP

TQFP
Thinner

Figure 1-1. Packaging trends
IC chip size

QFP
BGA
CSP
Flip chip

Figure 1-2. Comparison of IC packages

Supplemental
solder0.15mmtsolder ball

Pad

0
0.4
0.75
Substrate
0.6

1.5 (mm)Figure 1-3. Self-alignment effect (1.5-mm pitch)
Displacement to the rightSelf-alignment

Displacement
toward the bottom

1.

(a) QFP(b) BGA

Figure 1-4. Self-alignment function

2.

BGA/CSP post-mounting problems3)

As mentioned above, BGA/CSP greatly
contribute to the downsizing and performance
improvement of various electric/electronic devices.
However,they
involveproblems
withpost-mounting connection reliability; the following
two points are the keys to ensuring connection
reliability.
1) Thermal expansion coefficient (linear
expansion coefficient)
Packages must reduce stress, and this is closely
related with the thermal expansion coefficient ofeach material. The structural question is how tohandle the gap between the thermal expansion
coefficient of the silicon chip (approx. 2.6 ppm/°C)
and that of glass epoxy, which is used as thecommon circuit board (12-16 ppm/°C). Unless thestress due to this gap in the thermal expansion
coefficient between the chip and circuit board is
successfully released through a BGA/CSP structuralcontrivance, the connection reliability will becompromised.
For example, as shown in Fig. 2-1, in which analumina package is bonded to a glass-epoxy circuitboard, the bonding produces high stress due to thelarge difference in the thermal expansion coefficient,and the solder balls have cracks and may cause
connection failure as a result. In pin-insertion-type
packages, the pins work to reduce such stress, whilein lead-type packages the leads reduce the stress. In
surface-mount-type packages such as BGAs and
CSPs, however, the bonding distance is so short thatthe bonding is more likely to break due to stress.

2) Solder-ball bonding strength
BGA/CSP are mounted on a circuit board using
solder balls. The solder bonding is so thin, at φ
0.2-0.5 mm, and so weak that BGA/CSP may fall
off the board when external stress is applied due toshock and bending. In such a case, the cracks in thesolder bonding may reduce the connection
reliability.

3.

Underfill agent for BGA/CSP
mounting

To solve the above problems and improveconnection reliability, a sealing resin called"underfill agent" is used to fill the gap between the
package and the circuit board. As such, as theunderfill agent for BGA/CSP mounting, one-partheat-curing epoxy resin is normally employed. It iscoated around the BGA/CSP (1-3 edges) after
mounting and penetrated into the gap between the
package and the circuit board by capillary forces forthe process of heat curing (Fig. 3-1). The coating
shape and underfill agent amount vary dependingon the package size, pitch, and gap. To avoid
bubbles, it is important to release air from thepackage. (If underfill agent is applied to all foursides of a package, the underfill agent will be cured
with the bubbles remaining between BGA/CSP and
the circuit board.)

Underfill agent

BGA/CSP
Solder ballElectrode

Substrate

Figure 3-1. Using underfill agent
Figure 2-1. Schematic description of the package(PKG)-circuit board (PWB) bonding
breakage due to a difference in thethermal expansion coefficient33-1. Effects of using underfill agent

4-2. What is repairability?

Through the use of underfill agent, connection
reliability can be increased by improving thefollowing:(1) Resistance to thermal stress caused by heat
cycles(2) Resistance to physical stress caused bymechanical shock and bending
This is due to the fact that the underfill agent
reduces external stress. The stress is dissipatedthroughout the package, and the concentration ofstress on the roots of solder balls is avoided.

Repairability is defined as follows. When a defectis found in a BGA/CSP itself or in the connection
with the circuit board after the underfill agent has
cured, repairability allows you to:(1) Easily remove the defective BGA/CSP from thecircuit board(2) Use the other normal BGA/CSP or the circuitboard again

3-2. Requirements of underfill agent
Underfill agent is required to:(1) Spread sufficiently to the gap between the
package and the circuit board and cure there(2) Maintain connection reliability against
thermal/physical stresses(3) Provide excellent electrical characteristics (e.g.,
insulation)(4) Cure at low temperatures in a short time (toprevent damage to the other electronic
components and shorten the line tact time)(5) Repairable (to be discussed later)

4.
Underfill agent with repairability
4-1. Problems with underfill agent
The use of underfill agent significantly improvesconnection reliability, but causes a new problem.
Specifically, when a defect is found in a BGA/CSP
itself or in the connection with the circuit board
after mounting using underfill agent, it will bedifficult to replace (repair) it with another
BGA/CSP (or circuit board). Without the underfill
agent, the BGA/CSP can easily be removed bymelting the solder balls. In bonding using
thermocure resin underfill agent, however, it is very
difficult to remove the BGA/CSP. Therefore evenwhen a defect is found in one of the BGAs or CSPs
on a circuit board, the entire module including the
problem BGA/CSP must be replaced and disposedof. This increases the manufacturing cost.

44-3. Underfill agent with repairability
Repairability is not always compatible withbonding reliability. It is therefore necessary to
decide which will be given priority. For example, if
only repairability is more important, a thermoplastic
resin may be used as the underfill agent for easy
repair, while if only bonding reliability is more
important, a thermocure epoxy resin may be used as
the underfill agent. However, ThreeBond
2274/2274B are underfill agents (Table 1) that aresuitable for BGA/CSP and feature both repairability
and bonding reliability. They can cure at low
temperatures (ThreeBond 2274 cures at 70°C or
higher; ThreeBond 2274B cures at 80°C or higher;
they belong to a category of being curable at thelowest temperature of any one-part heat-curing
epoxy compounded resin), and are designed to be
repairable even if a defect is found.

4-4. Repair techniques
Some examples of repair techniques are shown
below. In them, the cured underfill agent can beremoved by heating and softening it. ThreeBond
2274/2274B harden at room temperature to work as
an underfill agent, but soften when heated forremoval.
1) Removing a defective BGA/CSP
The defective CSP/BGA can be peeled off using
tweezers or the like by hot-blowing the CSP/BGA
from its top with a local heater such as a heat gun(approx. 270-280°C), to soften the underfill agent(softens in 5-10 seconds).

• Be careful not to apply heat for an prolonged
period, or the underfill agent will harden due tocrosslinking and CSP/BGA will be difficult toremove.
• Pinpoint the heat so as not to affect the othercomponents.
• Be careful to avoid damaging the othercomponents.
2) Coarse grinding
Grind off the underfill agent remaining on thecircuit board using two soldering irons (use around-top iron and a blade-top iron). (Soften theunderfill agent using the round-top iron, and scrape
it off using the blade iron. The iron temperature
should be approximately 350°C.)• Be careful to avoid damage to the circuit patternon the board. (Do not attempt to finish this workin one session; rather, it should be performed over
several sessions.)
3) Finish grinding
Using the blade iron, completely scrape off theremaining underfill agent.
• Be careful to avoid damaging the circuit patternon the board. (Do not attempt to finish this workin one session; rather, it should be performed over
several sessions.)

for a longer time or at a higher temperature thanspecified in the standard conditions, they become
harder due to crosslinking and become difficult tosoften or repair.

Heat gunTweezers

Soldering iron(round top)

Soldering iron(blade top)4) Removing solder residual
Remove the solder residual on the circuit board
using a solder cleaner wire or the like.
5) Finish cleaning
Finish the repair by cleaning the repair face using
alcohol or the like.
6) Mounting new BGA/CSP
After the finish cleaning has been performed,
check the pattern on the circuit board for any
problems. If no problem is found, apply solder flux
and mount a new CSP/BGA.
After mounting, apply the underfill agent andcure it.
<IMPORTANT>Do not apply excessive heat to ThreeBond
2274/2274B. If ThreeBond 2274/2274B are heatedFigure 4-1. Example of repair techniques55.

Grades of the ThreeBond underfill
agent for BGA/CSP mouting

Table 1 shows the grades and characteristics of
the ThreeBond underfill agent for BGA/CSP
mounting. All grades of the underfill agent aredesigned to cure at low temperatures in order toprotect electric/electronic components from heat.

* As in the case of normal one-part heat-curing
epoxy compuonded resin, the performance(except for repairability) of the underfill agent
tends to improve when it is cured for a longer
time or at a higher temperature than specified in
the standard conditions (e.g., tensile-shearing
adhesive strength, Figs. 5-1 to 5-3.)

Table 1. Properties and characteristics of Grades of the ThreeBond underfill agent for BGA/CSP mounting
Unit (for reference
purposes)ThreeBond 2202

ThreeBond 2274

ThreeBond 2274B

Appearance

-

Black

Black

Black

ViscosityPa•s (P)

13 (130)

12 (120)

4.7 (47)

Specific gravity

-

1.14

1.14

1.13

-

70°C × 50 min

70°C × 50 min

85°C × 45 minMonths7

6

6

In a refrigerator(5-10°C)

-

D88

D86

D84

3TS-215-01

MPa (kgf/cm )10.0 (102)11.0 (112)16.0 (163)

°C

105

65

97

-5

7.4

Standard curingconditions
Stability duringstorage
Hardness
Tensile-shearing
adhesive strength
Glass transition
temperature
Linear expansion
coefficient2× 10 /°C

7.3
113TS-201-01(Visual check)3TS-210-02(BH-type viscometer)
3TS-213-02(Specific-gravity cup)-

2TS-301-11(Fe/Fe: SPCC-SD)
3TS-501-05(TMA, 10°C/min)
3TS-501-05(Tg or lower)

14

Volume resistivityΩ•m (Ω•cm)

1.3 × 1017
(1.3 × 10 )Surface resistivityΩ

1.5 × 10

-

0.017

0.010

0.012

3TS-405-01 (1 MHz)

3.46

2.97

3.36

3TS-405-01 (1 MHz)

No

Yes

Yes

Repairability-- Curable at low
temperatures- Moderate
viscosity- High adhesivity- High bonding
reliability

Tensile-shearing adhesivestrength (MPa)

Features-17

2.5 × 1013
(2.5 × 10 )8.6 × 10

11

- Curable at low
temperatures- Moderate
viscosity- High adhesivity- High bonding
reliability- High repairability

4.5 × 1016
(4.5 × 10 )8.4 × 1014

3TS-402-01
3TS-401-01-- Curable at low
temperatures- Low viscosity- High adhesivity- High bonding
reliability- Repairability

Tensile-shearing adhesivestrength (MPa)Dielectric loss
tangent
Dielectric
constant

6

9.3
15

Test method

Time (min.)Time (min.)Figure 5-1. Tensile-shearing adhesive strength ofThreeBond 2202 cured at various
temperatures

Figure 5-2. Tensile-shearing adhesive strength ofThreeBond 2274 cured at various
temperatures

Tensile-shearing adhesivie
strength (MPa)Time (min.)Figure 5-3. Tensile-shearing adhesive strengthof ThreeBond 2274B cured atvarious temperatures

5-1.

Characteristics of the underfill agent ofeach gradez ThreeBond 2202 (standard type)1) Curable at low temperatures
Curable at 70°C or higher, with minimum thermal
damage to neighboring components.
2) Moderate viscosity
Moderate viscosity enables easy spreading(filling) into the gap between BGA/CSP and thecircuit board, while limiting its reach to neighboring
components.
3) High adhesivity
High adhesivity at low-temperature curing.
Adhesivity increases with curing at higher
temperatures or for longer periods than specified in
the standard conditions.
4) High bonding reliability
High connection reliability against thermal
stresses such as heat cycles, and physical stresses
such as mechanical shock and bending.
Bonding reliability increases with curing at higher
temperatures or for longer periods than specified in
the standard conditions.
<For application>
ThreeBond 2202 is the best choice if highadhesivity at low temperatures and high bonding
reliability are required rather than repairability.

z ThreeBond 2274 (repairable)
1) Curable at low temperatures2) Moderate viscosity
3) High adhesivity
4) High bonding reliability5) Repairability
If a bonding defetct is found after the underfill
agent has cured, BGA/CSP can be removed byheating and the cured underfill agent can beremoved as well.
<For application>
ThreeBond 2274 is the best at providing highadhesivity at low temperatures and high bonding
reliability, as well as repairability. Its properties
before curing are nearly the same as those ofThreeBond 2202, so it provides workability as highas that provided by 2202.
z ThreeBond 2274B (repairable andlow-viscosity)
1) Curable at low temperatures
Curable at 80°C or higher.
2) Low viscosity
Low viscosity and easy spread (filling), suited forlarge, narrow pitches and narrow-gap BGA/CSP.
3) High adhesivity
4) High bonding reliability5) Repairability<For application>
With low viscosity, ThreeBond 2274B is the best
choice for a narrow-pitch, narrow-gap BGA/CSP.
When cured for a longer period or at a higher
temperature than specified in the standard
conditions, it provides the highest bonding
reliability (and less repairability).
* The spread (filling) and curing conditions ofunderfill agent vary depending on the BGA/CSP
type (material, size, pitch, gap, etc.), circuit-board
surface conditions, heat capacity of theneighboring components, and amount of underfill
agent. Determine the optimal conditions in
preliminary tests using actual components.

7

Conclusion
Not all of the BGAs or CSPs used in various electric/electronic products require reinforcement with theunderfill agent. In some cases, bonding reliability can be ensured through an improved package structure or
design.
However, the importance of reinforcement using underfill agent will increase due to the fact that bonding
reliability and package strength may decline as the semiconductor chips (packages) are made more compact andcircuit boards are made thinner.
Developing new products in addition to the above three grades of underfill agent, Three Bond continues to makeefforts to meet diverse customer needs (including higher workability, bonding reliability, and repairability).
Ken Nazuka
Industrial Materials Development Section-1,
Development Department
Research Laboratory
Three Bond Co., Ltd.
Referenced Documents
1) Roadmap on semiconductor packaging technologies, Electronic Devices Department, Electronics Industries
Association of Japan2) Eiji Hagimoto, All about CSP technologies, Kogyo Chosakai Publishing Co., Ltd.
3) Koichi Yamaguchi, Development in high thermal expansion ceramic, Electronics Packaging Technology

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

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