-
- News
- Books
Featured Books
- smt007 Magazine
Latest Issues
Current IssueComing to Terms With AI
In this issue, we examine the profound effect artificial intelligence and machine learning are having on manufacturing and business processes. We follow technology, innovation, and money as automation becomes the new key indicator of growth in our industry.
Box Build
One trend is to add box build and final assembly to your product offering. In this issue, we explore the opportunities and risks of adding system assembly to your service portfolio.
IPC APEX EXPO 2024 Pre-show
This month’s issue devotes its pages to a comprehensive preview of the IPC APEX EXPO 2024 event. Whether your role is technical or business, if you're new-to-the-industry or seasoned veteran, you'll find value throughout this program.
- Articles
- Columns
Search Console
- Links
- Events
||| MENU - smt007 Magazine
Lead-free: Reduce Joint Cracks in Automotive Electronics
December 31, 1969 |Estimated reading time: 6 minutes
The RoHS Directive comes into force in one month, and many manufacturers have adopted SAC solder alloys for lead-free consumer appliance production. However, durability is an issue as cracks in SAC joints progress quickly. A higher-reliability lead-free solder was designed to withstand harsh on-board automotive electronics environments.
By Atushi Irisawa
As the European Union’s (EU’s) RoHS Directives come into force in July 2006, the transition to lead-free solder is occurring at an increasingly rapid pace. Many manufacturers have adopted Sn/Ag/Cu (SAC)-type solder alloys as a representative lead-free solder for consumer appliance production. The mechanical properties of SAC solder differ greatly from those of conventional tin/lead solder. At the beginning of lead-free substitution, SAC alloys displayed high creep strength. It was believed that this would be effective for solder reliability, but as many users conducted reliability tests and market data became available, concerns about its durability were raised. It is now known that cracks in SAC-based joints progress quickly. After years of working with automotive suppliers and manufacturers, a higher-reliability lead-free solder was designed to withstand the severe environments of on-board automotive electronics.
Demands of Automotive Electronics
A wide range of on-board electronic devices are used in automotive applications, including car audio, car navigation and global positioning systems (GPS), engine control, power windows, and monitors for electronic toll collection (Figure 1). For solder mounting on vehicles, environmental tests that assume use at severe temperatures and humidity conditions are conducted in consideration of device-use in various regions. These tests include high- and low-temperature repetition (i.e., heat cycling), high temperature, and high humidity. Resistance to heat cycles in automotive applications is demanding - 3,000 cycles or more at -40° to 125°C, especially for the engine compartment, which is exposed to severe conditions such as rainwater, or for critical devices whose failure could lead to injury or death of vehicle occupants.
Figure 1. On-board automotive electronics.
null
Thermo-cycling and Cracks
Consider a case in which a 3216 chip resistor is subjected to a heat cycle of -40° to 125°C. Using the lower temperature as a reference, there is a temperature difference of 165°C, up to 125°C. The thermal displacement of parts and board (1) is the product of the difference in thermal expansion coefficients between parts and board (◽), and temperature difference (T) and package size (1):
1 = ◽ × T × 1 μμ
Figure 2. The mechanism of solder cracking.
In this case, the solder between the parts and the board must absorb a thermal displacement of 3.7 µm. This displacement works as shearing force, and cyclic shear occurs to promote metal fatigue - resulting in solder cracking. The larger the part, or the larger the difference in thermal expansion coefficient between the part and the board, the more the stress load that is applied to the solder. This leads to solder cracking. In addition to thermal displacement, a change in the crystalline state occurs because of solder alloy heating, making it difficult to estimate the fatigue life of the solder (Figure 2).
Material Choice
While Sn/3Ag/0.5Cu (SAC305) is used as a lead-free solder for consumer electronics applications, there are many concerns about its mechanical properties and thermal-fatigue characteristics for on-board automotive devices. Research points to a high-durability, anti-crack SAC-based alloy with special metals added to increase durability for such applications. By integrating trace elements into the SAC alloy, the resulting anti-crack alloy improves strength and shock absorption. This SAC-based alloy* is made by adding trace elements of nickel (Ni) and indium (In) to the conventional composition of SAC. There are three reasons why this alloy is a suitable candidate for a high-durability, damage-resistant, anti-crack alloy:
Figure 3. Cross-section comparison of SAC vs. SAC-based solder alloy.
• The structure should not be too different from the original composition of SAC. The preponderance of recent lead-free developments is based on SAC; market data has been accumulated on the material, and results are available to manufacturers. If one investigated compositions radically different from SAC, data on the design, reliability, and functionality would not be useful. New data would have to be generated and analyzed from the beginning for the new material. The SAC-based alloy composition can be handled in nearly the same way as a SAC alloy.
• Elements added to the composition should be ordinary and harmless. Even if an element has good characteristics, it cannot be established as an environmentally friendly, lead-free solder if it is harmful. Use of an unknown metal may raise serious concerns about reliability. The SAC-based alloy uses well-recognized metals (Ni and In). Nickel has been used for plating, and has been closely related to soldering for a long time. Demand for it is increasing for tin-indium oxide (TIO) used for liquid crystal panels. While this is expensive due to small reserves, this will not be a problem because the added quantity is minimal. Sn/Ag/Bi/In-type solders using indium were also used for portable mini-disc players, a mass-produced product using lead-free solder.
• The quantity of the additional elements should be minimal. This is related to the first reason; and the aim is to hold the added quantity to a minimum necessary amount to ensure durability, and make it equivalent to those of SAC alloys.
Test Results
Figure 3 shows a cross-section of a 3216R chip after 1,000 cycles at -40° to 125°C. The SAC solder develops large cracks in the fillet and chip back. Cracking for the high-durability, anti-crack SAC-based alloy is controlled, though some strain is found. Figure 4 shows cross-sections of the same chip after 3,000 cycles. The SAC alloy is completely broken through at about 2,000 cycles, resulting in conductivity failures. In contrast, the joints of the SAC-based alloy retain continuity after 3,000 cycles, although some crack progress is observed.
Figure 4. Cross-section comparison of SAC vs. SAC-based solder alloy at 3,000 cycles.
null
Developing a Corresponding Flux
When the high-durability alloy is formed into solder paste, the paste characteristics may change due to effects of the added metal. For the SAC-based alloy, because Ni and In are added, they have a positive effect on the alloy’s joint durability. However, they also have some negative effects with regard to solder paste. It is known that In is highly reactive. Even a very small amount (e.g., 0.5%) added to the alloy may have some effect. When developing a flux for the SAC-based alloy, reaction-controlling technology was developed for an earlier In-based flux was used. As a result, the shelf life was improved and viscosity changed when printing was controlled (Figure 5).
Figure 5. Composition of a SAC-based solder alloy.
It is difficult to develop a flux with crack-free residue. Flux used for soldering contains rosin, a material with insulating properties and an acid value (related to wettability). However, rosin use results in a flux residue that tends to crack because of inherent hardness and brittleness. To make the flux residue crack-free, the brittleness inherent to rosin must be improved upon. To do so, the ratio of rosin is reduced and an additive for plasticity is included. For the reduced portion of rosin, an activator must be applied to compensate for reduced wettability, to the extent that electrical reliability will not be compromised. The resulting paste made from mixing solder powder with this flux is the crack-free solder paste.
Conclusion
For flux used in on-board automotive devices, it is necessary to secure reliability of insulation in high-temperature and high-humidity atmospheres, as well as those in which dew occurs. To meet these requirements, a crack-free flux that does not crack in the flux residue throughout the heat cycle was developed. This can also control electromigration caused by moisture entering residue cracks; and can prevent residue from entering switches or other elements, which can cause contact failures. The flux also has a beneficial coating effect (Figure 6).
Figure 6. Flux residue after 1,000 cycles; conventional (a) vs. crack-free flux (b).
* S3XNI alloy, licensed by Koki Company.
Atushi Irisawa, manager, the Soldering Technology Division, Koki Company Ltd., may be contacted via e-mail: info@ko-ki.co.jp.