Download Development of pre-molded leadframe package technologies to suit

Development of pre-molded leadframe package
technologies to suit Photonic and RF applications
Andy Longford, Bob Radloff*
PandA Europe, Lambourn, Berks, RG17 8YP, UK – [email protected]
* Handy & Harman Electronics Materials Group (HHEM), Fontana Ca. 92337 USA [email protected]
The development of both Opto “photonic” and RF technology into silicon based chip
solutions is creating a demand for smaller, more cost effective solutions to house the
devices. The use of Thermoplastic mold compounds with pre-plated leadframes is one of the
most viable solutions for volume take-up at low cost. HHEM have developed novel mold
design assembly and process techniques to produce such package solutions for these new
markets. The issues involved and the solutions achieved are reviewed, including a novel
patented process, which provides isolation between interconnect levels in more than one
plane. New approaches to design of application specific packages are used to show how
matching function to environment can provide novel housing solutions. The capability to
provide higher performance characteristics such as heat transfer and hermeticity in
’plastic’ packaging remains the challenge for both RF and optical devices. Some of the
work ongoing and results obtained with commercially available and proprietary process
material is reviewed.
INTRODUCTION
The growth of Opto “photonic” technology and
its development into silicon based chip
solutions has created a new demand for Chip
packaging technology to provide smaller, more
cost effective solutions to house the devices.
In fact many photonic applications would be
improved if the housing could be specifically
designed to suit both the interconnection
between the chip and the control circuitry, and
the physical aspects of the location of the
optical sources in its overall application.
Additionally the rapid growth of wireless
products has increased the demand for higher
power, lower cost RF components. Devices
previously designed for military and aerospace
applications now need to be produced in a
manner and at a cost more suited to consumer
market products.
Packaging of these Opto and RF components is
one of the most costly parts of the whole
manufacturing process. Package costs in excess
of 50% of the overall device cost are not
unusual, whereas in traditional IC packaging a
cost of less than 5% is more the requirement.
Handy&Harman Electronics Materials Group
(HHEM) in the USA have for many years
provided a capability to design and
manufacture innovative leadframe and
Thermoplastic molded parts for medical and
electronic applications. However the
Semiconductor “chip” industry, until recently,
has not been in favour of injection molded or
insert molded packaging due to an outdated
view that the technology involved is incapable
of meeting the quality needs for performance
and reliability.
It has been the advent of chip based sensor
products, especially “opto” parts that has
prompted a change, in terms of needs for cavity
packaging. This has in turn created needs to
use latest technologies and materials to reinvent the thermoplastic package capability.
HHEM have become one of the new pioneers
in developing these technologies to suit RF and
Photonic packaging applications.
CAVITY PACKAGES OVERVIEW
Due to the nature of the chips being used in
these specialised applications, packaging them
has generally required the use of a cavity style
of carrier. There are a number of options for
cavity packaging as shown in table 1. These
range from machined metal cavity through
formed “cans” and Ceramic types to leadframe
and plastic options. As shown they have
relative cost and assembly issues which need to
be considered for volume applications.
Table 1
Cavity package options
There are a large number of suppliers and
manufacturers offering capability in all these
types but only a few companies, mainly
Japanese Ceramic package suppliers, can offer
products for volume manufacturing. In plastic
overmold assembly some new, high cost mold
designs and processes are being developed for
some of the volume applications, but these are
only feasible if the chips are not affected by the
heat and pressure stresses exerted in the
molding processes.
So these new markets have at last seen the need
to change to match the demand for cost
effective packages for silicon based RF chips,
Opto chips, MEMS and Sensors that need
specialist chip “packaging” with a cavity. The
relatively lower tooling costs coupled with
capability to match existing volume assembly
equipment have been the key factors in reawakening the market to the capability of
injection molding technology. In addition other
benefits such as colour, shape, surface finish
and dimensional stability are offering
advantages previously not considered.
REQUIREMENTS FOR RF AND
PHOTONICS PACKAGING
Because of the lack of “standards” in the
emerging markets, RF and Opto companies
have generally developed their own application
specific or custom cavity packages.
In Photonic applications these packages have
generally been suitable for prototype and low
volume requirements but these solutions are
costly and create a wide range of problems
when moving up to volume assembly.
An example of this issue is related to the
manufacture of products such as Fibre Optic
transceiver units and VCSEL devices. The
assembly processes are based upon the need to
have trained operators to individually ‘hand’
align and set component positions. Assembly
lines for such devices resemble the IC industry
back 30 years, when wire bonding was manual
or at most only semi –automatic. In this
respect, device manufacturers have requested
equipment manufacturers and materials
suppliers to find ways of using available
automated lines and adapt them to meet the
high accuracy needs of opto components(1).
Such calls are now being responded to by some
of the major suppliers(2) who have recently
introduced a range of ceramic cavity package
parts based upon a leadframe strip (figure 1)
rather than being individual units. The move to
leadframe technology clearly demonstrates that
this approach is the realistic factor in meeting
the needs of high volume assembly.
Figure 1 – Kyocera Ceramic packages on lead
frames
On the other hand, the rapid growth of RF
transistors, MMIC’s and GaAs devices
necessary for the wireless and mobile phone
applications, has meant that the few ‘volume’
package suppliers have now generated “de-
facto” standard package parts in order to help
reduce costs. The cost trade off here is that the
chip designers are having size or layout
constraints that are either increasing design
costs or reducing functionality.
Because of the needs of these devices to handle
RF power, heat generation is the major concern
followed by issues related to the electrical
effects of the package parasitics reducing
speeds and limiting frequency functionality of
the devices. These ‘standard’ cavity packages
for RF are developed around gold plated heat
sink base plates and therefore are individual
packages that do not lend themselves to
automation in assembly. So as volumes
increase cost for procurement and assembly
become increasingly significant. The trade off
is that there are multiple sources of parts but
until recently none have really addressed the
requirements to automate assembly.
RF packaging brief
The key factors for RF device packaging
depend upon the applications and the operating
frequency requirement. The RF frequency
range is from above 400Mhz to the low
microwave frequencies of up to 10GHz (4).
Most of the new volume wireless applications
are in the 2 GHz – 5 GHz range but eventually
these will be going on up to 40Ghz and
beyond.
For these wireless frequencies the package
constraints are:
• Thermal Management
• Parasitics (L R C)
• Cost
These in turn dominate both package design
and the assembly process in terms of die attach
capability for thermal and electrical contact
through the package base. Wirebond needs
affect the lead finish (gold is the standard) and
package lead size affects the frequency due to
parasitic inductance. Interestingly, package
sealing is not an issue, as non-hermetic
environments do not now affect many devices.
Volume manufacturing of many RF power
products is, and will remain, in standard IC
power packaging now that thermally efficient
packages are prevalent. But the stresses
generated in thermo-set transfer mold
processes can be damaging to a number of
technologies in RF devices. In particular, RF
power transistors require metal based cavity
packages to meet the thermal and electrical
conductivity needs. Further as higher frequency
demands dominate, the packages must have
bigger interconnect leads to reduce parasitic
effects.
For these reasons there has been a significant
growth in the Ceramic RF and Microwave
package demand but this has so far failed to be
cost effective. Leadframe technology is now
being developed (2) for low power devices in
order to get cost effective solutions both in
material and assembly costs. Consideration of
molded plastic cavities is the next challenge.
Photonics Packaging brief
The developing range of photonic devices is
integrating more functions in a rapidly
shrinking component size. The more the
integration the more complex the package
becomes which inherently adds cost.
Current and emerging products also have the
same basic, unique needs for packaging. A
review of the assembly issues involved (3) has
identified the key factors (which also apply
equally to RF packaging) for consideration in
choosing the package, namely:
• Electrical conductivity
• Operating temperature range
• Expansion coefficient
• Surface finish
• Ease of installation
• Cost
Associated with these the major challenges to
packaging and assembly of Photonic devices
(3) are:
• Alignment of components to within
0.5 micron
• Substrate selection
• Thermal management
• Sealing
In reality, these issues are no longer exclusive
to photonics as they have now become
predominant in standard IC chip packaging.
The industry needs to package camera chips
and other opto sensors with accurate alignment,
it requires wirebond placement accuracies less
than 5 micron and it needs to manage the heat
increases from shrinking chip geometries.
Much of the equipment available on the market
can therefore now be used in the assembly of
photonics, but materials and packages used will
have to adopt standard semiconductor
assembly criteria to suit to these machines.
Leadframes are a natural choice, allowing
strip-to-strip assembly with standard magazine
storage and control. They also provide thermal
and electrical conductivity. The cavities, as
seen earlier in metal and ceramic can be
applied to frames, but these are not that cost
effective. The use of insert molded or premolded thermoplastic housings are being
considered as a cost effective choice. Wide
ranges of mold compounds and very accurate
mold capability can now meet the demands of
this new industry.
emerging markets for RF and photonics
devices.
Cavity package issues raised by the
Semiconductor packaging Industry in its early
years, when Thermo-set compounds became
the viable solution to encapsulate chips, still
exist as a mindset in many Chip companies.
These relate to package strength, sealing,
hermeticity, quality, dimensional accuracy and
cost. However Chip package manufacturers
rarely use cavity packages since the demise of
Metal Can, CERDIP and Ceramic Sidebraze
assembly capabilities.
Recent growth in Sensor technologies and
Opto components (LEDs etc) has seen a
resurgence in cavity packaging assembly
needs. Pre-molded frame technology has been
the key to the successful growth of these
industries and manufacturing techniques have
also been developed to support these industries.
Likewise, engineers at HHEM have developed
novel mold design, assembly and process
techniques to suit the demands of these new
industries and to enable use of new materials
now available.
Mold machine developments
Figure 2 – Profiling of cavity features. (courtesy
of Kyocera)
Additionally, the cavity package design can be
customised both internally and externally to
match the application at relatively low cost. As
shown in figure 2, the internal design of this
ceramic cavity can be adapted to accommodate
a fibre connector location or fibre ‘pigtail’,
which provides high accuracy alignment as
well as reduced interconnection costs. In this
application, plastic molded cavities can be
more dimensionally accurate with tighter
tolerances than ceramic.
Injection mold technology is widely used in a
number of industries where precision parts are
required. The medical industry has for many
years used such technologies and HHEM’s
background in this industry has enabled the
development of a high performance molding
capability.
MANUFACTURING TECHNIQUES
Advancement of Leadframe technology,
development of injection mold technology,
introduction of new materials and novel design
ideas are the tools that have been developed by
the package manufacturers to ensure that
today’s pre-molded leadframe cavity packages
can match the demands of these new and
Figure 3 – Three plate Mold system
The key development has been that of a
pioneer 3 plate mold system enabling the insert
mold capability of leadframes in a vertical
plane as in figure 3. This mold system ensures
that the frames are accurately aligned with the
mold features then firmly clamped in the
molds, enabling good fill on mold shots and
preventing bleed. Additional development of
“dry” mold technologies has resulted in an
accurate, clean component with extremely fine
surface features. To keep costs down, and
provide high volume capability mold machines
have been adapted to allow reel to reel
leadframe feed.
Table 2
Mold Compound options
The molds themselves are built upon
standardised mold bases incorporating 4,16 or
32 cavities. This is done to keep tooling costs
to an absolute minimum and provide fast turn
round of new designs. Most prototype and pre production can be done on a single cavity,
which allows very small size features and rapid
adaptive design changes before production
tooling investment is needed. Accuracies of
tool placement and cavity features can be under
5 micron.
Mold Compound issues
As one would expect, the development of new
thermoplastic mold compounds has increased
greatly in the last few years, as products in a
broad ranging number of applications from
toys to Aeroplanes require different material
performances. A list of compounds typically
used in electronic components is shown in
table 2. The choice of mold compound can
depend on many factors, including colour, but
the current prime option for high performance
cavities is the black Xydar product. This has a
CTE that closely matches the applications and
high melt point to match assembly needs. On
an aside, thermoplastic compounds are
recyclable, an important feature for the
industry in coming years.
The main driver in the electronics cavity
package market has been for higher and higher
melting point materials. These need to
withstand the assembly processes which
continue to push the temperature and stress of
components, such as solder reflow for lead(Pb)
free electronics which is now around 300°C.
HHEM have pretty much standardised upon
LCP, a material widely used in the connector
industry, as it has a good performance, high
temperature withstand and can provide fine
resolution geometry.
A new compound, known as “conduit” has
now been made available to HHEM for use in
high thermal stress applications. This has been
tested up to 500°C and is likely to be used in
RF package units which use eutectic die attach
reflow processes. Results of tests so far show
that it meets the same stringent environment as
High temperature ceramic materials, bonded to
metals.
Multi - Leadframe capability
Leadframe design is also a key part of the
package development cycle. Use of HHEM
capabilities to stamp and pre- plate frames
before molding for volume applications means
that all packages developed can benefit from
Lead (Pb) Free finishes. Packages are
generally tooled for multi-up parts – depending
upon mold tool designs – that ensure lower
cost in terms of tooling investment and unit
prices. Multi-up frame devices are pretty much
commonplace for standard Semiconductor
assembly equipment.
To utilise the capabilities of the cavity package
for Chip devices that require thermal
management, HHEM have developed a novel,
patented process adaption that will allow the
use of a second leadframe to provide the cavity
package with a metal base that is isolated from
the i/o configured leadframe. In fact the
process developed can simultaneously mold up
to 3 frames at the same time allowing
development of 3 dimensional package
options. The frames can be maintained at a
minimum distance apart of 0.4mm and frames
can be of varying thickness up to 2mm. This
isolation between interconnect levels in more
than one plane offers a new packaging
technique opening up more opportunities for
new types of optical and bio-metric sensors as
well as other devices such as power hungry,
radio frequency chips. The “stadium” package
shown in figure 4 is one such option designed
to increase basic chip packaging density that
could equally be applied to RF or Opto
applications.
approach contributes to a factor 10+
manufacturing cost reduction for a range of
devices.
Figure 5 – RF 5 Watt package
(Courtesy of Ericsson Microelectronics )
Figure 4 – Three frame “stadium” package
design
PACKAGE EXAMPLES
The main process for pre-molded frame parts is
the insertion of a functional plastic cavity
around a formation of metal lead ‘fingers’ to
which connection of single or multiple chips
and/or other components may be made. The
cavity itself can be shaped, round, square and
can have multiple depths depending upon
application and chip designs. The exterior of
the cavity can also be shaped as required or
incorporate features to house external leads or
mountings. The possibilities are endless.
For RF applications, the package requirements
of thermal management and electrical isolation
have now been achieved in pre-molded frame
devices using the 2-frame process with high
temperature LCP mold compounds. The device
detailed in figure 5 is a design developed for 5
- 10 watt power RF transistors. The device is
similar in dimensions to “off the Shelf”
ceramic parts but is supplied in leadframe
strips for use in automated assembly. This
Adaption of leadframe technology to other RF
applications is ongoing where pre-plated
frames are insert molded with high temperature
LCP so that applications for power devices up
to 150 watt are possible. Here the mold
compound is used to isolate the main pad from
power lead connections in the same plane.
Utilising a stamped leadframe and molding 16
parts in a single shot provides some of the cost
advantage. Strip leadframes are used in
magazine to magazine assembly lines and a
low cost thermoplastic cavity lid is also
included when non-hermetic sealing is used.
Matching function to environment can provide
novel housing solutions. One of the most
important package developments for the opto
market has been the application specific
designed Opto-Encoder as shown in figure 6.
This unit was designed to fit in an encoder
module for a steering position sensor in the
automotive market. Key factors for this were
the need to assemble detectors and emitters in
the same environment and provide features to
ensure accurate alignment, the same
requirement needed for “photonics” packaging.
The cavities are mechanically located to each
other after the electronic components are
assembled and tested. The manufacturing
process is fully automatic.
Materials for molding the cavities are
improving as suppliers develop new
compounds. Conduit is so far unique to HHEM
and a number of tests, being run with selected
applications, need to be completed before
production can be undertaken. The compounds
used also determine the dimension accuracy
achievable. New materials as well as new
processes are likely to be necessary for sub
micron tolerances.
Figure 6 – optical encoder assembly
A recent design exercise for a Fibre optic
transceiver application has explored the use of
all the techniques discussed to develop a low
cost package to replace machined metal cavity
units. This design runs 2 leadframes and molds
a deep cavity around a heatsink base. The
second frame, for the electrical I/O connections
is molded into the side walls of the cavity at
the same time. The smart thinking for this
design is to build part of the fibre connector
into the package lid, which will then interlock
with the other part which is designed as part of
the package wall. The lid is an injection
molded part meeting the same high tolerances
for dimensional accuracy as the cavity
package.
CHALLENGES ONGOING
As the demand for optical devices grows, the
capability to provide higher performance
characteristics such as heat transfer and
hermeticity in ’plastic’ packaging has become
an issue in search of a solution. As previously
discovered (3) the major challenges to
packaging and assembly of emerging
technology devices remain:
• Alignment of components
• Substrate/Material selection
• Thermal management
• Sealing
The alignment of optical parts in particular is
the biggest challenge for dimensional accuracy.
For small mold devices tolerances can be kept
very tight and accuracies down to 1 micron are
achievable. The knock on effect for such parts
comes down to getting associated tolerances to
match in the feed and take up equipment
necessary for high volume production.
Thermal performance is not only determined
by leadframe materials and thickness but also
by the compounds used for molding, especially
in RF products. These need to have higher
melting points to withstand the chip assembly
processes yet provide a good thermal
conduction to prevent stress and delamination
around the metallic structures.
Hermeticity and Sealing of these types of
packages are factors which are receiving the
most attention. Thermo-plastic compounds
such as ABS or PBT’s mold well but are not
renowned for sealing around metal leads.
Newer materials like LCP are rougher in
surface finish but give good seals and can
achieve JEDEC level 2 packages. Figure 7
shows a Camera chip package design that after
assembly with a sealed clear glass lid meets
Level 2. However level 1 is the demand for
many of the Photonics applications. New
liquid sealants can provide solutions but add an
extra process step and cost to the assembly.
The challenge ultimately will be to seal the
chips (as in direct die attach devices) and other
components before assembly so that hermetic
packaging is not needed, hence keeping costs
down.
Figure 7 – 20L “Opto” cavity packages in strip
form
ACKNOWLEDGEMENT
Overall package sealing is dependant upon the
chips and other structure within the cavity.
Hysol type filler compounds can really close
up cavity packages to Level1, but are useless
with MEMS, optical sensory and fibre
components. Lids can be plastic or metal
depending upon the system need. Plastic lids
can be designed for glues and epoxies, snap in,
where epoxy chemicals are an issue, weldable
and /or functional capability. Depending upon
materials selected, especially for cost
considerations, new lid sealing compounds
now being introduced are in need of evaluation
for both RF and Photonic package applications.
FUTURE CAPABILITIES
The cost factor being the major drawback to
volume take-up of any products, the aspects of
low cost package solutions as detailed above
are becoming the drivers for the new
technologies such as required for “last mile”
fibre optic telecom systems, 3G phones,
Bluetooth and Wan/Lan applications. Devices
are needed at lower costs and in higher
volumes. Thermoplastic mold compounds with
pre-plated leadframes will inevitably be one of
the most viable solutions for such low cost
cavity packages.
New thinking for the design of application
specific packages is the key for cost effective
packaging. Why have a molded connector at
the end of a transceiver fibre pigtail. Why not
use the package as the connector with a cavity
for the photonic circuit all in one molded
housing? Why not have multi level substrates
for packaging different types of chip
components with fixed cavities for the optical
interfaces all part of the package.
The capabilities of materials and design are
being continually developed to match the
electronics markets philosophy – Smaller,
Faster, Cheaper. It is now evident that Premolded frame technology can meet the
demands of emerging technologies and will
continue to match development needs for
future applications.
The authors are indebted to the support of
Ericsson Microelectronics for their assistance
in the development of some of the technology
described for RF packages and are grateful for
their permission to release the information.
They also acknowledge the assistance of the
engineering and design staff at HHEMG for
their contributions to this paper.
REFERENCES
(1) Carla Miner,Nortel, Trends in Optoelectronics
– consequences for SEMI, SEMI Standards
conference, Dresden, 25-27 October 2000.
(2) Kyocera Corporation – Packaging networks
2000 – Sales and Marketing presentation OEsalestool Rev 1.
(3) Steve Riches, Micro Circuit Engineering,
Practicalities of Opto-Electronic Packaging,
IMAPS UK Opto-Hybrid Microelectronics
Conference, IOP, 12 October 2000
(4) Ericsson Microelectronics website:
http://www.ericsson.com/microe/products/rf_p
ower_transistors.shtml