Technical Library | 2012-10-23 14:25:38.0
Tin-Silver-Copper alloys are the primary choice for lead-free SMT assembly. Although there are other options available such as alloys containing bismuth or indium and other elements, tin-silver-copper solders, also known as SAC alloys are by far the most popular. They are used by approximately 65% of users, as last surveyed by Soldertec in 2003.
Technical Library | 2017-06-13 17:14:59.0
For tin-rich solder alloys, 200 C (392 F) is an extreme temperature. Intermetallic growth in tin-copper systems is known to occur and is believed to bear a direct relationship to failure mechanisms. This study of morphological changes with time at elevated temperatures was made to determine growth rates of tin-copper intermetallics. Preferred growth directions, rates of thickening, and notable changes in morphology were observed.Each of four tin-base alloys was flowed on copper and exposed to temperatures between 100 C and 200 C for time periods of up to 32 days. Metallographic sections were taken and the intermetallics were examined. Intermetallic layer thickening is characterized by several distinct stages. The initial growth of side plates is extremely rapid and exaggerated. This is followed by retrogression (spheroidization) of the elongated peaks and by general thick-
Technical Library | 2017-07-06 15:50:17.0
Head-in-pillow (HiP) is a BGA defect which happens when solder balls and paste can't contact well during reflow soldering. Package warpage was one of the major reasons for HiP formation. In this paper, package warpage was measured and simulated. It was found that the package warpage was sensitive to the thickness of inside chips. A FEM method considering viscoelastic property of mold compound was introduced to simulate package warpage. The CTE mismatch was found contributes to more than 90% of the package warpage value when reflowing at the peak temperature. A method was introduced to measure the warpage threshold, which is the smallest warpage value that may lead to HiP. The results in different atmospheres showed that the warpage threshold was 50μm larger in N2 than that in air, suggesting that under N2 atmosphere the process window for HiP defects was larger than that under air, which agreed with the experiments.
Technical Library | 2019-06-20 00:09:49.0
It is well known that during service the layer of Cu6Sn5 intermetallic at the interface between the solder and a Cu substrate grows but the usual concern has been that if this layer gets too thick it will be the brittleness of this intermetallic that will compromise the reliability of the joint, particularly in impact loading. There is another level of concern when the Cu-rich Cu3Sn phase starts to develop at the Cu6Sn5/Cu interface and an imbalance in the diffusion of atomic species, Sn and Cu, across that interface results in the formation at the Cu3Sn/Cu interface of Kirkendall voids, which can also compromise reliability in impact loading. However, when, as is the case in some microelectronics, the copper substrate is thin in relation to the volume of solder in the joint an overriding concern is that all of the Cu will be consumed by reaction with Sn to form these intermetallics.This paper reports an investigation into the kinetics of the growth of the interfacial intermetallic, and the consequent reduction in the thickness of the Cu substrate in solder joints made with three alloys, Sn-3.0Ag-0.5Cu, Sn-0.7Cu-0.05Ni and Sn-1.5Bi-0.7Cu-0.05Ni.
Technical Library | 2023-08-04 15:27:30.0
A designed experiment evaluated the influence of several variables on appearance and strength of Pb-free solder joints. Components, with leads finished with nickel-palladium-gold (NiPdAu), were used from Texas Instruments (TI) and two other integrated circuit suppliers. Pb-free solder paste used was tin-silver-copper (SnAgCu) alloy. Variables were printed wiring board (PWB) pad size/stencil aperture (the pad finish was consistent; electrolysis Ni/immersion Au), reflow atmosphere, reflow temperature, Pd thickness in the NiPdAu finish, and thermal aging. Height of solder wetting to component lead sides was measured for both ceramic plate and PWB soldering. A third response was solder joint strength; a "lead pull" test determined the maximum force needed to pull the component lead from the PWB. This paper presents a statistical analysis of the designed experiment. Reflow atmosphere and pad size/stencil aperture have the greatest contribution to the height of lead side wetting. Reflow temperature, palladium thickness, and preconditioning had very little impact on side-wetting height. For lead pull, variance in the data was relatively small and the factors tested had little impact.
Technical Library | 2018-05-17 11:14:52.0
Intermetallic compound (IMC) growth is being studied in earnest in this past decade because of its significant effect the solder joint reliability. It appears that from numerous investigations conducted, excessive growth of IMC could lead to solder joint failure. Leading to this, many attempts has been made to determine the actual IMC thickness. However, precise and true representation of the growth in the actual 3D phenomenon from 2D cross-section investigations has remained unclear. This paper will focus on the measuring the IMC thickness using 3D surface profilometer (Alicona Focus G4). Lead free solder, Sn3.0Ag0.5Cu (SAC305) was soldered onto copper printed circuit board (Cu PCB). The samples were then subjected to thermal cycle (TC) storage process with temperature range from 0 °C to 100 °C for 200 cycles and up to 1000 cycles were completed.
Technical Library | 2023-11-14 19:52:11.0
The continuous drive in the Electronics industry to build new and innovative products has caused competitive design companies to develop assemblies with consolidated PCB designs, decreased physical sizes, and increased performance characteristics. As a result of these new designs, manufacturers of electronics are forced to contend with many challenges. One of the most significant challenges being the processing of thru-hole components on high thermal mass PCBs having the potential to exceed 20 layers in thicknesses and have copper mass contents of over 40oz. High thermal mass PCBs, coupled with the use of mixed technologies, decreased component spacing, and the change from Tin Lead Solder to Lead Free Alloys has lead many manufacturing facilities to purchase advanced soldering equipment to process challenging assemblies with a high degree of repeatability.
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 | 2018-07-18 16:28:26.0
Reduction of first pass defects in the SMT assembly process minimizes cost, assembly time and improves reliability. These three areas, cost, delivery and reliability determine manufacturing yields and are key in maintaining a successful and profitable assembly process. It is commonly accepted that the solder paste printing process causes the highest percentage of yield challenges in the SMT assembly process. As form factor continues to get smaller, the challenge to obtain 100% yield becomes more difficult.This paper will identify defects affecting SMT yields in the printing process and discuss their Root Cause. Outer layer copper weight and surface treatment will also be addressed as to their effect on printability. Experiments using leadless and emerging components will be studied and root cause analysis will be presented
Technical Library | 2014-12-11 18:00:09.0
The growth of portable and wireless products is driving the miniaturization of packages resulting in the development of many types of thin form factor packages and cost effective assembly processes. Wire bonded packages using conventional copper lead frame have been used in industry for quite some time. However, the demand for consumer electronics is driving the need for flip chip interconnects as these packages shorten the signals, reduce inductance and improve functionality as compared to the wire bonded packages. The flip chip packages have solder bumps as interconnects instead of wire bonds and typically use an interposer or organic substrate instead of a metal lead frame (...) The paper provides a general overview of typical defects and failure modes seen in package assembly and reviews the efforts needed to understand new failure modes during package assembly. The root cause evaluations and lessons learned as the factory transitioned to thin form factor packages are shared
