Electroless Nickel/Immersion Gold plating, or ENIG, is a versatile process and enables
fabrication of high-density flip chip BGA substrates needed for high-performance IC chips.
ENIG is used extensively in advanced IC packaging of microprocessors, ASIC and DSP
components. By its nature, ENIG plating forms brittle intermetallic compounds of nickel, tin,
and other elements in the solder after solder balls are attached to the package substrate.
Certain conditions of high strain and high-strain rate are known to cause ENIG solder joints
to fail. Therefore, care must be used to avoid excessive shock and bending of the PC board
during assembly, handling, and testing of FCBGAs with ENIG plating. This paper discusses
the use and handling of semiconductor packages which contain electroless ickel/immersion
gold BGA pad finishes.
Introduction
This application report describes the quality and reliability of flip chip ball grid array packages
using electroless nickel/immersion gold pad finishes. Also included is information with which to
make informed decisions regarding use and reliability of TI FCBGAs with ENIG finish.
1.1 Glossary of Terms/Acronyms
The following terms and acronyms are used throughout the document.
ASIC – application specific integrated circuit
BGA – ball grid array
DSP – digital signal processor
IC – integrated circuit
FCBGA – flip chip ball grid array
ENIG – electroless nickel/immersion gold
OSP – organic solderability preservative
PC – printed circuit, as in printed circuit board, or PCB
1.2 Background
Flip Chip Ball Grid Array (BGA) packages produced with electroless nickel/immersion gold
(ENIG) pad plating are at higher than normal risk for brittle solder-joint failure or the package-off
defect. The brittle solder-joint failures are attributed to a quality problem with the nickel
undermetal. This phenomenon is referred to in the industry as black pad.
The generally-accepted root cause for the poor quality of the ENIG finish, specifically black pad
defect, is corrosion of the nickel undermetal during plating of the immersion gold layer. The Ni
and Au plating layers are applied during the fabrication of the package substrate.
1.3 Key Points Regarding Electroless Ni/Immersion Au Surfaces Finishes
The following points are important to understanding ENIG processes and finishes:
• ENIG is an electroless plating process. In this process, nickel and gold are plated on copper
pads for IC components such as packages.
• ENIG plating is a versatile process and enables fabrication of high-density Flip Chip BGAs
needed for high performance IC chips.
• ENIG is extensively used in the industry in advanced IC packaging in microprocessors,
ASIC, and DSP.
• ENIG plating forms brittle intermetallic compounds of nickel, tin, and other elements in the
plating after the solder ball is attached to the package. Brittle materials can break easily at
high strain or under impact. Therefore, during assembly, testing, and handling, care must be
used to avoid shock and excessive bending of PC board.
• Research is ongoing in the industry to develop alternative finishes, with improved toughness,
to ENIG.
• Certain conditions of high strain and high-strain rate could cause ENIG joints to fail.
• Strain and strain rates on the PCB can be measured using strain gauges attached to the
PCB, and special instrumentation.
1.4 Quality and Reliability of FCBGA Packages Using ENIG Pad Finishes
TI has evaluated the second-level solder joint reliability for IC packages with ENIG pad finishes,
and has established minimum performance and reliability requirement for ENIG packages.
These reliability requirements meet or exceed those of similar package technologies which use
non-ENIG pad finishes such as electrolytic nickel/gold, and copper OSP.
Field reliability of ENIG packages is determined to be very good when the use and handling of
the IC package is as described within this document. ENIG pad finishes have been used
extensively by TI for FCBGA packaging since 1998.
To date, a high rate of customer returns after initial field deployment of the product into the
customer’s end application has not been observed. The occurrence of solder joint failure, as
seen by package-off defects, has been observed primarily at SMT assembly, ICT and system
integration, and not after extended field use by the customer.
1.5 Quality Control and Containment
Since the root cause for black pad defects is corrosion of the nickel undermetal, the
phenomenon which causes black pad will most likely affect an entire substrate lot, as opposed
to a single substrate or single BGA pad.
Identification of products affected with black pad is not possible by means of conventional,
non-destructive techniques such as external inspection of the package and substrate.
Destructive analysis techniques, such as microsection or ball pull test, are required to identify
affected material. Thus, methods for detecting black pad in a high-volume assembly
environment are very difficult to deploy.
Containment methods typically include quarantine of suspect material based upon observations
of the SMT assembly process and designation of specific substrate lots as known bad material.
Rework or repair of BGA pad metallization with black pad is not possible. Therefore, reballing or
resoldering of BGA pads with black pad should be avoided.
1.6 SMT Board Design and Assembly Considerations
TI has determined PCB thickness to be an important element in the reliability of ENIG solder
joints. To that point, TI has determined PCB thickness of 0.093” to be acceptable. Use of PCBs
of lesser thickness should be fully evaluated using the assembly process, testing, and system
integration procedures to be used by the customer or contract manufacturer. PCB thickness
greater than 0.093” may have a deleterious effect on long-term cyclic temperature performance;
thus, you should select a PCB thickness that is suitable to resist damage to the package during
PCB assembly, and ensure long-term reliability for its intended field application.
In addition to properly designing the PCB, it is necessary to keep the following factors in mind:
• Understand the reflow process that best fits your PCB system mounting requirements.
Follow the provided reflow profile and compare closely to the solder paste manufacturer’s
recommended reflow profile,
• Conduct appropriate strain and strain rate characterization on PCB assembly process prior
to component mounting on a new PCB design.
• Avoid excessive shock and bending of the PC board during assembly, handling, and testing
of FCBGAs.
• Focus on material handling procedures that result in flexure, dropping or stressing of the
PCB.
In order to extend the 2nd level solder joint reliability of an ENIG package, the product/process
engineer can consider the use of package−level underfill. Package−level underfill will distribute
mechanical stresses imparted on the solder joint interface with the packages substrate, and will
provide additional mechanical strength through adhesion to the package and PCB. |
