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Introduction
Post mold cure (PMC) is one of the most significant
processes in electrical industry. This process exposes part of a mold to elevated
temperatures in
order to speed up the curing process and to optimize
some physical properties of the material.
The PMC process will expedite the cross-linking process
and properly align the polymer's molecules to make a stronger part with better high temperature
characteristics. Much like tempering
steel, post curing thermosetting can improve physical properties above what the material would normally
achieve at room temperature, such as tensile strength,
flexural strength, and can modify the temperature of heat distortion. Moreover, post
mold cure process is the most common strategy used today for warpage problem
solution. Finally, it can also deal with the outgassing phenomenon during IC
package.
Given the benefits
of PMC processing, it is widely used in the electrical device industry. It is
easy to find the application of PMC in many facilities and there are a large
number of companies using PMC on their products. Figure 1 shows various chips that
have been packaged using PMC processes.
Fig.1 Products of PMC
PMC Process in IC
Encapsulation
There are two major manufacturing steps
in IC encapsulation industry. For the first step, the IC chip would be
encapsulated into a thermosetting epoxy mold compound (EMC), which is
the most common plastic material for IC package manufacture today. This
pelletized compound is injected into a hot mold die to form the body around the
IC die. And after the plastic injection, the mold is cured. The object of this
step is to achieve good fill and partial cure of the mold.
The second step
of the process is the post mold cure (PMC) process. The goal of this step is to
increase degree of cure and reduce warpage in an IC chip. In order to optimize
properties, the PMC process provides a critical completion of the cure process to
complete the chemical cross-linking of the material. During PMC, the material experiences additional molecular
rearrangement and greater efficiency of molecular collisions resulting
in a greater degree of cross linking. The heating can also cause any
residual peroxide to break apart and initiate some additional chains.
Generally, a PMC
heating process can be divided into three heating stages, assuming that heating
process in the furnace is uniform. In the first stage, products are heated from
room temperature 25
oC to post mold cure temperature 175 oC in a short time. In the second stage,
the temperature is held constant for several hours. In the final stage, the
product is cooled from 175 oC to room
temperature in a short interval.
Applications
of PMC
Post mold cure has been applied by many
electronics companies, especially semiconductor designers and manufacturers. Table
1 shows various products that have benefited from PMC to achieve better
performance as well as some companies associated with them. From the table, it
is easy to see PMC technique is widely used in electronic market by a wide
range of corporations. If your company requires better device performance, PMC
processing is an easy choice.
Table1. Some
products and companies using PMC process
Products
|
Companies
|
IC
|
Intel; Samsung; Toshiba; SK Hynix;
IBM; Sony; AMD; Freescale; Marvell; Nvidia; Qualcomm; Anadigics; Cree;
Infineon; ST’s; Microsemi; Silicon Labs; TI; Vishay; IR; NXP;
Intersil; Amkor; Spansion; Renesas;
|
Packaging
|
ASE; Intel; TSMC; Microsemi; SPIL;
QuickLogic;
SMIC;
UMC; Globalfoundries; Amkor; UTAC;
|
RF
power amplifier
|
Skyworks; TI; TriQuint;
RFMD; Cree; Anadigics; Maxim Integrated; Infineon; NXP; ST’s;
Avago; Semtech; ADI; Linear; Macom; Freescale;
Microsemi; ONsemi; Lattice;
|
Memory
|
|
FPGA
|
|
ASIC
|
LSI Logic; Toshiba;
|
Attenuators
|
Skyworks;
TriQuint; RFMD; Avago; Peregrine; Analog;
|
RF passive
|
Skyworks;
TriQuint; RFMD; PPI; Macom; Microsemi; ONsemi; RCD;
|
Transistor
|
TriQuint;
NXP; ST’s; Avago; ONsemi;
|
Diodes
|
|
Mixers/
multipliers
|
Skyworks;
TriQuint;
RFMD;NXP; Avago; ADI; Linear; Peregrine; TI; Vishay; Analog;
|
Filters
|
Skyworks;
TriQuint; Avago; Macom; TI; ONsemi;
|
Modulator/
demodulator
|
Skyworks; TriQuint; RFMD; NXP; ST’s;
Micron; Avago; Linear; Microsemi; Analog;
|
Switches
|
Skyworks; RFMD; Infineon; NXP; Maxim Integrated; ST’s; Avago; ADI; Macom; Peregrine; Microsemi; TI;
Vishay; ONsemi; Analog;
|
Die/wafer
|
TSMC; TI; Fairchild;
Vishay; IR; Cypress; SPIL;
SMIC; Amkor;
|
Influences of PMC on
Material Properties
It is the epoxy molding compound (EMC)
that determines the properties resulting from PMC process. Epoxy is the vital
part of EMC. It will directly affect the flow characteristics of EMC as well as
impact the EMC thermal performances and electrical characteristics. Table 2
shows some commonly used epoxy resins and their characteristics.
Table 2. Some
epoxy resins and their characteristics
Epoxy resin
|
Characteristics
|
O-cresol-type
epoxy resin
|
High thermal
stability and chemical stability
|
Bisphenol A
type epoxy resin
|
Low
shrinkage and low-volatile component
|
Multi-functional-type
epoxy resin
|
Excellent
thermal stability, fast curing and high Tg
|
Biphenyl-type
epoxy resin
|
Low
viscosity, high filling
|
Tea-type
epoxy resin
|
High Tg,
high-heat-resistant
|
Modified
epoxy resin
|
Good
flexibility
|
A PMC
process involves placing the molded articles in a forced-air furnace and
thermally treating them to a series of increasing temperatures for various
times. The program of times and temperatures is referred to as the cure
schedule or cure cycle. During the PMC process, the molecular weight
of the polymer increases by chain extension. And as the molecular weight
increases virtually all mechanical, chemical, and thermal properties are changed. Figure 2 illustrates how physical properties change during
a PMC process. After PMC, the physical properties of objects are substantially
increased.
Fig.2 Physical
properties changes of cure circle: (a) Tensile strength; (b) Flexural strength;
(c) Heat deflection Temperature; (d) Shrinkage; The specimens are Torlon 4203L,
3 mm (1/8 inch) thick.
PMC can
prevent problems such as warpage during encapsulation in chip packages. In IC
encapsulation, one of a prevalent and troublesome EMC defect is warpage. Fortunately,
PMC is an efficient method to alleviate the warpage problem during
encapsulation. PMC is also one of the principal tools to mitigate
outgassing. PMC can remove the volatiles from the cross-linked plastic
material. If the volatiles are not removed and the EMC is exposed to elevated
temperatures with poor ventilation, one will observe deteriorations in
strength, elongation, compression set properties accompanied by chemical
decomposition. Insufficient or poor PMC can result in “smoke”,
bubbling, delamination and unsightly sticky surface deposits.
To achieve a satisfactory PMC process,
the furnace must be tuned to optimize the cure process, which can only be
achieved through high quality temperature control.
Furnace
Selection of PMC
Selecting a suitable furnace requires
knowledge of the temperature, time and atmospheric conditions of the process.
Basically, PMC furnaces can be divided into batch furnaces and continuous
furnaces. Batch furnaces are suitable for any part size but limited with
respect to production volume. As batch furnaces use the same door to load and
unload the part, these furnaces can only produce one batch at a time. A
continuous furnace uses a conveyor-belt to continuously move parts through the
furnace. So it is suitable for high volume production. Figure 3 shows a Hengli
continuous belt furnace.
Furnace technology, economics and part
quality influence the decision of using a continuous or a batch operation. The
economics questions center around cost of ownership, which can include initial
cost, operating costs, repair costs, product yields and return on investment.
Quality issues often are associated with process stability, quality and
consistency, while technology focuses on ease of operation, process definition,
thermal cycles, temperature requirements, atmosphere conditions, weight of
product and desired throughput. Issues and their relative importance depend on different
situations.
Most producers are still using batch
furnaces for PMC process today, but as many studies and discussions have pointed
out, it is better to use continuous furnaces for PMC
process if one wants to get a steady flow of incoming parts. This is because continuous furnaces are
extremely versatile and can be employed to perform a multitude of processes.
They are an excellent choice for manufacturing medium and high volume products.
There are a great number of advantages of converting the batch process to
continuous, such as:
1) Superior art to
part temperature uniformity;
2) Increased
throughput;
3) Process
combination;
4) Lower up-front investment;
5) Reduced
changeover times;
6) Part loading
flexibility.
A continuous furnace is ideal for
processes requiring high production volumes, process consistency, and precision
control. All components can go through the furnace smoothly. And during the
furnace process, as the continuous furnace can serve consistent heating
process, the consistency of products could be ensured in a high level. The
defects will also be effectively prevented and eliminated. Furthermore, a
continuous furnace can greatly improve the production efficiency by being
continuously available, rather than intermittently (as is the case for the
batch furnace, which must heat up and cool down). In addition, a good
continuous furnace is often much more compact than
a batch furnace, which is beneficial for floor space considerations and
facility costs. Moreover, a continuous furnace is easily used for automation
offers.
When choosing continuous furnace, the air
convection heating function should be considered, too. Unlike the traditional
furnace using radiative heating, a hot air convection furnace can elevate the
temperature through convective heating, which offers an extreme uniformity to
PMC process. Figure 4 illustrates the difference between traditional radiative
heating and convective heating. More important, the hot air convection furnace
can get higher energy efficiency than conventional ovens. Additionally, with no
noise and pollution operation, a convection furnace is environmental friendly.
Figure 4. Radiative heating process and convective heating process
Furnace Control of PMC
For most PMC process, the longer
cure profiles require longer furnaces. These furnaces are more suitable for the
inline, integrated manufacturing line used by many printed circuit board
assemblers. The PMC process needs specific
control of temperature and time, which is critical for getting excellent
performance after PMC process. If the temperature of furnace during PMC is
higher than the setting point, components are likely to be damaged, which will
cause production failures. And if the furnace temperature cannot reach the
required point, the post curing would be insufficient which will lead to a
great reduction of PMC quality. Consequently, it is vital for furnace
temperature control to achieve high quality PMC performance.
As the PMC process is sensitive to the temperature,
uniform furnace temperatures are essential for the PMC process. For most cases,
5.6°C (10°F) is the greatest temperature difference between the hottest and
coldest point in an oven that can be tolerated. Generally, a hot point occurs
near the air intake while a cold point near the exhaust vent.
Controllers programmed to raise the
temperature by 0.3°C (0.5°F) per minute are recommended. Automatic shut-off and
manual reset features are also desirable. A good oven is supposed to cut off
automatically when the temperature reaches 2.8°C (5°F) above
the set point. This is required to avoid distortion of the parts which can
occur if the temperature exceeds the deflection temperature of the part.
Belt Furnace for PMC
The HSF series hot air convection furnace
is an efficient belt furnace designed and used for
post mold cure process. This furnace can make temperature to 400°C. It can
heat by infra-red and/or hot air circulation heating depending on your process
parameters and requirements and the temperature profiles
are able to run at
the desired heating
rate to meet
the required PMC temperatures
under controlled atmosphere. Its
air or nitrogen atmosphere can serve a completed curing process. Temperature control zones offer precise control allowing
the furnace to run
at the proper
heating rate to
meet the needs
for curing. The HSF series belt furnace can offer uniform temperature
distribution to meet the qualifications of PMC process. Its conveyor system allows
proper heating across the belt with little temperature variations. And the
HSF series furnace comes with an ultra-clean heating chamber, which can give
rapid thermal response. Figure 5 shows a HSF series furnace.
The furnace is long enough to handle PMC
process. To
ensure proper practice
for continued use, technical information and training will
be given upon installation of
the furnace. A microprocessor
based PID controller provides appropriate system control. Type K thermocouples
are used in determining the zone temperatures. The central processing unit
(CPU) is located at the entrance table and is available with a Windows operating
system for ease of use and the program is installed ready to control furnace
parameters such as belt speed, zone temperatures, and atmospheric conditions.
Temperature profiles can be stored and retrieved as well for future purposes. There
are programs for capturing/storing, displaying, and printing out the furnace
profile which is already included in the software. Additionally, the furnace is
equipped with a redundant overheat safety protection system which incorporates
an additional type “K” thermocouple in the center of each controlled zone and
the multi-loop alarm.
Conclusion
PMC is an important technology for
electrical industry. It can highly improve the properties of chips and is
widely used by a great number of companies. A large number of electronic
products and companies have used PMC for better performance. The quality of PMC
process is deeply influenced by the temperature and time, which are strongly
influenced by the furnace. A good continuous belt furnace with precise control
and convective heating will offer great conditions for PMC process.
For more information, please check http://www.beltfurnaces.com/index.html