Technical Library | 2024-06-23 21:57:16.0
Two extremes of reflow time scale for copper pillar flip chip solder joints were explored in this study. Sn-2.5Ag solder capped pillars were joined to laminate substrates using either conventional forced convection reflow or the controlled impingement of a defocused infrared laser. The laser reflow joining process was accomplished with an order of magnitude reduction in time above liquidus and a similar increase in solidification cooling rate. The brief reflow time and rapid cooling of a laser impingement reflow necessarily affects all time and temperature dependent phenomena characteristic of reflowed molten solder. These include second phase precipitate dissolution, base metal (copper) dissolution, and the extent of surface wetting. This study examines the reflow dependent microstructural aspects of flip chip Sn-Ag joints on samples of two different size scales, the first with copper pillars of 70μm diameter on 120μm pitch and the second with 23μm diameter pillars on a 40μm pitch. The length scale of Pb-free solder joints is known to affect the Sn grain solidification structure; Sn grain morphology will be noted across both reflow time and joint length scales. Sn grain morphology was further found to be dependent on the extent of surface wetting when such wetting circumvented the copper diffusion barrier layer. Microstructural analysis also will include a comparison of intermetallic structures formed; including the size and number density of second phase Ag3Sn precipitates in the joint and the morphology and thickness of the interfacial intermetallics formed on the pillar and substrate surfaces.
Technical Library | 2024-01-08 18:36:01.0
The following aims lie behind the investigations described: The circuit board is an integrated structure made of metal and plastic. Like most integrated components enclosed in plastic, it absorbs water. When it is rapidly heated as, for example, in soldering technology temperature processes, it is a well known fact that the water will evaporate abruptly, leading to destruction. It is therefore essential that the circuit board be dried before these soldering processes. Circuit board manufacturers are extremely hesitant at providing instructions on drying their circuit boards. Information from the ZVEI [1] should also be regarded critically. The cardinal problem is the high temperature which is recommended for baking. If this is applied, the result is often de-lamination and distortion of the circuit boards. Corrosion and the formation of intermetallic phases of the metallic surfaces are also to be expected. The following investigates whether gentle drying at 45°C or 60°C and at low relative humidity achieves the same result as baking at high temperatures. The industry provides novel dry cabinets which are suitable for rapid drying at relative humidities below one percent.
Technical Library | 2012-12-13 21:20:05.0
First published in the 2012 IPC APEX EXPO technical conference proceedings. We investigated the micro-void formation of solder joints after reliability tests such as preconditioning (precon) and thermal cycle (TC) by varying the thickness of Palladium (Pd) in Electroless Nickel / Electroless Palladium / Immersion Gold (ENEPIG) surface finish. We used lead-free solder of Sn-1.2Ag-0.5Cu-Ni (LF35). We found multiple micro-voids of less than 10 µm line up within or above the intermetallic compound (IMC) layer. The number of micro-voids increased with the palladium (Pd) layer thickness. Our results revealed that the micro-void formation should be related to (Pd, Ni)Sn4 phase resulted from thick Pd layer. We propose that micro-voids may form due to either entrapping of volatile gas by (Pd, Ni)Sn4 or creeping of (Pd, Ni)Sn4.
Technical Library | 2014-10-30 01:48:43.0
The ultimate life of a microelectronics component is often limited by failure of a solder joint due to crack growth through the laminate under a contact pad (cratering), through the intermetallic bond to the pad, or through the solder itself. Whatever the failure mode proper assessments or even relative comparisons of life in service are not possible based on accelerated testing with fixed amplitudes, or random vibration testing, alone. Effects of thermal cycling enhanced precipitate coarsening on the deformation properties can be accounted for by microstructurally adaptive constitutive relations, but separate effects on the rate of recrystallization lead to a break-down in common damage accumulation laws such as Miner's rule. Isothermal cycling of individual solder joints revealed additional effects of amplitude variations on the deformation properties that cannot currently be accounted for directly. We propose a practical modification to Miner's rule for solder failure to circumvent this problem. Testing of individual solder pads, eliminating effects of the solder properties, still showed variations in cycling amplitude to systematically reduce subsequent acceleration factors for solder pad cratering. General trends, anticipated consequences and remaining research needs are discussed
Technical Library | 2015-02-12 16:57:56.0
Electronic systems are known to be affected by the environmental and mechanical conditions, such as humidity, temperature, thermal shocks and vibration. These adverse environmental operating conditions, with time, could degrade the mechanical efficiency of the system and might lead to catastrophic failures.The aim of this study is to investigate the mechanical integrity of lead-free ball grid array (BGA) solder joints subjected to isothermal ageing at 150°C for up to 1000 hours. Upon ageing at 150°C the Sn-3.5Ag solder alloy initially age-softened for up to 200 hours. This behaviour was linked to the coarsening of grains. When aged beyond 200 hours the shear strength was found to increase up to 400 hours. This age-hardening was correlated with precipitation of hard Ag3Sn particles in Sn matrix. Further ageing resulted in gradual decrease in shear strength. This can be explained as the combined effect of precipitation coarsening and growth of intermetallic layer. The fractured surfaces of the broken solder balls were also investigated under a Scanning Electron Microscope. The shear failures were generally due to ductile fractures in bulk solders irrespective of the ageing time.
Technical Library | 2020-08-05 18:49:32.0
The evolution of internet-enabled mobile devices has driven innovation in the manufacturing and design of technology capable of high-frequency electronic signal transfer. Among the primary factors affecting the integrity of high-frequency signals is the surface finish applied on PCB copper pads – a need commonly met through the electroless nickel immersion gold process, ENIG. However, there are well-documented limitations of ENIG due to the presence of nickel, the properties of which result in an overall reduced performance in high-frequency data transfer rate for ENIG-applied electronics, compared to bare copper. An innovation over traditional ENIG is a nickel-less approach involving a special nano-engineered barrier designed to coat copper contacts, finished with an outermost gold layer. In this paper, assemblies involving this nickel-less novel surface finish have been subjected to extended thermal exposure, then intermetallics analyses, contact/sheet resistance comparison after every reflow cycle (up to 6 reflow cycles) to assess the prevention of copper atoms diffusion into gold layer, solder ball pull and shear tests to evaluate the aging and long-term reliability of solder joints, and insertion loss testing to gauge whether this surface finish can be used for high-frequency, high density interconnect (HDI) applications.
Technical Library | 2021-05-13 16:03:25.0
Sn-based lead-free solders such as Sn-Ag-Cu, Sn-Cu, and Sn-Bi have been used extensively for a long time in the electronic packaging field. Recently, low-temperature Sn-Bi solder alloys attract much attention from industries for flexible printed circuit board (FPCB) applications. Low melting temperatures of Sn-Bi solders avoid warpage wherein printed circuit board and electronic parts deform or deviate from the initial state due to their thermal mismatch during soldering. However, the addition of alloying elements and nanoparticles Sn-Bi solders improves the melting temperature, wettability, microstructure, and mechanical properties. Improving the brittleness of the eutecticSn-58wt%Bi solder alloy by grain refinement of the Bi-phase becomes a hot topic. In this paper, literature studies about melting temperature, microstructure, inter-metallic thickness, and mechanical properties of Sn-Bi solder alloys upon alloying and nanoparticle addition are reviewed
