|BTU's Pyramax 98 reflow oven. |
The mandated removal of lead from the environment motivated the surface mount industry to search for a material to replace eutectic tin-lead solder. The new material had to be compatible with most of the existing components while maintaining good electrical conductivity, strength, and flow ability during soldering. An important requirement of the new material was that the liquidus be close to that of tin lead solder, 183°C. After extensive study, various industry groups focused on the Tin-Silver-Copper (SAC) alloy with a liquidus temperature of 217°C.
During the initial SAC studies, engineers focused on reliability issues surrounding the time above liquidus (TAL), peak temperatures, atmosphere, and solderability. Much of this was because the higher liquidus temperature of SAC meant that peak reflow temperature increased from 225°C to 240°C. This was dangerously close to the maximum component temperatures, thus control of the reflow oven becomes critical. Additionally, the surface of the SAC alloy exhibited a tendency to oxidize. This oxidation displayed the attributes of a poor quality joint with tin-lead solder.
Because of the questionable visual quality, atmosphere discussions centered on joint reliability, solder wet-ability, cosmetics, and process cost. At first, many reflow engineers used nitrogen when processing SAC. The cell phone and consumer electronics people resisted the use of nitrogen because of the added cost, thus there was significant pressure to find inexpensive solutions. As solder paste manufacturers developed new fluxes and industry consortiums discovered that non-shiny joints were reliable, most consumer products were processed in air. The notable exceptions are very expensive boards, medical devices and military applications. Early lead-free profiles consisted of a simple ramp to peak to conserve flux, and minimize the heating rate. Later a slight soak was added to assist in minimizing the delta T across various components at the peak. This also provided the flux a bit more time to clean the parts.
Higher Temp Capability Needed
The new SAC lead-free solders succeeded in demonstrating that some low-end process equipment was not up to the task. In the case of reflow ovens, the major issues were higher temperature capability, uniformity, repeatability, and handling of flux. Some of the temperature issues even affected the design of profiler thermal barriers, thermocouple (TC) attachment methods, and TC insulation. Most SAC alloy issues have been resolved, but some of the reliability studies show that the shear strength of SAC is slightly lower than eutectic lead solder. It is common knowledge in the material sciences that materials with large grains usually have decreased strength — especially when they are multi-modal or anisotropic. It is also known that fast cooling produces smaller grains. Thus, a number of people are asking for fast cooling rates to minimize the grain size of SAC. (One needs to remember that the current IPC-JEDEC J-STD-020D and proposed 020E limits the cooling rate limit to 6°C/sec.).
On the other hand, people that reflow large BGAs are advocating slower cooling rates. It is believed that increased ball shearing has surfaced with SAC because of the lower strength (increased brittleness) of lead-free solder. The difference in thermal contraction between the board and BGA develops stress during cooling. By limiting the cooling rate, the thermally imposed stress can be minimized
BGAs Add Complexities
At first look, the measurement, control and reporting of cooling rate is simple. However, this becomes complex with BGAs, non-uniform boards, and product with varying mass. As in most reflow applications, temperature measurement is complicated by TC size, placement, attachment method, accuracy, and response. In addition, the cooling rate between the center and edges of large BGAs can vary widely.
Therefore, we have a dilemma. Fast cooling to obtain theoretically higher strength, slow cooling to minimize the stress, and questions about how we measure the temperature. The answer lies in experience and common sense. Cool quickly if you have small components, slowly if you have large ones, and make sure you use consistent measurement techniques.
|Part of the cooling system on the Pyramax 98. |
Cooling (and heating) is about the transfer of BTUs/Calories in a controlled manner. In the case of solder reflow we have little control over the heat capacity of the board and components, or even the surface area of the product. Since we are looking at rates, time is somewhat fixed, thus the temperature differential between the product and the atmosphere is the major factor. Large temperature differentials create fast cooling and small differentials create slow cooling.
From an equipment viewpoint, fast cooling requires increased convection rates, heat barriers, cooling jets, increased belt speeds, and colder water for the cooler.
When a reflow oven has efficient cooling capability, decreasing the temperature of the water with the use of chillers will lower the gas temperature and increase the cooling rate. Increased convection rates (gas flow) are important because they allow for replenishment of the spent gas at the surface thus maintaining maximum temperature differentials where the work is being done — like blowing across the surface of coffee to cool it. Heat barriers between the spike zone and cooler separate the hot area of the oven from the cold area, thus allowing larger temperature differentials. Increased belt speeds move the product through the hot-cold transition faster thus lowering the time the board is in the heat. Finally, cooling jets can add a boost to the cooling. Recent results succeeded in doubling the cooling rate on 780 gram board from 2.7°C/sec to over 5.9°C/sec. by the use of increased convection and special cooling jets. This cooling rate was even higher on lighter mass boards. For slow cooling, you do the opposite of fast cooling. Slow cooling needs decreased convection rates, increased water temperatures, decreased belt speeds, and in extreme cases supplemental heating in the cooler. Some people have gone to extreme measures and moved the profile spike one or two zones away from the cooler so they can use the last zones in the reflow as heated-cooling zones. Moving the peak zone usually has an adverse affect on process throughput because it shortens the heated section of the oven by giving some heated zones over to slow cooling.
Lowering the convection rate in the cooler is quite simple on ovens that have convection control in the cooler. When convection gets too low restrictions are needed in the blower outlets so the blower fan motors do not stall.
Supplemental heating (heaters in the cooler) does a great job of limiting the cooling rate. Rates as low as 0.5°C/sec are achievable with supplemental heating, but the concern then becomes the exit temperature of the board. This is usually addressed by increasing the length of the cooling area.
In extreme cases, people opt for an oven with increased heated and cooling lengths that was designed for high throughput. They run them at reduced speed using the last heated zones for slow cooling and having more time to get the board temperature to where it can be handled.
Whether you need fast or slow cooling, there are a number of alternatives available to the reflow process engineer. In some cases, the desired rates can be obtained by adjusting the convection rates or temperature in the cooler. But if that is not enough, there are a number of new options offered for reflow ovens that enhance their ability to process at the extremes of cooling rates for SAC solder.
For more information, contact: BTU International, 23 Esquire Rd., North Billerica, MA 01862 978-667-4111 fax: 978-667-9068 E-mail: Fdimock@btu.com Web: http://www.btu.com