ultralow residue (ulr) semiconductor grade fluxes for

36th International Electronic Manufacturing Technology Conference, 2014

Ultralow Residue (ULR) Semiconductor Grade Fluxes for Copper Pillar Flip-Chip

SzePei Lim, Jason Chou, Maria Durham, Dr. Hyoryoon Jo, and Dr. Andy Mackie

Indium Corporation

Clinton, NY, USA

[email protected], [email protected], [email protected], [email protected], [email protected]

Abstract

Copper pillars topped with solder microbumps are

emerging as a standard flip-chip solderbump replacement in

the semiconductor assembly industry. The relentless drive

towards finer pitch, combined with reduced copper pillar

height, makes aqueous cleaning of flip-chip flux residues

more difficult. An emergent failure mode is joint damage

during aqueous jet impingement.

The move towards Semiconductor Grade ultralow

residue no-clean fluxes and away from cleaning processes is

therefore inevitable, and this paper discusses the testing of

assembly materials for this purpose.

1. Introduction

The most recent edition of the International Technology

Roadmap for Semiconductors (ITRS) [ref. 1] shows that

package pitches, cost per I/O, die thickness and overall

package thickness will all see significant reductions in the

remainder of the millenium [Fig. 1].

Year of Mass production = 2014 2016 2018 2020 Units

Transistor (T1)

MPU 1/2 Gate Length

after etch 18 15 13 11 nm

Wafer

Die thickness (min) -

Extremely thin packages 10 5 5 5 microns

Cost per I/O for OSAT

production (minimum cost) 0.36 0.34 0.32 0.3 (USD$/pin/100)

Chip Area 100 100 100 100 mm^2

Package pin count 207 - 1100 218 - 1150 252 - 1150 278-1150 number of I/O

Package profile or

thickness (minimum) 0.22 - 0.40 0.20 - 0.35 0.19 - 0.35 0.19 - 0.35 mm

Junction temperature - Tj

(maximum) 105 105 105 105 degC

Operating temperature -

ambient (maximum) 45 45 45 45 degC

Mobile Device

Packages

Single Chip Package Technology Requirements for Mobile Devices

Figure 1: Package Changes 2014-2020 from ITRS

The stress caused by the CTE mismatch between the

substrate and die is exacerbated by reduced die/substrate

clearances, and for this reason, as well as avoidance of

electromigration issues, electroplated copper/pillar

microbumps are becoming standard, replacing the more

common solder bumps [ref. 2].

Flip-chip assembly onto substrates using solder has

typically been carried out by either spraying oxide-removing

flux onto the substrate; dipping the die bumps into flux; or

occasionally, both [Fig. 2]. In most instances, the flux used

has been able to be cleaned away using water or an aqueous-

based solvent system. However, the combination of large

die, finer pitch, and reduced clearances has led to increased

complexity in ensuring adequate cleaning [ref. 3].

Figure 2: Typical Flip-Chip Flux Application Methods

2. Characteristics of Ultralow Residue Fluxes

Ultralow residue (ULR) no-clean fluxes have several

characteristics that are critical to their functionality, such as

rheology, residue level, solderability, the ability to retain die

in place, and compatibility of the final residue with molded

underfill (MUF) and capillary underfill (CUF).

Rheology of the flux is important in several different

aspects of the flux behavior, such as flux-reservoir height

(that is, the actual measured flux height); ability to remove

die from the flux dipping reservoir; consistent dipping

performance; and ability to hold the die in place during

placement and reflow (so-called “MDR” – movement during

reflow).

If the viscosity is too low, flux may wick up the

bump/copper pillar and contaminate the die surface. If

viscosity is too high, it may lead to bridging [Fig. 3]. If the

tack (extensional viscosity) is too high, it may even make it

impossible to remove the device from the flux dipping

reservoir.

Please note that viscosity is only one of the rheological

characteristics of a flux, and only for a Newtonian material is

the viscosity independent of the shear rate; fluxes are mostly

non-Newtonian. Therefore the use of this single point

measurement, although common, is not recommended for

complete characterization, but may be used as a “shorthand”

for many different rheological parameters [ref. 4].


Figure 3: Flux Rheology for Fine-Pitch Dipping

All fluxes leave behind a certain amount of material after

reflow, called “flux residue”. ULR no-clean flux is designed

to provide the same functionality as both standard (high-

residue) no-clean fluxes and watersoluble types, and leaves

almost undetectable amounts of flux residue. Because the

level is so low, free flow of the CUF/MUF is enabled,

minimizing the risk of void formation and delamination. If

the residue chemistry is compatible with the underfills, then

delamination during subsequent thermal cycling and other

stress testing is avoiding.

As with most fluxes, the purpose of the flux is to promote

good solderability (wetting) between two surfaces. It does

this by reacting with metal oxides, making a salt that then

dissolves into the remainder of the flux residues (away from

the joint), leaving clean metal surfaces, and allowing the

formation of intermetallics that drives the formation of a

strong bond between the cooled solder and the underlying

metal.

If the solderability of flux is insufficient, it will lead to

weak joint formation and/or voiding, whereas, if the flux

solderability is excessive it may, in certain instances, lead to

bridging. Hence the solderability of flux must be optimized

to promote good wetting without causing solder bridging

between adjacent I/Os.

It is important for the flux to be able to retain the flipped

die in place during reflow, to minimize die-skew. A test

method (the MDR test) designed to study the movement of

solder on flux during the reflow process is studied in this

paper.

Underfill compatibility is becoming a growing concern as

clearances and the pitch of flip-chips is getting tighter, as

discussed previously.

3. Experiment and Discussion

Rheology: A Brookfield Cone and Plate Viscometer was

used to measure the viscosity of the flux as a function of

time. The spindle used was a CP-51 and measurements were

taken at 25°C at 20RPM. The results for a few versions of

the ULR no-clean fluxes are shown in Figure 4.

Figure 4: Flux Stability shown by Viscosity versus Time

For this study three different versions of the ULR no-

clean fluxes were tested and have consistent viscosity over a

period of 10 hours, which is more than a single shift. After

each shift, the user is advised to clean the flux tray and

replenish with fresh flux, this is to ensure consistent flux

dipping performance is achieved. The rheology (viscosity or

tack) of a flux can be fine-tuned to suit the specific

application, package configuration and equipment capability.

Residue Level: The ULR no-clean flux was compared to

a standard flux residue before and after reflow was explored.

Figure 5 shows the results.

Figure 5: Visual Comparison of Flux Residue Types

Thermal Gravimetric Analysis (TGA) (equipment: TA

Instruments SDT Q600) is used to measure the amount of

flux residue after reflow. This test is done by using a ramp

rate of 10°C/min, with 10mg±1mg sample size, and nitrogen

flow of 100ml/min. The results are shown in Figure 6. Two

different ULR no-clean fluxes (NC-826 and NC-26-A) and

one standard residue flux (Tacflux007) are used to show the

differences in residue levels.


Figure 6: Results of TGA

As shown in Figure 5, there was little or no residue

visible from the ULR flux, whereas with the standard residue

material there was clearly still some flux present. Figure 6

confirms this: at a standard reflow peak temperature range of

235-250°C, the ULR no-clean fluxes have about 10% or less

residue in comparison to about 50% for the standard residue

flux. However, the TGA test is unrepresentative of real

situations for flip-chip flux. The exposed surface area is

much smaller in TGA than for most real flip-chip

applications. In most reflow conditions, the flip-chip fluxes

should therefore leave lower residue levels than shown here.

Solderability: Testing for solderability was done by

printing flux onto a metallized coupon using a stencil, then

96.5Sn3Ag0.5Cu (SAC305) 28mil solder spheres were

placed onto the flux using an automated pick & place

machine. The metallized coupon with the printed flux and

sphere was then reflowed in a BTU oven in a nitrogen-

purged environment at <500ppm O2.

After reflow, solder wetting is calculated from

measurement of the height of the solder bump, and the solder

spread ratio (%) is calculated using the below equation:

S = [(D-H)/H] * X100

Where: S = Spread factor

D = Initial sphere diameter

H = Post-reflow solder height (1)

The results of the solderability test are show in figure 7.

Figure 7: Results of Solderability Test

Three different ULR no-clean fluxes (NC-26-A, NC-510,

and NC-826), one standard residue no-clean flux

(TacFlux007), and one water-soluble flux (WS-575-A) were

used in this test. Typically the standard residue no-clean

flux and water-soluble flux will have better wetting. As

shown in figure 7, NC-826 ULR flux wetting behavior is

close to the WS-575-A water soluble flux. Depending on the

application and the solderability requirement, a suitable ULR

no-clean flux can be chosen.

Movement During Reflow (MDR): The ability of the

flux to hold die in place during reflow was studied by using a

proprietary “movement during reflow” (MDR) test. This

used the same test setup as the solderability test. The results

are shown in figure 8.

Figure 8: Results of the MDR Test

Same as the solderability testing, three different ULR no-

clean fluxes (NC-26-A, NC-510, and NC-826), one standard

residue no-clean flux (TacFlux007), and one water-soluble

flux (WS-575-A) were again used. The results shown in

figure 8 show that Tacflux007 has the least movement during

reflow, as is typical for a standard residue flux. However a

standard flux will have a high residue, partially-filling the

area under the chip, and so making it impossible to get a

void-free underfilling process.

Two of the ULR no-clean fluxes, NC-26-A and NC-826

show a similar amount of movement to that seen when using

the water soluble flux WS-575-A, which is generally

acceptable for most applications.

Compatibility with MUF/CUF: In this study for

underfill compatibility, two different test methods were used.

The first test (Test 1) is used as a more visual aid with a

copper lead frame and glass slide, whereas the second test

(Test 2) was used to evaluate the shear strength.

Test 1a: Tape Test (No T/C): To test the compatibility of

underfill using an adhesive tape test method, a thin film of

flux was spread onto a glass microscope slide. Flux was

then placed onto a copper leadframe and heated on a hot

plate at 240°C for 10-15 seconds. Two leadframes were

used: one with a ULR no-clean flux (NC-26-A) and one as a

control (no flux). After being removed from the hot plate

the leadframes were then placed on a microscope slide. CUF

is then deposited and cured under the recommended

conditions. A visual check (eye and optical microscope) was

used to check the sample surface. A peel/adhesion test was

then performed by placing adhesive tape on the sample

surface then tearing it off to check the adhesion of the

interface.


This test showed no voiding or delamination between the

interface for both the ULR no-clean flux sample, and the

control sample.

Test 1b: Tape Test (with Thermal Cycle): The samples

were initially prepared in the same way as in Test 1a. The

glass slide was then treated with either no bake (control),

baking at 180°C for 1 hour, or baking at 180°C for 3 hours.

After the bake treatment the underfill was deposited and

cured under the recommended curing conditions.

After cure, the underfill was again checked by eye and

OM. An adhesive tape test was performed, tearing it off the

sample surface to check the adhesion of the interface. SEM

cross-sections are then used to check the interface.

Checking by eye and optical microscope showed no void

or delamination between the interface for flux on glass slide

sample. The adhesive tape test also showed before and after

the test a smooth surface with no film delamination.

Test 1b: SEM Inspection: The SEM cross section on the

three post-reflow treatment condition samples: the control

(no baking), baking at 180°C for 1 hour, and baking at

180°C for 3 hours the results are shown in figure 9. The

interface between underfill and glass slide shows no voids or

delamination. Post thermal cycle also shows the good

compatibility result with no voids or delamination issue.

Figure 9: SEM Cross Sections: Before and After Thermal

Cycling

Test 2: Shear Strength Test. The interfacial shear

strength test of the ULR no-clean flux NC-26-A was also

used to evaluate the compatibility between flux residue with

the MUF and CUF by using a MTS Alliance RT/10. The

compatibility is related to the chemistry between flux residue

and the underfill. Therefore the degree of compatibility can

be varied by applying different materials. For each shear

strength measurement, one combination of flux and underfill

(five samples of each) was tested.

For this test there were three test parameters: open

surface of flux, sandwich shape, and a blank substrate for the

control.

Open: For the open surface of flux test, flux was applied

on the top of the substrate, then reflowed under a typical

lead-free reflow profile using a BTU reflow oven. Underfill

was then applied between the substrate and cured under the

recommended conditions.

Sandwich: As in the previous tests, flux was applied on

each of two substrates. These substrates were then plcaed

together in a sandwich, then treated at 180°C for 3 hours.

After treatment, underfill was applied between the substrates

and then cured.

Blank: For the blank substrate (control), no flux was

applied to the substrate, but underfill was applied between

the substrates, then cured.

With this test method, ULR no-clean flux NC-26-A was

evaluated using two underfills: one from Hitachi and one

from Shin-Etsu [Fig. 10].

Figure 10: Underfill Shear Strength

There was almost no influence from the residue of the

ULR no-clean flip-chip flux NC-26-A. The open surface

sample allows easy escape of flux and shows high strength,

but even with the sandwich samples (two flux/underfill

interfaces) good compatibility with the underfills was also

seen.

4. Conclusions

Simple test methods for flux materials alone (that is, tests

outside the auspices of system-level tests like those from

JEDEC [ref. 5]) are, in many instances, inadequate to

quantify the critical to functionality parameters of ULR no-

clean fluxes. The authors have worked together to develop a

suite of novel test methods and associated data that have

given customers confidence that the materials are suitable for

use in emerging copper-pillar flip-chip applications at the

sub-100micron pitch node.

The successful implementation of flip-chip fluxes such as

the NC-26-A and the NC-826 at customer sites in China,

Taiwan, and Korea underlines their utility in the high-

volume package, and high-yield assembly processes

demanded by today’s manufacturers of portable electronic

devices.

References

1. http://www.itrs.net/

2. Gerber et al., “Next generation fine pitch Cu Pillar

technology”, ECTC 2011

3. Lee et al., “Flux study for ultra fine pitch flip chip

packages”, Microsystems, Packaging, Assembly and

Circuits Technology Conference, 2009. IMPACT 2009.

4. S. P. Lim et al., “Ultralow Residue Semiconductor

Grade Fluxes for Copper Flip-Chip”, SEMICON

Taiwan 2014

5. http://www.jedec.org

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