Plastic IC packaging subjected to moisture is vulnerable to delamination and cracking during the solder reflow process. Modeling the response of the delaminated plastic package is an essential step to fundamentally understand failure mechanisms and mechanics involved in the cooling, moisture absorption, and solder reflow process. A nonlinear finite element model was developed for predicting the deformation, stress, and fracture behavior of delaminated plastic packages induced by mechanical and hygro-thermal loads. The model consists of a sequentially coupled hygro-thermo-mechanical analysis considering moisture absorption, evaporation and interface contact and fracture analysis. A Lagrange Multiplier method was utilized to model the delamination interface condition. A general contact model was adopted which can handle complex contact conditions, such as arbitrary slippage and discontinuous curvature. A thermal contact was utilized to model the contact along delamination surfaces. Mixed mode fracture modes were elaborated. The model was verified by comparing existing analytical results with the predictions in the case of pulsed heating of the IC chip. Packaging responses and failure mechanisms due to encapsulation cooling, moisture absorption and evaporation, wave soldering, and interfacial moisture pressure loading are investigated, with consideration of temperature-dependent material properties change around the glass transition temperature for both die attach and molding compound. Packaging deformation and stress, crack tip driving force, doming of the delamination, and delamination growth stability as a function of time are discussed. >

Behavior of delaminated plastic IC packages subjected to encapsulation cooling, moisture absorption, and wave soldering

The interfacial-adhesion performance between the lead frame and molding compound was studied after temperature cycles and hygrothermal aging, simulating a typical package-assembly process. The hygrothermal aging involved a treatment at 85°C and 85% relative humidity (RH) for 168 h and three cycles of infrared (IR) solder-reflow condition. The interfacial-bond strengths were measured using shear and lead-pull tests. The lead-frame surface finishes studied include a bare Cu, microetched Cu, spot Ag coating, Ni, Pd/Ni, and Au/Ni coatings. Special emphasis was placed on the study of the changes in surface characteristics and the corresponding interfacial adhesion after various manufacturing processes. It was found that moderate thermal cycles enhanced the interfacial adhesion for all coated lead frames, except the Ni coating. Hygrothermal aging was detrimental to the interfacial-bond strength, especially for hydrophilic or polar surfaces, such as bare Cu, Ag, Pd/Ni, and Au/Ni coated lead frames. The introduction of tiny dimples etched on the lead frame was effective in mitigating the reduction in interfacial-bond strength arising from hygrothermal aging. This result confirms the important role of the mechanical-interlocking mechanism provided by dimples in retaining the interfacial adhesion in a humid environment.

Effects of moisture and elevated temperature on reliability of interfacial adhesion in plastic packages

This study attempts to elucidate the interaction of three main material components in a plastic package: mold compound, die attach, and die coating. The investigation was divided into three parts, addressing the combined effect of the previous three components, the influence of the thickness of the polyimide coating, and the effect of the mold compound alone, respectively. Five mold compounds (first generation low-stress, second generation low stress, third generation ultra low-stress, moisture resistant, and anti-crack compound for thin packages), two die attach resins (low-stress epoxy and ultra low-stress silicone-modified epoxy), and two die coats (standard polyimide and photo-sensitive polyimide) were tested. The authors discuss how combining different high- and low-stress materials affects the failure rate and the types of failures. Three main results were obtained: (1) die attach is the most important factor since its compliance during thermal shock affected the interfacial integrity of the die; (2) thicker coatings of polyimide (>8 mu m) give better protection to the die surface; and (3) the anticrack molding formulation with the lowest CTE produces the fewest failures. >

Effect of material interactions during thermal shock testing on IC package reliability

When a plastic package is subjected to repeated thermal excursions such as during thermal cycling or thermal shock, progressive damage to the silicon die occurs. Damage can be initiated in the form of interfacial delamination accompanied by passivation cracking, and subsequently, by dielectric fracture that may ultimately lead to device failure. The extent of damage depends on the interaction between the various components in the package. In this study, thermal cycling of PLCC packages indicated that the die design configuration, the nature of the coating and its thickness, the formulation of the molding compound, the preconditioning of the packages, and the thermal excursion conditions all govern the electrical failure rates observed. Thus, careful selection of the proper combination of parameters can offer improved device reliability. >

Effects of die coatings, mold compounds and test conditions on temperature cycling failures

This work benchmarks the current reliability tests used by the electronics industry, examines those tests that affect and are affected by molding compounds, discusses the relevance of accelerated testing, and addresses the major reliability issues facing current molding compound development efforts. Six compound-related reliability concerns were selected: moldability; package stresses; package cracking; halogen-induced intermetallic growth at bond pads; moisture-induced corrosion; and interfacial delamination. Causes of each failure type are surveyed and remedies are recommended. >

Molding compound trends in a denser packaging world: qualification tests and reliability concerns

http://ieeexplore.ieee.org/iel3/5087/13879/00639753.pdf

L.T. Nguyen

Papers

Molding Compound Trends In A Denser Packaging World. I. Technology Evolution