Microprocessor RoHS Requirements: Demands a need for lightweight high stiffness AlSiC Microprocessor Lids

The Problem
      Recent RoHS requirements for epoxies and solders used to attach microprocessor lids to the printed circuit boards means changes to microprocessor assemblies behavior during thermal cycling.  Often times these new “safer” materials lose desirable properties and become less compliant with the formulation change.   As a result, thermally induced stresses are not compensated by the epoxy and the microprocessor can bow and flex due to thermal expansions differences between the assembled materials.

       Flexing of a microprocessor assembly has deleterious consequences.  In the best case the system will have performance issues as the system can flex causing the thermal interface distance to change as position of the lid and printed circuit moves with respect to the die or chip.  An increase in TIM1 thickness will increase the thermal path and degrade the thermal resistance and cause the thermal interface material to “pump out” from between the die and heat sink material.   A decrease in TIM1 thermal interface distance ultimately improves the thermal dissipation performance, but ultimately will “pump out” the TIM1 thermal interface material eventually degrading the thermal dissipation the performance.  In the most severe cases bowing and flexing of the circuit assembly if too great can stress the die, solder ball connections to the point of failure.

The Solutions

       The best way to counteract these thermally induced stresses without the aid of this stress relief mechanism is to choose materials that are compatible in terms of their thermal expansion coefficients.  This is not always possible, because there are many materials components in these assemblies with different CTE values making a “one compatible CTE solution” very difficult to manage.

       An alternate approach is to increase the stiffness of the system.  This can be done by choosing a stiffer material(s) or by increasing the thickness of the materials in the system presently.   Increasing thickness to for increased stiffness may not meet the critical height requirements of the system.  Also the increased thickness will increase weight of the system which can result in an assembly that is less tolerant to shock and vibration.  Also the weight of the material may create orientation dependence to avoid damage created by material creep.  Copper has a density of 8.9 g/cm3; increasing thickness for improved stiffness will add significant weight.

       Increased material stiffness without a increase in material density (decreased material density is more desirable) would be the solution.  AlSiC, which has a density of 3 g/cm3 – 1/3 the density of copper, has a stiffness value that is greater than the copper equivalent.   And since AlSiC has significantly lower density increase thickness to achieve  greater assembly stiffness is possible without significant weight penalty (it will always be 1/3 lighter weight than the copper equivalent).

       And, by the way, No Restricted Hazardous Substances (RoHS) are used in the fabrication of AlSiC Heatsinks and Microprocessor lids.

      Please visit the CPS website for more information www.alsic.com also contact Bo Sullivan for discussion of your application.

AlSiC for Hybrid Electric Vehicle Power Modules

AlSiC for Hybrid Electric Vehicle Power Modules

Hybrid electric vehicle power generation and recovery systems also use IGBTs for power conversion (see also IGBT Thermal Management Baseplates).

The need in these applications is obvious.  Often times, especially in the case of the mild hybrid, these systems are designed to fit in the existing engine compartment, with limited space.  As a consequence the power density is very high and requires active cooling usually off the existing automotive cooling loop system.  And there is another constraint - the active cooling is operating at about 80°C and there cannot result in a significant pressure drop.  Systems need to be lightweight and it also needs to provide a significant level of reliability (10 year lifetime).

CPS AlSiC Power Module Pin Fin Coolers

In this case AlSiC is an ideal and proven solution for thermal management reliability for high density power electronic and IGBTs.  The AlSiC fabrication process also allows for extended pin fins in an open structure for low pressure drop for these active cooling requirements.  AlSiC is ideal for these applications because it is lightweight, with a high strength and stiffness so that it meets the weight budget and it is tolerant of the shock and vibration.

CPS AlSiC-9 Material Properties

CPS is in volume production AlSiC power module coolers for production vehicles today.  For more information please contact me, Mark Occhionero at 508 222-0614 x 242 or marko@alsic.com.  In Europe, contact Gio Schouten at +31 (0) 75 64 76 961 gio.schouten@semidice.nl.

Also please visit the CPS website at www.alsic.com. 
For a data sheet on AlSiC pin fin application please click the following link:
CPS AlSiC IGBT Coolers

AlSiC Hermetic Packaging

CPS AlSiC-9 Hermetic Packaging in production

AlSiC at about 63 volume percent SiC (CPS AlSiC-9) provides similar thermal management to traditional thermal management materials like Copper Molybdenum (CuMo) and Copper Tungsten (CuW).  All these materials have thermal conductivities (measured at room temperature) of about 200 W/mK and instantaneous thermal expansion coefficients of about 7 -8 as measured at room temperature.   

CoMo and CuW are traditionally used in military applications, these materials we an answer to Kovar that has a low CTE, but is not a good thermal dissipation material as it has a low thermal conductivity (~ 20 w/mK).   These applications include radar Tx/Rx modules, optical communications and also power conversion modules.  Application of these materials is primarily for reliability, and are used to enclose the electronics from the outside environment  and are hermetic to 10x 10^8 cc/sec He Leak up rate or better (MIL STD-883, method 1014).  Also the AlSiC HP is compliant with MIL-PRF-38534 subgroups 1-4 and 6. To get signals into and out of the package there are glass or ceramic isolated pins that are also hermetically sealed in the wall.  As a result Hermeticity requirement these packages are typically referred to as "Hermetic Packages" (HP).

The difference between the traditional HP materials is that AlSiC has a very low density of 3 g/cc and Kovar, CuMo and CuW have densities around values of 9, 16 and 18 g/cc respectively.  AlSiC is therefore more appropriate in weight sensitive or shock and vibration applications as compared to the traditional HP materials.

CPS AlSiC-9 base alloy 49 sealring with brazed ceramic feedthrus

The solution to use AlSiC in these applications, and make it transparent to the customer's down stream processing, is to make a composite package consisting of an AlSiC base with an Alloy 48 seal ring.  In this configuration, the AlSiC package can be conservatively 40 to 60% the weight of the traditional package.   

Alloy 48 seal ring is an Iron Ni Alloy that is similar to Kovar.  It can be integrated with either brazed ceramic feedthroughs or sealed with glass feed throughs, and can be plated using standard plating processes.  Additionally the Alloy 48 seal ring can be sealed by solder, brazing or laser welding processes used today with standard hermetic packaging. 

CPS AlSiC-9 Material Properties

Another advantage is over traditional HP materials is that that AlSiC can be provided in traditional HP sheet stock thicknesses at a lower cost.  Thickness is not a limit with AlSiC and the manufacturing casting process can provide thicker functional shapes, pedestals, cavities walls and without costly machining as is necessary with the traditional HP materials. 


For more information on AlSiC please visit AlSiC-Hermetic-Packages, and for a summary on AlSiC Hermetic Packaging capabilities please see the Data Sheet.  Also CPS manufactures standard Hermetic Packaging too.  If you need to speak to someone on this subject pleas call Cheryl Oliveira at 508-222-0614 x247 or e-mail coliveira@alsic.com.

www.alsic.com

AlSiC For Microprocessors

Figure 1: CPS AlSiC Flip Chip Microprocessor Lids

AlSiC materials properties are ideal for microprocessor applications for more than just the thermal management issues associated with high thermal conductivity and compatible coefficient of thermal expansion (CTE).  Figure 1 shows a  number of AlSiC lid designs (notice the shape capability).

AlSiC also has the advantage of being lightweight with a high strength and stiffness making it great for flip chip microprocessor applications that see shock and vibration.  Shock and vibration considerations should also be considered in automated assembly lines too.

Figure 2: Flip Chip Lid Assembly.
The heat spreader is the AlSiC lid.

Figure 2 shows the typical flip chip lid assembly.  The heat spreader shown in this illustration is the lid.  The lid protects the die and needs to provide a good interface between the die and the lid for thermal dissipation.  The Thermal Interface Material (TIM) 1 is usually a thermal grease material.  This needs to be as thin as possible to minimize the thermal path of heat from the die through the lid.  It is therefor critical that the lid cavity height and tolerance be carefully controlled as well as the flatness of the lid.  This is also influenced by the stiffness of the lid. 

There are two main assembly types for flip chip microprocessor systems: Ceramic and FR4 systems which have two different thermal expansion considerations.  This material is represented in green in Figure 2.  Ceramic systems require lower CTE AlSiC-9 lids and for the FR4 the designer may choose to use AlSiC-12 to match that of the printed circuit board or the designer may also consider using AlSiC-9 to match the CTE of the die.

Being lightweight and stiff makes AlSiC good for large lidded applications, and can enable some new integrated systems packaging concepts (System in Package).  In these applications the designer needs to have a material that can span long distances with out deformation for die protection.  It also needs to be lightweight so that there is minimal orientation dependency.  AlSiC has a strength and stiffness that is slightly greater than Cu yet is is 1/3 the weight of Cu (materials properties).

Figure 3: CPS AlSiC Large SIP lid 55 mm x 70 mm @ 17 g.
The Cu equivalent is 55 g.

Figure 3 shows a large AlSiC SIP lid that is 55 x 70 mm.  Thermal management was not the main consideration in this application and weight was.  A large copper lid is three times as heavy as the AlSiC equivalent, and copper had an orientation dependency in service.  A lid with high mass can also affect the assembly.  The weight of the lid can cause the solder balls in the BGA or under the die to creep during reflow, which may result in solder balls shorting.  The other issue during automated assembly is the shock of acceleration and deceleration that can cause solder balls to debond.  This AlSiC solution enables the designer to consider incorporating more functionality under the lid for shorter electrical distances (faster speed).  All of the electronics are then protected by one large die in this application.

visit the CPS website www.alsic.com for more information.

Properties of Material Used In Electronics

Many materials are required for today's electronics and electronics systems.  In thermal management we need to consider how all these properties affect the system not only during power cycling during the service life, but also in the assembly and testing.

Thermal Conductivity and Coefficient of Thermal Expansion are the most common considered in power electronics.  But the careful design engineer must also consider the weight (density), strength and stiffness too as these have an influence in system reliability.  

Figure 1: CPS AlSiC Products Illustrating Design Functionality

Other considerations, that are not material properties, are what is the materials capability to be fabricated into something that is useful and design capabilities to hold the necessary dimensional tolerances.  These issues will be discussed in the applications in later posts.  Figure 1 shows a wide range of AlSiC products illustrating design functionality.

Figure 2 is a table of physical properties for common materials used in electronic assemblies and their material properties.


I have color coded this chart with the light blue for the packaging materials commonly used.  AlSiC-9 and AlSiC-12 are on top.  AlSiC-9 is applicable for direct mounting of die and ceramic substrates.  AlSiC-12 is good for microprocessor lid assemblies that are mounted on FR-4 boards.


Also included are materials like Copper and Aluminum.  Copper is used in Low power IGBTs.  I can be lower cost since the material is stamped.  Aluminum is used as a low cost heat sink. It also can be stamped but most commonly extruded to have fins for extended surface area for convective or forced air cooling.  The drawbacks of these materials is that both have very high CTE values which will not match that of the electronic die materials (Green) or the substrate materials that are used for electrical isolation.  

Figure 2:  Physical Properties of Materials used in Electronic Systems

Copper Molybdenum (CuMo) and Copper Tungsten (CuW) are considered thermal management materials because they have CTE values that are compatible with the electronic materials and substrate materials.  Both these materials have been used extensively in the military and aerospace industry in hermetic packaging because of the CTE thermal management reliability required in these systems.  The draw back of these materials is that the material is more expensive, it is expensive to machine (most parts are consequently flat) and they are heavy (high density).  They high density is not desirable in weight sensitive aerospace applications or where shock and vibration are a consideration.


In contrast AlSiC materials are thermal management materials having a good thermal dissipation and CTE that is compatible with the electronics and dielectric substrates.  Additionally these materials are light weight, and the fabrication technology allows for functional shapes to be considered. 

Electronic materials are in green contrast in this table.  The notable properties of these materials is that they have very low CTE values and that they have very low fracture strength.  The low strength means that the design engineer really needs to be cognizant of thermal stresses that may be induced during power cycling, and at operation temperatures.


In yellow are some common dielectric substrate materials.  These materials are used for electrical isolation the electronics to the rest of the world.  Their strength and CTE values are relatively low.  So choosing choosing compatible CTE values is necessary when considering heat sink attachment.  

Usually these dielectric materials are metallized with a very thin (~ 300 µm) Copper layer that provides electrical connection between devices and also a solder or braze Cu attachment surface on the side opposite to the electronics to integrate these substrates with a heat sink.  These copper layers are attached by an activated attachment process called Direct Bond Copper or DBC. DBC substrates are commonly used in IGBTs.   In IGBTs the CTE of the assembled materials is very important.


for more information please visit CPS at www.alsic.com.

IGBT Thermal Management

Figure 1 IGBT Baseplates for Traction and Power Conversion.

Insulated Gate Bipolar Transistors (IGBTs) are used in power conversion applications where AC is converted to DC and DC to AC.  Applications include traction (Trains, Subway Systems),  Power Generation (Wind Energy, Hydroelectric, Power Conditioning) and also in hybrid electric vehicles where power is generated by conventional motors, and power is recovered from breaking.  Hybrid Electric applications will be discussed elsewhere.  

Figure 2 Schematic of IGBT Module Assembly.

These power IGBT applications generate a lot of heat, so heat and thermal dissipation are critical to the reliability of the the electronics.  AlSiC provides this thermal management for these applications with a CTE value that is compatible with the assembled electronics and substrate materials.  Examples of AlSiC IGBT bases are shown in Figure 1.

Figure 2 shows a schematic of the Power IGBT assembly.  The electronics (the actual IGBT) is attached to a metallize ceramic substrate which provides the electrical isolation needed and electrical connections.  The IGBT and ceramic substrate assembly is attached the the baseplate by soldering.  The baseplate provides the functional attachment of the IGBT assembly and electronics to a cold plate that will ultimately provide the heat sink for the thermal dissipation.   

Power IGBTs use either Aluminia (Aluminum Oxide) or AlN (Aluminum Nitride) that is metallized with a thin layer of copper (~300 µm).  The Copper is thermally applied by direct bonding process (often times this is referred to as direct bonding - DBC for direct bond copper).  

Alumina and AlN have a very low thermal expansion values of 6.7 and 4.5 ppm/C which are very compatible with the electronic IGBT silicon die (see material properties).  These materials are however compatible with materials like Copper and Aluminum that have CTE values of 17 and 23 ppm/C over the same temperature range.  AlSiC is compatible with DBC substrates with a CTE value of 8.0 ppm/C over the same temperature range.

Figure 3 Ultrasonic Image of Cu Baseplate @ 400 thermal cycles

To illustrate the need for CTE compatibility in power electronics systems the next set of pictures show ultrasonic images interrogating the solder interface between the DBC and the baseplate material contrasting Copper and AlSiC from a paper entitled "The new 6.5kV IGBT module: a reliable device for medium voltage applications" by Thomas Schuetze, Herman Berg, Oliver Schilling Aug 1, 2001.


In the case of the Copper IGBT baseplate the solder layer fails by delamination after about 400 thermal cycles (Figure 3).  The thermal dissipation path continues to degrade as the delamination continues.  As a consequence the electronic system is subjected to higher and higher thermal load that will ultimately cause the device to fail.

Figure 4 Ultrasonic Image of AlSiC  Baseplate @ 30,000 thermal cycles

For AlSiC IGBT baseplate there is no delamination even after 30,000 thermal cycles.  In power electronic systems this equates to long term reliability. 

AlSiC (200 W/mK) has a lower thermal conductivity than Cu (400 W/mK).  But as illustrated above in the Cu system the designer must consider stress compensation for these systems to manage the CTE difference between the Cu and the attached substrates.  Stress compensation layers increase the length of the thermal dissipation path, so the benefit of Cu high thermal conductivity may never be realized.  Often times the AlSiC system can be optimized in design to have very thin solder layers for a shorter thermal dissipation path.  As a result AlSiC baseplate solution can have equal or better thermal dissipation as compared to Cu baseplate solutions.

Long term reliability is very important in systems where failure is not an option or where replacement of the IGBT can be expensive. 

Windfarm in Iowa - changing out IGBTs is costly - there is a lot of corn in this picture.

please visit the CPS website www.alsic.com for more information.

What is Thermal Management - and Why do I need it?

Most materials expand and contract when they are heated or cooled respectively.


A materials ability to expand and contract is usually expressed in terms of thermal expansion sometime expressed as coefficient of thermal expansion (CTE).  The CTE is a measure of the expansion difference as a function of temperature (expansion rate).  Most commonly CTE is expressed as a value at a given temperature value.   It should be more carefully expressed over a temperature range as an "Average CTE".  If you are really careful you want to look at instantaneous CTE (RATE) as a function of temperature.  I will discuss CTE in greater detail later.

If the combined materials are not matched in thermal expansion, stresses can develop that can cause the assembly to

• warp
• delaminate at the attachment interface
• cause the weaker of the assembled materials to crack

as a consequence these behaviors threaten the reliability of a product.  

In electronics, warping through many thermal cycles can cause the device to fail by breaking electrical connections.  Delamination failures disrupt the thermal heat dissipation path, causing the devices to heat beyond the design limits and also causing greater thermally induced thermal expansion stresses.  In this case the electronics will eventually fail thermally, if no other failure occurs.  The case where the thermal stresses cause the die to crack ultimately results in failure of the device.

So when designing a device that generates a lot of power, which needs to dissipate heat you will need to consider the assembly of the components and make sure that the thermal stresses does not introduce one of the failures described above.  

I will discuss the various failure mechanisms in greater detail with typical product applications: IGBT baseplates; power module coolers for HEV applications, and Microprocessor Lids.

to see more please visit www.alsic.com.