Effect of high-temperature requirements for lead-free solder: although the thought of processing lead-free solder moves us out of our traditional comfort zones, the information and techniques needed to be successful are available.

Lead-free solder processing is a major topic at industry conferences and in many electronics magazines' articles and advertisements. Like it or not, lead-free solder processing is here, and it will affect us all sooner or later.

The thought of lead-free processing is causing apprehension because we are being forced to move from our comfort zones into the unknown. But, managing this unknown is becoming less and less of a problem because our industry is learning more about lead-free alternatives every day. The early adapters have demonstrated that good lead-free circuit boards can be made economically, and new standards for lead-free processing are being established. But some industry professionals may be unknowingly (and sometimes knowingly) spreading rumors and misinformation. Deciding what is real is always hard when new technologies evolve.

Reliable Lead-Free Recommendations

Groups such as the National Electronics Manufacturing Initiative (NEMI, Herndon, VA) and the Massachusetts Lead-Free Solder Consortium (Lowell, MA) have brought together resources from different parts of the industry. Using a team approach, they are identifying the best materials and methods for implementing lead-free solder. This team approach generally means that a single agenda can be put aside, and the checks and balances of professional interactions get to the real answers. These groups have identified the best materials and are moving to subjects such as rework and reliability testing.

The results from these industry groups comprise the new knowledge base for lead-free solder implementation. Many of the advanced solder materials and reflow oven suppliers are using this information to develop a fundamental understanding of lead-free solder needs and are willing to assist you in getting started. But remember the adage that no golden drop-in replacement for the eutectic tin-lead solder exists.

Much of the focus on lead-free solder has been on the tin-silver-copper system (Sn-Ag-Cu, or SAC). Both NEMI and IPC (Northbrook, IL) have officially named slightly different variations of SAC as the new standard for lead-free solder. Both systems contain about 4.0% silver and 0.5% copper. From an equipment viewpoint, the implementation differences are negligible, but challenges do exist. The major questions we hear are in regards to the oven's ability to process lead-free solder and atmosphere requirements.

The answers to the oven capability question are yes, no and maybe. If you have an inexpensive oven that is just barely adequate for processing eutectic solder, you will need a new oven. If you have a top-of-the-line oven with an excess of power, good temperature control and the ability to work at higher temperatures, you can probably use what you have. In-between ovens may be able to be upgraded with simple retrofit kits (consult with the manufacturer) or can be used if you have light boards.

[FIGURE 1 OMITTED]

Do I Need a New Oven?

To begin answering the question for your particular oven, we must first understand the difference between a eutectic tin-lead and SAC profile (Figure 1). The major differences are the peak temperatures required for reflow. Tin-lead has a liquids of 183[degrees]C, while SAC is at 217[degrees]C. This 34[degrees]C difference translates directly to a profile requiring a higher peak temperature. If we normally process tin-lead at 210[degrees]C, we begin looking at a peak of 240-250[degrees]C for SAC. This peak is very close to the danger zone of 260[degrees]C for many components and in some cases exceeds the component maximum temperature.

What used to be a fairly comfortable process safety factor of 40[degrees]C has now shrunk to about 10[degrees]C. The small safety factor and higher temperature has many manufacturers asking if their current reflow oven is capable of processing lead-free solder. Reflow oven manufacturers would like to say that you need a new oven with the latest bells and whistles, but in reality it depends upon the quality of the oven you have and the trade-offs you are willing to make.

Higher temperatures mean you need more power in the heated sections and slower belt speeds to maintain the same ramp rates (Figure 2). At ramp rates of 3[degrees]C per second, an additional 11.33 seconds is needed to heat the extra 34[degrees]C and another 11.33 seconds is needed for added cooling time. This 20 seconds or so may not sound like a lot of time, but, with a five-minute cycle, that 20 seconds translates to a 7% decrease in throughput. Go ahead, tell your boss that you want to slow down the line speed by 7% and see what happens!

The oven atmosphere question is also not easy to answer because it depends upon your specific market requirements. Over the years many reflow houses have learned to process tin-lead solder without nitrogen, and they will probably not return to nitrogen processing because of the added cost. Much of the recent consortium work has shown that lead-free solders need a nitrogen atmosphere for cosmetic reasons. Consortia have also said that, although the joint does not look good when processed in air, its strength and reliability are good. Many of the manufacturers that are buying new ovens for lead-free processing are purchasing the nitrogen options just-in-case they are needed; however, these manufacturers are typically planning on running in air. Upgrading an air-only oven to nitrogen can be more costly than purchasing a new oven.

Other Questions to Consider

While the major questions concern atmosphere and temperature capability, you should also ask yourself questions about flux collection, temperature control and board support for large boards.

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 the solder suppliers have developed fluxes that work well at the higher lead-free processing temperatures. From an oven perspective, the flux collection system needs to be able to handle the increased temperatures and larger quantities of flux.

[FIGURE 2 OMITTED]

The well-known mantra of temperature uniformity and consistency becomes all the more important when dealing with lead-free solders. As previously mentioned, the safety factor has shrunk from 40[degrees]C to 10[degrees]C; thus, temperature control becomes critical. A slight deviation in temperature that would not affect performance in the past now causes damage to components. Unfortunately, many times, the damage is not visible and becomes apparent only after the board is placed into service. Many older ovens do not have the control necessary for a safe lead-free process.

With lead free on the near horizon, a process with precision temperature control is needed. As noted earlier, the process window shrinks for lead-free solder, and tighter process control is necessary to avoid over-heating components, especially smaller ones. This tighter window requires control systems that can maintain consistent temperatures and react to changes in load without the dreaded overshoots and temperature oscillations. This point is where sophisticated PID algorisms are needed. Most modern ovens have these in place, but some may need adjustments or upgrades due to the tighter requirements.

While differences in temperature always exist on different points on a board, especially large boards with various sized components, increased convection rates minimize the temperature differential. At times increased convection is the only way to ensure that all points on a board can fit into the required tight process window, and reflow can take place without damage to any components. Higher pressure (convection rate) also increases the peak temperature (Figure 3), so, if you have an oven that is marginal, this feature might help your current oven process lead-free solder.

Higher convection rates by themselves help uniformity and peak temperature, but convection, without control, can make for an unstable process. The best way to establish complete control is to regulate the static pressure of the gas through closed-loop convection feedback.

Regarding precision temperature control for lead-free processing, controlling the factors that affect the transfer of thermal energy is important. With more items being controlled, repeatability and reliability are increased. One such item is closed-loop convection. Process engineers looking to achieve the most effective and reproducible thermal transfer process, especially for high temperature lead-free processing, should be taking a serious look at closed-loop convection.

Closed-loop convection provides temperature control. It ensures that constant heating and cooling transfer rates are maintained and are repeatable. Closed-loop convection, in brief, is a forced convection process that employs static pressure to maximize thermal uniformity and repeatability. And no matter what pressure you use, controlling the convection rate guarantees better temperature repeatability, stability and uniformity.

Instead of the increased temperature only affecting the profile, oven, flux collection and components, some manufacturers processing large boards on rails have found that their boards are sagging. The answer to this problem is to use an oven with a center support. A center support rides under the board, and it gently supports the board so it remains flat. The trick is to find an area on the bottom of the board without components for the support to contact. One or two manufacturers are redesigning their double-sided boards with a path to accommodate a center support.

[FIGURE 3 OMITTED]

If you run boards with components on both sides, you know that the components on the bottom of the board sometimes fall off if your temperature is too high or the hold is too long. This challenge does not change with lead-free solder. The compositions for no lead solder are at or near a eutectic, so you do not have to deal with the large mushy zones that are prevalent with high lead solders. However, the increased temperatures are still there.

While oven control is an important factor, the consistency of the solder paste is also critical. One recent article describes the alloy melting and solidification criteria used by NEMI. (1) It discusses the effects of compositional changes on the melting range (solidus vs. liquidus) of SAC and shows that slight shifts in solder composition can affect the liquids of the solder by up to 13[degrees]C. This finding highlights the need for solder manufacturers to ensure that the compositions remain consistent from lot to lot so the process requirements do not shift.

Conclusion

Although the thought of processing lead-free solder moves us out of our traditional comfort zones, the information and techniques needed to be successful are available. The best places to start are to connect to a group like NEMI and then use your solder supplier and oven manufacturer to get practical information. The technology is already well along, and soon lead-free processing will be within all of our comfort zones.

FIGURE 2: Ten-zone lead-free profile (ramp/spike).

          Peak   Rising   Total
                 Time     Time
                 Between  Above
                 50/219   219

1.1       248.0  144.35   39.35

2.2       247.5  144.14   39.73

3.3       247.7  144.21   40.11

4.4       247.7  145.21   39.96

5.5       248.4  143.21   41.32

6.6       248.7  145.04   39.05

7.7       248.3  144.62   39.20

8.8       248.9  144.03   40.51

10.9      247.3  144.45   39.64

11.10     248.6  145.01   39.77

12.11     248.2  144.99   39.66

TC Mean   248.0  144.48   39.85

TC Range    1.6    2.00    2.26

Reference

(1.) A. Rae and C. Handwerker, "NEMI's Lead-Free Alloy," Circuits Assembly, April 2004.

Fred Dimock is a senior process engineer with BTU International, North Billerica, MA; email: [e-mail].