Previous Page  24 / 52 Next Page
Information
Show Menu
Previous Page 24 / 52 Next Page
Page Background

22

Chip Scale Review November • December • 2017

[ChipScaleReview.com]

Cleaning fine-pitch copper pillar flip-chip packages

By Mike Bixenman

[KYZEN Corporation]

he miniaturization of modern

ele c t r on ic s c on t i nue s t o

challenge the effectiveness of

common cleaning processes and the ability

to obtain desired cleaning performance and

optimal yield. When soldering the flip-chip

die to the ball grid array (BGA) interposer,

water-soluble paste fluxes are used to ensure

a strong metallurgical bond. Ineffectiveness

in removing f lux residues can lead to

contamination, underfill voiding, poor yield,

and reliability issues in the field.

Devices utilizing copper pillar technology

have more interconnects per surface area,

which results in tighter pitch and lower

standoff gaps. As standoff gaps lower, flux

residues have less area to outgas during

reflow. This results in more active residues

under the die. A longer wash time using water

with a low concentration of cleaning agent is

typically required to properly clean die under

these lower standoff gaps.

Aqueous saponified cleaning agents diluted

in deionized (DI) water have commonly been

used to remove flux residues under the flip-

chip bottom termination. Saponified cleaning

agents are mildly alkaline to form an attractive

force to the flux residue. However, alkaline

cleaning agents can react, consume and

corrode reactive metals. Longer time in the

cleaning solution increases chemical attack,

which can cause failures due to mechanical

stresses. Potential metal incompatibilities

and insufficient rinsing represent the most

common challenges for engineering improved

cleaning agents for cleaning copper pillar flip-

chip packages.

Current saponified cleaning solutions

adversely react with many metals (Al, Cu,

Sn, Ag, Ni, etc.) that are present on the

copper pillar die. Especially in alkaline

solutions, Al is readily attacked causing

galvanic corrosion reactions. Also, there is the

potential for defects and discoloration on Cu

and SAC alloys during the cleaning process.

Crevice corrosion from cleaning solution

reactions with exposed metals can weaken

interconnects and reduce mean time to failure.

Cu pillar technology

Copper pillar bumping is a growing design

trend in electronics packaging. Copper

pillar technology offers many advantages

in speed and line pitch. Building integrated

circuits with copper and aluminum reduces

the potential for electromigration while

improving current carrying capacity.

Likewise, copper pillar is more cost effective

than Au stud bumps for high bump designs.

Finer pitch can be achieved, which translates

into higher performance over a smaller

surface area (

Figure 1

).

Unlike the traditional bumping process,

the copper pillar design exposes a number

of reactive metals to the cleaning process.

Many of these metals react and dissolve

when exposed to saponified cleaning agents.

Corrosion inhibitors can reduce, and in some

cases, prevent this interaction. The challenge

is designing cleaning agents that are effective

at inhibiting the different exposed metals

with which the cleaning agent comes in

contact. If longer cleaning time is needed,

the risk of metal interaction is greater.

Aluminum is highly reactive when

exposed to alkaline cleaning agents.

Common saponified aqueous cleaning

agents that are used to clean traditional

bumped die work well. These cleaning

agents are not suitable for cleaning copper

pillar die due to their propensity to attack

aluminum and copper. Aluminum is

a diffusion barrier metal for copper.

Common alkaline cleaning agents can

attack, dissolve and crack the aluminum

pad.

Figure 2

is an example of how a

traditional saponified cleaning agent

at tacks, cor rodes and dissolves the

aluminum pad. Copper is also reactive

when exposed to alkaline cleaning agents.

Cleaning solutions with poor copper

inhibition will tarnish and oxidize the side

of the copper pillar (

Figure 3

).

Nickel functions as the intermetallic

layer between the copper pillar and SAC305

alloy. Cleaning agents high in alkalinity

have been shown to undercut the copper/

nickel adhesion layer (

Figure 4

). SAC305

alloy can pit and darken when exposed to

highly alkaline cleaning agents. Longer

exposure time increases the interaction

(

Figure 5

). The problem is that cleaning

agents that react with the exposed metals

result in various forms of pitting, galvanic

and crevice corrosion. The corrosion effect

not only affects electrical performance

T

Figure 1:

Copper pillar design.

Figure 2:

Al pad dissolves, cracks and turns white.

Figure 3:

Copper pillar oxidation.

Figure 4:

Nickel adhesion layer undercut.