Attaining Lead-free Solder Joint Reliability
December 31, 1969 |Estimated reading time: 5 minutes
With the lead-free deadline approaching, EMS providers must offer OEMs transitional know-how to resolve several impending lead-free concerns, such as lead-free solder joint reliability.
By Peter Biocca and Zulki Khan
With the 2006 deadline for RoHS and lead-free compliance fast approaching, there is growing confusion among OEMs over a variety of issues and technologies aimed at resolving lead-free concerns, particularly lead-free solder joint reliability. An efficient route for an OEM is to work in tandem with an EMS provider capable of offering transitional expertise to guide OEM engineers through these issues, as well as potential pitfalls. In effect, an arrangement like this can be an extension of an OEMs’ R&D; and an EMS provider can serve as their lead-free lab in which to run different processes. By taking this direction, they are alleviated of carrying excessive engineering overhead in attaining lead-free solder joint and assembly reliability.
An ideal lead-free transition team has trained personnel in purchasing, inventory management, fabrication and process control. Purchasing and procurement specialists are versatile enough to fully understand and track a variety of changing conventions associated with lead-free assembly. Inventory personnel understand differences between eutectic and lead-free components. Particularly, they are attentive to the adverse consequences of mistaking one for the other. They also maintain stringent control over incoming vendor components from a variety of supplier bases; and once in house, segregate those components and maintain proper inventory of them.
Fabrication engineers and technicians are well versed in all the differences, tradeoffs, advantages and disadvantages of an array of lead-free PCB surface finishes. Process engineers on a lead-free transition team are trained to know the differences and effects of lead-based and lead-free thermal profiles, and which ones to use to avoid catastrophic results. Therefore, when transitioning to lead-free, OEM concerns should focus on major areas, such as PCB surface finishes, solder paste, accurate process controls, newer and different thermal profiles and precise rework.
Surface Finishes
There are many surface finishes available. Deciding on the most suitable one for an OEM product can be daunting. Closely associated with that issue is an erroneous belief among engineers that the final finish must be fully solderable. This is not true. The primary function of a surface finish is to assure the solderability of the metal underplate - usually copper. However, it is nickel in the case of electroless nickel immersion gold (ENIG) and other finishes using nickel, that is a combination of nickel and 5 to 12% co-deposited phosphorous.
Regardless of the selected finish, it must maintain precise signal integrity and not degrade it in any way. Most comply with this requirement; however, some experts question ENIG because it does not perform well on PCBs running in excess of 5 GHz. The reason is that the nickel plate remains on copper traces. It is also due to the “skin effect,” the signal is slowed because copper is a better conductor than the nickel-phosphorous deposit.
Other concerns with selecting the correct finish include electromagnetic interference (EMI), contact resistance and high joint strength. Any applied final finish must significantly contribute to controlling EMI. It also must not degrade with aging, which could cause EMI leakage at any surface finish/gasketed interface.
Some systems require the finish to have low-contact resistance throughout a product’s lifetime. Contact resistance comes in two forms: touch pads, such as keyboards; and plated edge rails, which make electrical contact with the system’s chassis. Maintaining high joint strength between the PCB, solder paste and package components over time is also critical.
Differences
There are differences between eutectic and lead-free assemblies, not only in terms of different thermal profiles, inspection and rework criteria, but also because lead-free assemblies tend to have latent defects that are absent in eutectic assemblies. One of these defects is “whisker formation.” This phenomenon is created when tin whiskers or electrically conductive, hair-like structures grow outward from tin, zinc, gold, cadmium, indium or silver. Figure 1 shows a tin whisker growing on a QFP lead, resulting from the lack of nickel barrier on copper.
Figure 1. Tin whisker growing on a QFP lead, resulting from the lack of nickel barrier on copper.
Several whisker mitigation strategies can be used to make lead-free joints more reliable. These strategies include a conformal coating applied to some devices to retard whisker growth within the coating and to prevent whiskers from shorting exposed conductors. However, process effectiveness depends on factors such as material-coating thickness, type of coating used and type of electronic-device material.
Another latent defects that can cause solder joint reliability is the embrittlement of joints over time. This is due to the absence of lead in the joint, which makes the remaining alloy hard. Over time, this hardening alloy can show signs of cracking and micro-cracking. Signs of fatigue are more prevalent in PCBs subjected to motion- or vibration-based applications. These latent defects, as well as others, place greater pressures on process controls and require tighter assembly, rework and inspection tolerances. They also demand newer, more detailed EMS provider guidelines.
Temperature Zones and Thermal Profiles
Lead-free solder and assembly reliability is achieved if trained specialists carefully implement several process requirements. There are a number of variables that must be taken into consideration to achieve lead-free solder joint reliability. These include alloy melting temperature; alloy wetting and surface tension properties; solder balling and bridging potential increases; and cosmetic effects of flux at higher reflow temperatures.
The most common alloys used in lead-free SMT are tin/silver/copper (SAC) with a melting temperature range of 217° to 220ºC. These alloys melt at higher temperatures than traditional eutectic solders such as 63/37, which has a melting point of 183ºC.
Lead-free Transition Guidelines
Investigating how progressive and savvy an EMS provider is in these new areas of lead-free solder reliability is a foremost guideline for OEMs. Keeping updated on these technologies and current R&D also is vital because many lead-free solder reliability issues continue to be resolved.
Another important guideline is to check for proper component labeling and a clear distinction between leaded and lead-free equipment. This includes solder stations; solder tips and flux jars; and solder bars and solder wires - labeled specifically to show eutectic and lead-free equipment and components. Labeling such as this is especially critical if an EMS provider or an OEM is running a dual process, such as eutectic and lead-free assemblies side-by-side.
Before transitioning to lead-free, a prudent OEM must take time to closely inspect an EMS provider’s process flow and lead-free procurement, design, fabrication and assembly; and check on how well trained the engineers and technicians are in these categories.
Peter Biocca, senior market development engineer, Kester, may be contacted at (972) 390-1197; e-mail: pbiocca@kester.com. Zulki Khan, president and founder, Nexlogic Technologies, may be contacted at (408) 436-8150; ext. 102, e-mail: zk@nexlogic.com.