Lead-free Solder Wafer Bumping
OVEN CONTROL AND SOLDER MATERIAL CONSISTENCY ARE CRITICAL
BY FRED DIMOCK AND KRISTEN MATTSON
Over the past 30 years, we have learned that lead negatively affects the health of humans and seen strong legislation remove it from gasoline and paints. More recently, governments in Europe and Asia have set deadlines to remove lead from consumer electronic devices that use PCBs. The ban currently is not being applied to high-reliability applications such as military or medical devices, be we all know that will come someday soon. Likewise, many believe that lead-free solder is coming to wafer bump reflow and are beginning to make the transition.
The transition to lead-free solder will affect the entire wafer bumping process from beginning to end. Changing the bump material affects the reflow temperature, under bump interface chemistry (unstable intermetallic compounds), process atmosphere, plating procedures, cleaning methods, fluxes, etc. Add to this the requirement to shrink geometries, as components get smaller and closer together, and you realize that the move is not trivial at all (Table 1).
Table 1. Decreasing micron size forecast from 2003 to 2018.
The lead-free transition is much further along for the board assembly process. At first, the thought of lead-free processing caused apprehension among many surface mount engineers because they were forced to move into unknown territory. But groups such as NEMI and the Massachusetts Lead-free Solder Consortium have brought together resources from different parts of the industry to help identify the best materials and methods for implementing these new materials.
Much of the lead-free focus for surface mount has been on the tin-silver-copper (Sn-Ag-Cu) system, also known as SAC. Both NEMI and IPC officially named slightly different variations of SAC as the new standard for lead-free solder, with about 4.0% Ag and 0.5% Cu. The new materials require a peak temperature of 240° to 250°C for reflow, instead of the current range of 210° to 215°C for eutectic tin lead solder.
Early adaptors have demonstrated that these SAC compositions are viable for surface mount applications by showing that they can economically and reliably produce good products. There have been challenges of new fluxes and efforts to better understand the effect of nitrogen on cosmetics, but there is no reason to believe that lead-free solder compositions will be different for wafer bump reflow.
Reflow Process Capability Requirements
Oven control and the consistency of solder material are more critical with lead-free solder. The alloy melting and solidification criteria used by NEMI have been described by Rea and Handwerker. In their report, they discuss the effects of compositional changes on the melting range (solidus vs. liquidus) of SAC and demonstrate that slight shifts in composition can affect the liquidus of the solder by as much as 13°C. This large difference highlights the need for solder manufacturers to ensure that the compositions remain consistent from lot to lot, and places more emphasis on oven repeatability and stability.
Tightening the process control on the reflow process can compensate for these expected slight variations in solder material composition. This can be achieved through a perfectly centered peak temperature with minimum variation run to run. If the process window is ±10°C around peak temperature, and you expect a 13°C variation in liquidus temperature, the effective process window becomes only 7°C. Total thermal variation of 3°C or less across a 12-in. wafer at peak temperature will be required for a lead-free process.
Oven stability and repeatability requirements necessitate control systems that can maintain consistent temperatures and react to changes in load without overshoots and temperature oscillations (Figure 1). Sophisticated PID algorithms are needed. Most modern ovens have these in place, but some may need adjustments or upgrades.
Figure 1. High-performance wafer bump reflow line.
Another temperature control factor is convection rate. Higher plenum pressure (convection rate) produces increased and more uniform process temperatures. Although higher convection rates by themselves help uniformity and peak temperature, convection without control can make for an unstable process (Table 2). The best way to establish complete control is to regulate the static pressure of the gas through closed loop convection feedback.
Table 2. Convection rates.
Closed-loop convection provides temperature control, ensuring that constant heating and cooling transfer rates are maintained and repeatable. Closed-loop convection is a forced convection process that uses static pressure to maximize thermal uniformity and repeatibility. No matter what pressure you use, controlling the convection rate guarantees better temperature repeatability, stability and uniformity.
Questions about atmosphere requirements are not easy to answer for surface mount processing, because it depends on the needs in the specific market being sold to. Much of the recent work has shown that SAC solders need a low oxygen nitrogen atmosphere for cosmetic reasons, not performance. Air results in a discolored surface with wrinkles. Wafer bumping requires a smooth surface, thus a stable nitrogen atmosphere with low oxygen content is important. Many ovens can provide a stable nitrogen atmosphere, but to obtain this atmosphere at a reasonable cost, you need a sophisticated gas management system with gas recirculation and precision control.
Flux is an important lead-free solder issue. The higher reflow temperatures mean that fluxes need to be less susceptible to burning and must stay on the parts longer. Many of these solder suppliers have developed no-clean fluxes that work well at the higher lead-free processing temperatures. From an oven perspective, the flux management system must be able to handle increased temperatures and larger quantities of flux. Older or low-end ovens may have inadequate flux collection systems that can quickly become plugged and even drip flux onto expensive wafers. Flux management, however, has significantly improved in recent years.
Figure 2. Reflow process roadmap.
The challenges organizations will face depend on which side of the wafer bump reflow equation they are coming from. If they are currently using tin-lead eutectic solder with a nitrogen reflow process, they understand most of the flux issues but will have to deal with equipment capability concerns. If they are currently processing high lead in hydrogen, the move will require extensive learning because they will need to deal with flux and nitrogen reflow in addition to equipment issues (Figure 2).
Eutectic Lead to Lead-free Transition
The major difference between eutectic tin-lead solder and SAC lead-free solder is the reflow temperature. The temperature increase of 30°C can affect many aspects of the process and equipment.
Higher process temperatures mean there is a need for higher furnace set points and higher convection rates. If they have a top-of-the-line oven with an excess of power, good temperature control and the ability to work at higher temperatures, the current ovens will work. Low-end ovens that already are working at their limits will need to be replaced, while in-between ovens may be able to be upgraded with simple retrofit kits (consult with the manufacturer).
Higher process temperatures also mean there is a need for slower belt speeds to maintain the same ramp rates. At ramp rates of 3°C/sec., it takes an additional 10 seconds to heat the extra 30°C and another 10 seconds of additional cooling time. With a 5-min. cycle, 20 seconds translates into a 7% decrease in throughput. Higher process temperatures could affect other components and materials on the wafer. If the UBM is temperature sensitive or alloys with the lead-free solder, it may need to be changed.
High-lead to Lead-free Transition
Much of the world's supply of bumped wafers currently is produced using a dedicated high-lead wafer bump reflow process in a hydrogen atmosphere. For these companies, the switch to lead-free will be dramatic. Along with changes in materials and issues with eutectic solder, the largest challenge will be learning to deal with flux.
In the past, manufacturers of large, expensive wafers moved to high-lead solder with the hydrogen-based process because there was little to no residue from the reflow operation. This clean process had distinct advantages, especially when the pitch between balls was small. They went in this direction because flux management at that time was poor and a clean environment was necessary to maximize yields.
Much of the dedicated high-lead wafer bump reflow is done by large OEMs with large-scale manufacturing. A front-end type batch reflow furnace is used by a number of these manufacturers. This type of tool will not be able to make the transition to lead-free processing. The system originally was designed to control at 360°C and does not control well at the lower temperatures required for lead-free reflow (~120°C less). In addition, a major problem is the use of flux for the lead-free process. These batch systems were designed for a clean environment and cannot withstand the addition of flux residue. They are not suited for cleaning or managing the flux contamination.
The lead-free mandate will soon apply to many of our processes and materials. With the lead-free transition on the horizon for wafer bumping, there is a lot of work to do.
A process with precision temperature control is needed. When it comes to precision temperature control for lead-free processing, it is important to control all of the factors that affect the transfer of thermal energy. With more items being controlled, the need for repeatability and reliability increase. One such item is closed-loop convection. Process engineers trying to achieve the most effective and reproducible thermal transfer process, especially for high-temperature lead-free processing, should take a serious look at this.
Those of us in the wafer bump reflow environment can use this knowledge base to help speed up our implementation, but each of us will have our own unique set of issues to deal with.
FRED DIMOCK, senior process engineer, and KRISTEN MATTSON, product manager for Semiconductor Packaging, may be contacted at BTU International, 23 Esquire Road, N. Billerica, MA.01862; e-mail: email@example.com and firstname.lastname@example.org.