Reinhard Pufall

Studies on the reliability of power packages based on strength and fracture criteria

Thermal cycling induced failure in power packages has been studied comparing thermal cycling (TC) with thermal shock (TS) tests by thermo-mechanical analyses. The major question for the time reduction is that after the failure mechanism and if it changes with changed loading type and duration. These questions were addressed by theoretical analyses including coupled transient thermal-mechanical analyses and interface fracture mechanics applying the cohesive zone approach. By the latter, critical states for delamination failures could be derived. Finally, critical interface fracture parameters of the package were compared with results from the button shear test.

Degradation of moulding compounds during highly accelerated stress tests. A simple approach to study adhesion by performing button shear tests

High temperature storage can degrade moulding compounds for chip encapsulation to such an extent that the adhesion to surfaces like copper (lead frames) or polyimide (chip coating) decreases drastically causing delamination. Also during normal operation of electronic components heat is generated locally (bond wire or chip surface) degrading the moulding compound and reducing the adhesion which in extreme cases can destroy the metallisation or the bond wires.

High temperature storage influence on molding compound properties

An electronic device cannot perform its designed functions until it is packaged such that it is interconnected with the rest of the system and protected. As an encapsulation material, thermosetting polymers are widely used. It is well known that properties of polymer-based composites like molding compounds are highly affected by the influence of temperature, relative humidity and degree of conversion. The effect of above mentioned internal and external circumstances are investigated extensively in the past. Surprisingly the effect of high temperature storage on the mechanical properties is scarcely studied. From literatures research it is concluded that high temperature storage and postcure treatments increases the glass transition temperature. Also a weight loss during high temperature storage is reported [1], [2].

Modelling of metal degradation in power devices under active cycling conditions

Metal degradation has recently received increased attention as a failure mechanism in power devices under active cycling conditions, i.e. under repeated pulsed voltage/current loads [1, 2]. Both electro-thermal and thermo-mechanical simulation are indispensable for understanding this mechanisms. The paper presents experimental and simulation data for a dedicated test structure. A suitable lifetime model has to go beyond a simple Coffin-Manson type model to capture the essential influencing parameters.

New aspects in characterization of adhesion of moulding compounds on different surfaces by using a simple button-shear-test method for lifetime prediction of power devices

The knowledge of moulding compound adhesion on different surfaces inside a semiconductor package is fundamental to ensure high reliability of the devices. The most susceptible interfaces are the boundary surfaces between moulding compound/chip and moulding compound/lead frame. The adhesion properties of these interfaces are influenced by various material and process parameters for chip surface and bond pad conditioning. The knowledge of the zero hour adhesion state and the possible degradations during reliability stress tests like e.g. high temperature storage (HTS) or temperature cycling (TC) is the base to perform a trusty lifetime prediction. This paper shows the principle of button-shear-test method offering the determination of adhesion in a quantitative way. The influence of different moulding compound materials, polyimide layers and bond pad conditioning on moulding compound adhesion on silicon is described by corresponding force-displacement diagrams. Degradation effects of moulding compound adhesion on copper lead frames after performing HTS stress tests is presented and will be discussed. For high accelerated HTS tests the possibility of test time reduction by a factor of 10 compared to automotive qualification standards AEC-Q100/101 is discussed in the paper based on performed experiments (Automotive Electronic Council, 2003, 2005 [1,2]).

Investigation of chip-package interaction-looking for more acceleration in product qualification tests

Chip-package interaction is a major concern for product reliability. Temperature cycling is a commonly used stress test to address this issue. The paper shows that temperature shock can substitute temperature cycling for certain failure mechanisms, thereby reducing stress times by a factor of 28 or even more.

Exceptional operative gate voltage induces negative bias temperature instability (NBTI) on n-type trench DMOS transistors

Abstract In some automotive applications, high negative bias is used to faster switch off n-type devices. This exceptional operative gate voltage at relative high temperature can induce instability of device parameters (e.g. threshold voltage, transconductance, saturation current, etc. In this work we will show that positive charge trapping generated under exceptional negative bias can induce large threshold voltage shift. Even if the effect can partially recover during the standard operative condition, nevertheless large Vth, shift are still present and can affect the correct functionality of the device.

Acoustic detection of micro-cracks in small electronic devices

Abstract We demonstrate the feasibility of in-situ acoustic detection of micro-cracks in small electronic devices. Applying precisely controlled damage to test vehicles using a nanoindenter, we record brittle fracture of thin layers by means of an ultra-sound piezo sensor, which is able to detect micro-cracks in the moment they emerge. This robustness test does not require further preparation effort that may induce additional stress to a sample or modify it physically, inhibiting unambiguous failure analysis. With regard to its applicability and limitations, we put acoustic emission into context with standard ex-situ experimental procedures for crack characterization in micro-electronic structures.

Determination of adhesion and delamination prediction for semiconductor packages by using Grey Scale Correlation and Cohesive Zone Modelling

Abstract Adhesion of moulding compounds on different surfaces like silicon, copper or silver is the most important parameter to optimise with regards delamination behaviour of a semiconductor package. Standard material characterisation methods like Thermo-Mechanical Analysis (TMA) or Dynamic Mechanical Analysis (DMA) do not fully cover this essential parameter. This paper presents results from moulding compound adhesion measurements obtained by using the Button-Shear-Test (BST). The interface cracking of a specimen was analysed by Deformation-Measurement-by-means-of-Correlation (DAC) technique which is based on Grey Scale Correlation. Using knowledge of the crack opening derived from DAC, a methodology to determine fracture energies which are necessary to remove the button from the dedicated surface is shown. Based on these energies the possibility of delamination prediction of semiconductor packages by using the Cohesive Zone Modelling (CZM) is described. This simulation approach is demonstrated for a TO220 package and compared with temperature cycle test results. The aspect of material pre-selection by evaluating adhesion in reliability stress tests like High Temperature Storage (HTS) is also discussed.

Delamination modeling for power packages by the cohesive zone approach

Ensuring reliability of interfaces of dissimilar materials is one of the most critical design aspects of electronic assemblies. A study on the interface delamination phenomenon of different power packages was undertaken. Thermally induced stresses caused by the thermal mismatch between molding compound (MC), Si-die, and Cu- heatsink act as combined shear and normal loadings on the respective interfaces. Interface fracture- or damage mechanics was implemented to avoid the difficulties linked with the singular stress field at bi-material interface edges and to allow for inclusion of different interface adhesion parameter as well as damage progress. The cohesive zone approach was adopted for these purposes. A parametric study with different adhesion properties was performed to determine the critical interface energies causing delamination of different power packages. It turned out that delamination failure is not an issue for good adhesion properties, but depends on the package type. For lower critical fracture energies, delamination sensitive areas were found at the sawing edge of the die and at the MC-heatsink interface. Delamination onset was calculated based on a scalar damage metrics based on the critical interface energies. Delamination progress was followed and the delaminated areas were compared with experimental analyses. It turned out that at the critical interfaces MC-heatsink different types of delamination growth can occur which are stable and unstable delamination growth.