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Lead (Pb)-free packaging is
part of a concerted effort in the
electronics industry to eliminate
Pb in electronic assembly.
Only ~0.2% of the Pb worldwide
is used for electronic
assembly; most of it is used in
automotive applications. But
when a hazardous substance
ban is implemented, all products
containing the material are
equally affected.
The Pb-free movement in
the electronics industry has
accelerated since the European
Union (EU) released a directive
calling for the removal of Pb
and other hazardous materials
from electronics by 2006 as
part of the RoHS (Restriction
of Hazardous Substances).
The requirement for Pb-free is industry-wide and not exclusive to Xilinx.
Although we wish to lead Pb-free implementation
efforts to secure advantages in
winning future designs, it is equally
important to align with the rest of the
industry in terms of Pb-free technical solutions,
such as solder ball composition and
lead finishes.
Xilinx has made a conscious effort to
remain within industry-standard boundaries
for lead finishes and therefore leverage
the infrastructure to satisfy industry needs.
However, there are some differences. As a
PLD company, Xilinx is in a unique position
because it has the industry's largest die
sizes; the resulting stresses in packages are
much higher. As a result, the most common
package construction materials, such as die
attach and mold compound offered for Pb-free
packages, required a significant overhaul
to ensure the same levels of
performance and reliability for Xilinx applications.
Technical Challenges
Figure 1 shows the application of Pb-based
alloys in electronic materials. Pb in tin
(Sn)Pb solder is used for electronic assembly.
The alloy is called eutectic solder,
which consists of 63% Sn and 37% Pb.
This material has been very well characterized
and understood for the last 40 years,
and a drop-in replacement of SnPb for soldering
applications in electronics has not
yet been found.
Typically, when the parts are placed on
PCBs and sent through the board assembly
process, the SnPb alloy melts to form a solder
joint. This is what is referred to as the
reflow process. The alloy melts at 183°C
and the peak reflow temperature for this
alloy is restricted to 220°C.
But Pb-free replacement alloys in the
electronics industry have higher melting
temperatures than the current SnPb eutectic
alloy. As a result, the peak reflow temperatures
have gone up by 25 to 40°C.
This poses a significant restriction on the
material capability of existing electronic
packages today, as they must be able to
withstand temperatures of 245 to 260°C.
Figure 2 illustrates this issue.
Today's Pb-free packages are capable of
withstanding higher reflow temperatures
during assembly. As a result, all package
construction material such as die attach,
substrate, and mold compound have
improved significantly so that they too can
withstand higher reflow temperature
requirements.
Leader of the Pack(age)
The Pb-free program at Xilinx was established
in 1999 as a proactive effort to
ensure future leadership. Xilinx quickly
formed partnerships with customers like
Sony and Matsushita Electric Industrial
for Pb-free beta-site implementations.
The initial focus for Pb replacement
was unclear, and mostly viewed as an environmentally
friendly product introduction
offering a marketing advantage. Our
implementation strategy today is far more
serious than it was two years ago, given the
EU directive with its "hard" compliance
date of 2006.
The Xilinx plan is a global strategy of
implementation and industry standardization.
Xilinx has closely followed the activities
of industry consortiums MEPTEC
(Microelectronics Packaging and Test
Engineering Council) and NEMI
(National Electronic Manufacturing Initiatives Inc.) and standards organizations such as IPC and JEDEC (Joint
Electron Device Engineering Council) to
ensure that the solutions for Pb-free are
accepted worldwide.
The primary assembly partner of choice
was Amkor, closely followed by
Siliconware Precision Industries. The first
Pb-free prototypes were shipped to beta
customers for evaluation in 2001; since
2002, Xilinx has shipped Pb-free products
in volume.
The extra letter "G" in current package
designations easily identifies Xilinx Pb-free
parts. For example, the Pb-free version of
the VQ100 standard package is VQG100.
This unique identification of the product
simplifies inventory and supply chain
management.
Backward Compatibility
In the electronics industry, the ideal conversion
to Pb-free is a drop-in replacement
of solder finishes and associated materials
in electronic packages that mimic the current
SnPb finishes relative to assembly and
reliability. Unfortunately, no such product
exists today that allows the industry to
quickly switch over to Pb-free products.
There will always be a transition period
for any major change requiring industrywide
implementation. Customers can
expect today's standard and Pb-free products
to coexist until the date of transition.
From the perspective of component suppliers
like Xilinx, this calls for carrying
inventory of both packages for all product
families targeted for the Pb-free market.
This is a significant effort in terms of operations,
with a supply chain that must be
managed skillfully until all products are
Pb-free.
From a user perspective, Xilinx would
like to ensure that all system components
are Pb-free compatible, including all components,
passives, and connectors. This
presents a significant challenge, however,
as all suppliers are clearly not ready all at
once to implement a Pb-free solution.
Customers will have different timeframes
for implementation dates using the Pb-free
package solution, and companies supplying
parts to the industry will be forced to
carry dual inventories of packages for the
same product.
A creative solution to this problem is
backward compatibility – using Pb-free
packages directly on existing PCBs with no
changes in the assembly process or longterm
reliability.
The schematic in Figure 3 illustrates
the requirements and restrictions of backward
compatibility. Lead frame Pb-free
packages (TQ, PQ, VQ, SO, VO, and so
on) are fully backward-compatible with
the proposed Pb-free solutions.
The PBGA (plastic ball grid array)
packages are a different story. To date, no
obvious backward-compatible solution
exists in the industry. This mandates a dual
inventory of Pb-free PBGA parts until the
industry is fully converted to Pb-free
assembly solutions.
Lingering Legacy Issues
The Pb-free lead frame solution has a matte
Sn finish on the leads. For the last 30 years,
Sn plating has been used on terminal finishes
for passive components. There are a
few legacy issues relating to a whisker-like
structure formation on Sn-plated leads.
When the electronics industry was in its
infancy, bright Sn finishes were common.
This bright Sn plating was shown to be susceptible
to whisker growth (single crystals)
in appropriate temperature/time conditions.
The whiskers would eventually grow
large enough to short out adjacent leads.
The new solution using appropriate
matte Sn plating is implemented using an
advanced, well-controlled Sn-plating bath
with special additives resistant to whisker
growth. Although no standard tests in the
industry exist for whisker growth, Xilinx
has worked very closely with industry consortiums
and assembly partners to exhaustively
test for whisker growth in the
Sn-plating offered for Pb-free lead frame
packages. So far, none of the known tests
have shown significant whisker growth.
Although most of the industry, including
large suppliers of microprocessors and
controllers, have made clear commitments
to move towards this preferred industry
solution, the telecom, networking, storage,
and aerospace industries are cautious about
matt Sn as a single alternative for Pb-free.
As a result, new tests are being proposed with several whisker growth mitigating
solutions, although as yet no clear agreements
exist among the industry players.
Pb-Free Flip-Chips
By definition, flip-chip packages are ball
grid array packages with interconnect solutions
based on area array solder bumps.
The internal interconnection is through
solder bumps, which are similar in composition
to external solder balls.
Xilinx is evaluating its flip-chip packages
with large die sizes, and our current
focus is to introduce Pb-free flip-chip
packages in two phases. The primary goal
is to be compliant with RoHS.
The first phase for the introduction of
Pb-free flip-chip packages will be based on
eutectic Sn/Pb solder ball replacement with
Pb-free solder balls only. The current schedule
for this implementation is by the end of
2004. This will allow packages to be on customer
boards until the 2006 deadline.
The second phase will ensure complete
compliance with RoHS mandates by 2006.
The primary focus during this stage will be
compliance with the RoHS directive for
the solder bumps inside the package at the
silicon-to-substrate interconnect level.
Xilinx plans to have a solution established
at least a year before the 2006 deadline.
Industry Compliance
Europe
The European Union RoHS legal directive
calls for the restriction of six primary
materials from electrical and electronic
products by 2006. These are Pb, mercury,
hexavalent chromium, cadmium, and two
types of flame retardant used in packages
abbreviated as PBB and PDE.
There are some key exemptions in
RoHS that have made conversion to Pbfree
very complicated from an operations
and business perspective; Figure 4 lists one
example in which network and storage
products are exempted. Similar exemptions
have also been granted to high-Pb
materials and ceramic packages.
Figure 4 - RoHA Key Exceptions
RoHS Key Exemptions
- Lead in high melting temperature type solders (i.e., tin-lead solder alloys containing more than 85% lead)
- Lead in solder for servers, storage, and storage array systems (exemption granted until 2110)
- Lead in solders for network infrastructure equipment for switching, signaling, transmission as well as
network management for telecommunication
- Lead in electronic ceramic parts (e.g., piezo-electronic devices)
Xilinx has successfully introduced Pb-free packaging solutions for
wire-bonded parts, shipping in volume since 2002.
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Most companies in Europe are committed
to implementing Pb-free solutions by the
RoHS deadline. Evaluations are ongoing
with samples of Pb-free parts on test boards
to understand the manufacturing issues.
Japan
Japan has clearly been the leader for Pb-free
implementations in most consumer products.
Many companies such as Sony and
NEC have issued mandates stating that all
consumer products in 2004 will be Pb-free.
One key challenge has been to develop
every component on the board (connectors,
passives, boards, and associated materials) as
Pb-free and capable of higher reflow temperatures.
This elusive capability has forced
many companies to push their implementation
timelines back several times.
Nevertheless, it has been clearly established
that doing business in Japan requires a Pbfree
solution for electronic packages.
North America
In North America, laws banning or restricting
the use of Pb are already in place for
many products, and there is an increasing
demand for a total ban. However, the
North American electronics industry has
been slower than other global regions to
adopt the move to Pb-free. This pattern is
changing, however, with RoHS timelines
for implementation set for July 2006.
Companies are scrambling to understand
the implications of Pb-free solutions
and have lobbied to extend the
timelines for implementation to ensure
that the robustness of the solution is
proven to their satisfaction. Lobbying
has yielded significant exemptions
in RoHS, as described earlier. Many
companies are working through industry
consortiums such as NEMI and CALCE
(Computer Aided Life Cycle
Engineering) to understand the robustness
of industry solutions, proposing
new tests and evaluations to address specific
concerns. Until these concerns are
addressed beyond a doubt, Pb-free
implementation timelines will inevitably
be pushed back.
Conclusion
Xilinx has successfully introduced Pb-free
packaging solutions for wire-bonded parts,
shipping in volume since 2002.
Yet current RoHS exemptions, coupled
with non-backward-compatible PBGA
solutions, pose a significant issue for the
supply chain. These exemptions and restrictions
may result in carrying dual inventory
on specific products for an extended period
of time and hence could have significant
cost and logistical implications.
In the long term, it is expected that the
industry will convert to a full Pb-free implementation
on PCBs and convert all packages
to Pb-free solutions.
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