Increasingly downsizing solder joints in microelectronics devices lead them to withstand more mechanical, electrical and thermodynamic load with their requirement in terms of reliability uprising. Electronics packaging technology including SMT (Surface Mount Technology), CSP (Chip-Scale Package) and BGA (Ball Grid Array) technology need to implement electrical and rigid mechanical connection between different materials through solder joints so that connection quality and reliability determine the quality and reliability of electronic products. Failure of even a single solder joint possibly cause complete breakdown of electronic products. Therefore, how to ensure quality of solder joints is an extremely essential concern for modern electronic products.
Traditional SnPb solder contains Pb (lead) which, together with lead chemical compound, is such a highly toxic substance that their long-term application will bring extreme damage to people's life and environment. Up to now, lead-free solder is constantly replacing lead solder for its merits of environmental protection. However, lead-free manufacturing differs from lead manufacturing in PCBA (Printed Circuit Board Assembly) process with parameters modified. Therefore, it's of much significance to get fully aware of comparison between lead soldering and lead-free soldering manufacturing procedure in PCBA so that electronic products' performance and functions won't be compromised by environmental concern.
Property Comparison between Lead-Free Solder and Lead Solder
• Lead-free solder features higher melting point than lead solder.
a. Melting point of traditional lead eutectic solder (Sn37Pb) is 183°C.
b. Melting point of lead-free eutectic solder (SAC387) is 217°C.
Since lead-free eutectic solder (SAC387) features a 34°C higher melting point than traditional lead eutectic solder (Sn37Pb), the consequence is:
1).The rising temperature afterwards leads solder to be easily oxidized with chemical compound quickly growing between metals.
2).Some components such as those with plastic package or electrolytic capacitors tend to be more affected by soldering temperature than other elements.
3).SAC alloy will bring larger stress to components so that components with low dielectric constant will have more access to failures.
4).Numerous types of soldering surfaces are available on lead-free solder surface of components. Application of tin in solder is more applied due to its low cost. Nevertheless, a thin oxidation layer tends to be generated on the surface of tin. Plus, stress will be possibly made after electroplating. As a result, tin whisker tends to be generated.
• Lead-free solder features worse wettability than lead solder.
Compared with lead solder, lead-free solder features obviously lower wettability than lead solder. Bad wettability tends to make solder joints perform too incapably to meet requirement in terms of self-calibration capability, tensile strength and shear strength. Bad wettability possibly lead to a higher reject ratio of solder joints when modifications aren't implemented to compensate for this disadvantage.
• Comparison between lead-free solder and lead solder on physical characteristics.
Below table demonstrates physical characteristics difference between lead-free solder and lead solder.
|Melting Point (°C)||183||217||227|
|Electrical Conductivity (IACS)||11.5||15.6||/|
|Thermal Conductivity (W/m·1k·1s)||50||73||/|
|Surface Tension 260°C (mN/m)||481||548||491|
|Shear Strength (MPa)||23||27||20-23|
As is depicted in above table, lead-free solder will definitely call for bad influence on solder joint reliability due to solder performance difference compared with traditional lead solder manufacturing. From the perspective of mechanical influence, typical lead-free solder is harder than lead solder. Moreover, generated surface oxide, flux contaminant and alloy residue will possibly lead to bad performances on electrical contact and contact resistance. Thus, electronic products' conversion from lead to lead-free manufacturing is never pure replacement in either electrical or mechanical aspects owing to the following reasons:
a. Because lead is relatively soft, solder joints generated by lead-free manufacturing are harder than those generated by lead manufacturing, leading to higher intensity and smaller transformation, which, however, will definitely lead to high reliability of lead-free solder joints.
b. Because lead-free solder features bad wettability, more defects will be aroused including vacancy, displacement and tomb standing.
Comprehensive Concern on Lead-Free Manufacturing
When transforming from lead solder to lead-free solder, the most protruding difference lies in high tin content (>95%Wt). As a result, the following issues should be focused on at first hand.
• Tin Whisker Growing
Tin whisker grows from weak section of tin oxide layer as monocrystal tin, performing in columnar shape or cylindrical filament. Its damages include:
a. Shortcuts may be caused between neighboring pins.
b. Bad influence may be generated to high-frequency features.
Pressure stress is available in tin soldering layer, which is regarded as the essential reason for the generation of tin whisker. For example, when lots of irregular Cu6Sn5 metallic alloy take place, a lot of defects will be formed including pressure stress accumulation on tin layer, component pin deformation and dismatching of CTE all of which will result in the generation of tin whisker. High tin alloy will lead to the generation of tin whisker, which especially works on pure tin. Lots of metal alloy like Pb or Bi, however, can stop or hindering tin whisker from growing.
Tin whisker possibly cause shortcuts on fine line components, QFP for example. Therefore, pure tin can be plated on low-grade products and components whose life time is less than 5 years. When it comes to high-reliability products and components whose life time has to be over 5 years, a nickel layer should be plated first with thickness less than 1μm and then comes a tin layer with thickness of 2-3μm.
• Growing of Metal Dendrite
Metal dendrite features different growing procedures from tin whisker. The former results from ionic electromigration in electrochemistry. Metal dendrite will lead to shortcuts that will further cause circuit failure.
• Generation of CAF
Conductive anodic filament (CAF) is another failure type as a result of electrochemical reaction. CAF takes place inside PCB board, caused by anode conductive filament containing copper growing from anode to cathode.
CAF grows to an extent when anode and cathode are connected with short circuits taking place between two poles, finally leading to catastrophic disaster. CAF is a disaster for high-density PCB assembly and lead-free solder with higher temperature makes this issue happen more easily.
• Tin Pest
Tin pest results from pure tin spontaneous polymorphism phase change. When temperature is lower than 13°C, pure tin will lead to conversion from white tin (density is 7.30g/cm3) deriving from central square structure to grey tin (density is 5.77g/cm3) in centered cubic structure. Theoretically, tin pest will lead to potential reliability risk but it is seldom observed because impurity is mixed into tin.
The above discussed issues are possible defects when lead-free solder is applied. Nevertheless, they can be eliminated as long as advanced soldering technologies are taken advantage of during PCBA process.