Flip-chip Reliability Under The Conditions Of Temperature Shock

In this thesis, the reliability and relevant questions of flip chip on board (FCOB) for harsh environment application were investigated by using thermal shock test. Different material/process combinations (FR4 and ceramics substrates, five different underfills, two types of flux, two different soldering atmospheres etc.) were designed to have a good understanding of the solder joint degradation. The failure modes and mechanisms were analyzed. A good competence regarding flip chip reliability design and evaluation was established including selection and optimization of materials/process, test methods, result analysis and finite element method (FEM). Plentiful practical experience was accumulated.A good evaluation of different material/process parameters on the solder joint reliability depends on very much whether the solder joint crack initiation and propagation process can be effectively monitored. The present investigation shows for assembly without underfill, both dye penetration and SEM fracture analysis can be used to determine quantitatively the crack area and distribution. And for underfilled assemblies, application of high frequency ultrasonic microscopy in flip chip inspection was developed. Especially, investigation indicates the fatigue crack initiation and growth directly coincide with the contrast variation of solder joint in C-SAM images. The results provide important and quite effective method to study effects of material/process on the solder joint reliability.The thermal fatigue lifetime of SnPb solder joints can be greatly increased by applying underfill encapsulant in FCOB (From 37 cycles without underfill to 1500 to 2900 cycles, a magnitude of 1-2 orders higher). The solder joint lifetime was governed by the mechanical properties of the underfill material rather than delamination. For the selected underfill materials, Young's modulus has thehighest influence on stress level in the solder joints. CTE of underfill has relative little influence. While improving the solder joint lifetime, it needs to be noticed that underfill also changes the stress distribution in the whole assembly, thus creating complete different failure modes relative to no underfill assembly. Failure modes and failure mechanism become more complicated. New failure modes like FR4 substrate crack arise. Unlike no underfill assembly, solder joint lifetime of underfilled assembly has little correlation with the height of the solder joint and distance to the neutral point (DNP). The thermal fatigue life of SnPb joints in Flip Chip lies on underfill mechanical properties and delamenation.Although proven to be not the dominating factor, delamination does show clear influence on the solder joint reliability. The underfill delamination could decrease the reliability of solder joints. Delamination initiation depends on the competition between interfacial stress and adhesion. So it is decided by packaging frame and material properties.Different fluxes and their quantity affect solder shape. Residues and volatile material may degrade the adhesion between the underfill material and its surroundings causing early delamination during reliability tests or device service. Compatibility of flux and underfill material and processes is another problem. So that selection of a given combination of material/process must be demonstrated by test.Periodic solder joint cracking is first reported by the present investigation. And it is verified to be related with the glass fiber distribution inside the PCB. Due to the glass fiber cloth in the PCB, the solder joints are modulated with a periodic stress, so that those with higher stress are more prune to crack initiation and propagation. This observation indicates, after underfill encapsulation, it might be no longer valid to model the PCB as a homogeneous material, instead, local effects on micro-structure and mechanical properties might have to be considered.The solder joints reliability under thermal shock was simulated with two-dimension finite element method. Based on the experimental the...