Embedded Files
Surface Mount Technology
Surface Mount Technology
SMT (Surface Mount Technology) is widely used in high-tech electronic printed circuit board (PCB) assemblies. Two major SMT technologies to assemble electronics components on PCBs are:
- Solder paste reflow
- Wave soldering
The assembly technology is chosen based on the PCB layout and whether there are through-hole components (THC) to be assembled. Wave soldering is used for PCBs with THCs. For the process the PCB with inserted THC are cleaned with liquid flux before soldering. On the other hand, in the reflow process the solder alloy (powder) and flux are pre-blended to form a solder paste.
Solder Paste
Solder Paste
Solder pastes are made by mixing a metal alloy powder (~ 90 wt%) with a cream-like material (flux) made of organic chemicals (~10 wt%). The solder paste serves a fourfold purpose:
- Elimination of oxide film from the surface of solder, substrate and component
- Prevention of re-oxidation during reflow
- Reduction of surface tension of the solder to enhance wetting
- Providing rheology and viscosity to make the solder powder printable
The rheology of the paste is carefully designed to facilitate stencil printing or paste dispensing process on the PCB.
Solder paste flux has a very complex chemical composition comprising:
- solvents, resins, activators, rheology modifiers, and additives.
Generally there are three flux categories with different activities:
- Rosin flux (R, RA, RMA), Water soluble (WS) & No Clean (NC).
For development of a solder paste a number of functions must be fulfilled:
- Good printability
- fine pitch, speed, continual
- Good workability
- tackiness, slump resistance, stencil life
- Good Solderability
- solder balling, wetting
- High reliability
- Corrosion, electromigration, SIR, ionic residue
At AIM Solder, a leading global manufacturer of lead-free solder assembly materials, we carried out intensive research and development of advanced solder paste fluxes with very complex chemical composition to deliver robust performance for high reliability requirements such as LED, automotive, and avionic applications. We developed halogen-free, lead-free solder pastes to resolve difficult issues including QFN voiding, low area ratio printing, and high reliability requirements for demanding applications.
Development of low silver SAC solder
Development of low silver SAC solder
The conversion to RoHS-compliant lead-free assembly has brought a challenging task to the electronics industry due to process compatibility, cost and reliability considerations. The Pb-free alloy SAC305 emerged as the industry standard during the RoHS transition. However, there are issues surrounding SAC305 implementation, for example; poor drop shock performance, unstable mechanical performance by aging, tin whisker growth, and high cost of silver. The solder industry is innovating to address these challenges. New options and new understandings are emerging to meet the demands of multiple markets as RoHS exemptions expire.
In an extensive study a systematic investigation on the effect of Bi on low silver SAC solder alloy was carried out (*). Properties such as thermal behavior, wetting performance, hardness and tensile strength, as well as tin whisker growth were investigated.
Based on soldering and mechanical evaluation performed on various Bi concentrations of low silver (Ag<1wt%) SAC alloys, it was concluded that alloy with Bi<3wt% was optimum composition in terms of melting, wetting, mechanical and reliability performance.
Tin whisker growth of SAC305 exposed to 85%RH and 60°C environment for 2600 hr.
Wetting balance test running on Cu coupon immersed in molten solder bath.
Up to 3% Bi in SAC-Bi the elongation fits with a power law equation. Elongation decreases more rapidly in alloy with higher concentration of Bi.
Variation of maximum wetting force with Bi content in SAC-Bi.
New Pb-free solder for harsh environment
New Pb-free solder for harsh environment
Reliability is undeniably a vital characteristic of any electronic device, and with increasing functionality and reliance on electronic devices the reliability requirements are increasing. For example, in automotive applications there are now autonomous brake control systems, electronically controlled shock absorbers, hybrid power control systems and LED lighting. Electronic devices ensure the safety of the vehicle and its occupants and reliability becomes a life critical concern.
Increased functionality in smaller devices generates higher temperatures inside the device inducing further thermal-mechanical fatigue due to the mismatch of the coefficient of thermal expansion (CTE) between electronic components, board and solder material. Moreover, higher heat density increases the kinetics of microstructure evolution of the solder joint and thus faster degradation of mechanical performance leading to solder joint failure.
Fatigue is a major cause of failure of surface mount components and is an important consideration in high reliability applications. Fatigue strength, which refers to the capacity of a material to resist conditions of cyclic loading, typically decays with increasing numbers of cycles. By increasing the tensile strength the fatigue resistance of material will increase.
In a research collaboration with universities with extensive experimental investigation a new SAC-Bi-Sb-X solder alloy (X represents micro alloying dopants) with superior solderability and reliability was developed. Thermal, mechanical, and soldering properties of new solder alloy is superior to the commercial SAC305.
Superior tensile property of new solder alloy against SAC305.
Higher toughness of new solder vs. SAC305.
Weibull plots of thermal cycling results of SAC305 versus new solder alloy for solder joints of LED components.
Systematic study on alloying of Pb-free solders
Systematic study on alloying of Pb-free solders
A systematic study on the effects of Bi, Sb, Ag, Cu and Ni alloying elements on mechanical and thermal behaviour of Sn-X binary alloys has been performed. It is demonstrated that this systematic approach is a useful tool to understand solder alloy behaviour and design new alloy for demanding applications.
Based on this systematic approach and other research presented elsewhere a new multicomponent solder alloy for demanding applications was developed. In this metallurgical approach, the evolution of microstructure and mechanical properties with alloying elements and thermal effects are emphasized as the key parameters when developing solder alloys for demanding applications.
Effects of alloying elements on mechanical & thermal properties of Binary Sn-X solders.
Creep behaviour and alloying impact
Creep behaviour and alloying impact
Creep behaviour of some lead-free solder alloys and the effects of varying amounts of alloying elements such as Sb, Bi, Ni,...on the creep resistance and high temperature microstructural evolution have been systematically studied.
Creep behaviour as well as microstructural evolution resulting from long-term isothermal treatment of multi-component solder alloys for electronic assembly applications are investigated. Mechanisms of deformation are evaluated via creep tests conducted at different temperature range and stress and in-depth scanning electron microscopy analysis.
Based on this systematic approach and other research presented elsewhere a new multicomponent solder alloy for harsh electronic environment applications is developed.
Steady state creep and SEM microstructure of SAC-Bi-Sb alloys
Page updated
Google Sites
Report abuse