When the Restriction of Hazardous Substances (RoHS) was implemented by the European Law in 2003, hazardous materials were no longer permitted for use in the manufacture of electronic products in most countries around the world. With lead being predominantly one of the most used materials within electronics, this presented a series of challenges to manufacturers of consumer electronics worldwide. However, there are some segments within electronics that are exempt from this rule, which include markets used in harsh environments and or have life/system critical functionality such as medical and aerospace equipment, defence and automotive electronics as well as products used in national security.
Today, as components are getting smaller and more complex, it is increasingly difficult to procure advanced components, especially when it comes to ball grid array (BGA) assemblies. Those who continue to use tin/lead solder within their soldering process are faced with the issue of how to use BGA components that are only available with lead-free solder spheres. Higher processing temperatures for lead free assembly increases the risks of internal delamination, internal cracks, bond damage, die lifting, thick film cracking as well as external packaging cracks. Focusing on some key challenges in this article, we examine several aspects of managing lead-free BGA components in a tin/lead soldering process.
The combination of a tin/lead solder paste alloy with a BGA component with lead-free solder balls results in a “mixed metallurgy” solder, often resulting in poor solder joint integrity. Today’s BGA’s consisting of lead-free solder spheres contain tin/silver/copper alloy. As the lead-free alloy has a melting temperature of 222°C, which is higher than that of the typical tin/lead alloy melting temperature of 183°C, the solder profile must be examined to determine appropriate manufacturing conditions. However, three industry solutions have emerged as acceptable methods to address the potential solder joint integrity issue.
Reballing requires sending the lead-free BGA component to an external service provider or depending on equipment and skill levels, this can be handled in house to be “reballed.” Martin Rework has over 30 years’ experience in rework technology. The lead-free solder balls are removed and replaced with tin/lead alloy solder balls. This method can be effective if there are strict controlled processes set in place. According to Dave Hillman 2017, the advantage of a reballed BGA component is that it is transparent to a tin/lead soldering process. The reballing of a BGA component requires the control of several key process parameters: moisture sensitivity level, solder ball removal / attachment temperature / time and cleanliness of the reballed BGA component.
Functional component testing is necessary to ensure that no process or component defects result from the reballing process. X-Ray scanning of the reballed part is also highly recommended. Reballing remains a viable solution to manage the obsolescence of tin-lead ball metallurgy.
Reflow profiling is to utilise a “hot” reflow profile during the tin/lead soldering process to harmonise the BGA solder joint microstructure and minimise solder joint microstructure segregation. Typically, the industry uses tin/lead solder paste reflow profiles with a maximum temperature limit of 200–225°C. Solder paste flux formulations and component fabricators have characterised and tested their construction materials to ensure they withstand this temperature range limit without degrading. However, the typical lead-free solder reflow profile has temperature excursions in the range of 235–260°C, which easily exceeds the 200–225°C tin/lead solder process reflow profile limit. Many component fabricators will void their tin/lead component warranties if the 225°C temperature is exceeded. Some high-performance product design teams have conducted testing and worked with their component fabricators to develop acceptable “hot” reflow profiles, that do not introduce component integrity concerns. The advantage of using a “hot” profile is minimal process parameters changes and low cycle time impact.
Underfilling is becoming increasingly popular because it gives back its structural integrity, reducing any shock and vibrations that can occur from handling the component. According to Dave Hillman (2017), using the lead-free BGA component in a tin/lead soldering process and then underfill it, results in the BGA solder joints having various degrees of segregated solder joint microstructure, dependent upon the size and density of the BGA component population on the printed circuit assembly.
Underfill advantages are Fritzson, M (2011):
The Sn63Pb37 and Sn60/Pb40 solder alloys have always been a popular choice within the electronics industry due to them being eutectic, meaning the alloys melt and freeze at one single temperature. Since the primary lead-free solders used today are not eutectic alloys, additional processing due diligence is required. Additionally, a non-eutectic metal alloy can often form solidification shrinkage voids.
A number of industry investigations have documented solder joints with high ionic content and/or corrosion situations that initiated tin whisker growth. These tin whiskers have been shown to originate at the edges of component pads or solder joint fillet/component lead interfaces, where the solder is very thin and behaves like tin plating rather than a bulk solder alloy. Investigated by J. Osenbach, J. DeLucca, B. Potteiger, A. Amin, R. L. Shook, and F. A. Baiocchi, (2007)
As we have mentioned in the article above, significant effort is needed to determine the optimum reflow solution. The PCB materials and electronic components must therefore be able to withstand the higher temperatures. The potential exists for non-uniformity of the finished solder connection which could have a negative effect on reliability. The key factor in determining appropriate reflow temperature profile is a temperature profile that matches the solder paste flux requirement. Some fluxes require a long dwell time below 180°C, while others burn up with a long dwell time. Out-of-bound solder paste flux temperatures can result in poor solder connections for all components on the board. Obtain the ideal reflow profile, which gives the best solderability, from your solder paste vendor.
Many companies are moving into thermal profiling. SolderStar makes the transition easy, as it is extremely simple to use yet offers full profiling capabilities, enabling users to establish and maintain the correct temperature and speed settings in all of their thermal processes, quickly and easily.
Another innovative development to help in the manufacturing of microelectronics is the SolderStar Automatic Profiling System (APS). This profiles each and every PCB soldered in a convection reflow oven and is ideal for customers who need complete traceability. The system tracks the profile of every assembly that passes through the oven – a necessity in many industries today.
SolderStar enables electronics manufacturers to control and optimise their thermal profiles, ensuring:
Circuit boards today come in all sorts of shapes and sizes, especially in compact and miniature devices. With 15,000 solder connections in a small chip package, they are not easy to verify with the naked eye. This is where X-Ray is employed as an option. X-Ray is capable of penetrating the solder joints, and identifies missing balls, solder deposition misalignment, and more. These images are then compared by experts to identify the PCB solder connections.
The Unicomp X-Ray is designed to provide high resolution X-Ray imaging primarily for the electronics industry. This versatile system is effective for many applications within the PCB manufacturing process. This includes BGA, CSP, QFN, Flip Chip, COB and other ranges of SMT components. The AX-8200 is a powerful support tool for process development, process monitoring and refinement of the rework operation. Supported by a powerful and easy to use software interface, the AX-8200 addresses both small and large volume factory requirements.
With 15,000 solder connections in a small chip package, they are not easy to verify with a naked eye. This is where X-Ray is employed as an option. X-ray is capable of penetrating the solder joints, and identifies missing balls, solder deposition misalignment, and more.