21.10.2021
Vehicle Electrification: The Heat is On
Transportation accounts for at least one-fifth of global carbon emissions1. Massive consumption of common fuels, gasoline, and diesel to power vehicles continues to generate vast amounts of pollutants like carbon monoxide that contribute heavily to air pollution.
As a response, the world is moving inexorably towards cleaner energy sources to gradually replace fossil fuel technologies. These alternatives are electric in nature and include configurations such as battery electric vehicles (BEV) or plug-in hybrid electric vehicle (PHEV). This wave of vehicle electrification necessitates car manufacturers using a much amount of semiconductor technology for their solutions, which will subsequently lead to the design and assembly of increasingly high-powered electronics (HPE) components to power the cars of the future.
However, “with great power comes great heat”, to paraphrase a well-known superhero caption! In the case of HPEs for automotive electrification, operating conditions can be brutal, with temperatures often at more than 150 degrees Celsius in a typical vehicle. To protect and enable these intricately designed and assembled components to continue generating high voltage and current to power and operate these vehicles, advanced encapsulation is key.
Understanding High Powered Packages: The Power Package Behind HPEs
An electric vehicle system is basically comprised of a power generator, electric motor, inverter, battery, battery charger, and electrical control unit (ECU). The ECU is where HPEs such as double-sided cooling (DSC) packages and single-sided cooling (SSC) packages reside.
In order to protect the DSC and SSC packages, a key process called transfer molding using epoxy material is typically employed.
In Figure 1a, a typical DSC internal structure is shown with a top lead frame and bottom direct bond copper (DBC) to dissipate the heat generated while supplying high voltage and current levels. To comprehensively remove the significant heat that is generated, the DSC is mechatronically integrated into a cooling system, such as a heat sink.
Figure 1a: Schematic of Double Sided Cooling (DSC) Package
This is in contrast with SSC in Figure 1b, which is generally attached to a pin-fins package to dissipate the heat generated.
Figure 1b: Schematic of Single Sided Cooling (SSC) Package
Whether DSC and SSC whoever, there are multiple obstacles to surmount in their molding. One such initial challenge is mold bleed & flash (MBF) or die crack, both of which cause damage and a decrease in solderability. Another potential problem is delamination and voiding, which typically occur when utilizing a sub-optimal process during design and setting.
Such defects are to be avoided at all costs, especially when aiming to produce a high-quality HPE. Generally, these problem areas are resolved by looking at four major aspects: indirect materials, process setting, incoming material, and tooling design (Figures 2 & 3). For the purpose of this article, the focus will be on MBF and die crack, as they are by far the most commonly faced.
In Figure 2’s mind map, one of the most notable causes for die crack is the nature of the incoming material. To address this, users need to perform stringent quality control on third-party material purchases, for example, the DBC substrate dimensional quality.
Subsequent challenges beyond this involve die integrity, which involves dimensional accuracy of die placement position; co-planarity of the die attachment; and finally, centering of the die above the die attach silver paste.
Figure 2: Mind Map of Die Crack
In Figure 3, the most significant cause of MBF - where the epoxy-moulding compound (EMC) seeps into the top and bottom of the exposed pad of the DBC - can be attributed to tooling design. This is consistent with ASM Pacific Technology’s significant experience in design and research in this area.
Figure 3: Mind Map of Mold Bleed Flash (MBF)
Figure 4 shows an illustration of the ideal moulding process with the moulding tool designed with optimal sealing capabilities.
Figure 4: Sealing for MBF Control
To sum it all up, the production of quality HPEs and package structures requires the addressing of many critical factors. Two of the most significant ones being die crack and MBF, and the factors contributing to them include incoming materials, tooling accuracy, and tooling design. With ASM Pacific Technology’s advanced encapsulation technology, and years of research expertise in this area, we are able to help fulfil all of your encapsulation needs.
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