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Thermal Dynamics and BGA Ball Reflow
By Greg Ayers
Abstract
The Reflow technology has undergone some major changes in recent times. This is largely due to the requirements in the industry to improve process control. Three major factors contribute to achieve this: Temperature, Time and Atmosphere. Not only do you have to control each segment, but you have to do it at the same time, as each interacts with the other. It is no longer tolerated that one of the ingredients are held to specs and the other can just be so so. Today’s furnace therefore takes a drastic divergence from the common Convection furnace designed for the SMT circuit Industry. It must be compact, and it must be GREEN. i.e. use less electricity, less Nitrogen or Forming gas. For the processing of plastic BGA’s for example a cycle rate of 10 seconds or less must be achieved and this should be accomplished single file. The furnace must be less than 5 feet long. Let us discuss how this can be achieved.
Introduction
Heat energy transferred by a soldering iron to a solder joint is an old and proven technology. Using the same concept we develop a lower conduction heat platen for the product to absorb heat from and an upper platen to flood the area with convection gas. Both platens provide energy (heat) at the same temperature. If you subject product (strip BGA) to a certain temperature, say 110 degrees C, it is only going to take a few seconds to attain that temperature and maintain that temperature, no lower and no higher. Figure 1 shows us a textbook profile. The temperature profile consists of four items: Pre-heat, soak, spike and cooling. To simulate this we set the Pre-heat zone for 140, the soak zone for 170 and the spike zone for 230. Figure 2 shows us how we can control a higher soak temperature by elevating it 10 degrees, from 170 to 180. In Figure 3 we see we can limit the peak temperature to 210 degrees. Cooling may be provided by either air or chilled water. The water temperature may range from 10 to 50 degrees depending on the ramp-down temperature, as illustrated in Figure 4. The process engineer selects all of these parameters depending on the task at hand.
Drive Mechanism
Figure 5 - shows the compact furnace from above with upper heat platens and hood removed. Typically, strips, either plastic or singulated BGA’s in stainless steel carriers, are shuttled into the furnace on a loading platform. Chilled water provides an even process “start” temperature. This is important as we always want to start of profile at the same place. Since the strip is lying directly on the surface, movement is provided by a sweeper bar. A series of these bars is connected by a drive chain and spaced 4 1/2 inches apart. Upon a signal, the DC motor moves the bars exactly 4 1/2 inches. This is how the product is moved from zone to zone. The dwell time may be from 7 seconds to whatever the process soaktime may call for. A plastic strip may cycle at 10 seconds, and a carrier fixture may take 30 seconds. Referring back to Figure 1 where we divided the profile into 5 parts, so have we divided the furnace into 5 zones; 4 heating and 1 cooling. Each zone is contained within an inert chamber. As we see our BGA strip shuttles from one zone to the next subjecting it to 4 different temperatures and then cooling. If we use 10 second dwell time, the process time will be 70 seconds from the time the strip enters the furnace till it exits for the next operation. The productivity is 360 strips or well over 2000 BGA’s per hour.
Furnace Construction
In Figure 6 - the side view of each zone shows the conduction heat stage and convection heat manifold with the sweeper bar acting as a “dam” at either end of the zone. The substrate or BGA strip is therefore in a very small, inerted chamber, usually, no more than 1/4 inch high. Fresh pre-heated gas (nitrogen or forming gas) pressurizes the chamber, thereby maintaining oxygen levels well below 10PPM O2. The expended gas is pressured out each side of each zone, collected by the hood, and vented out of the furnace. Gas cannot escape from one zone to the other, nor can there be any flux build-up because the zones are purged with fresh new gas continually.
Micro processor controlled DC motors and temperature controllers are located in the front panel. Three types of P.C. cards make up the guidance of the furnace that may either be manually or P.C. driven.
Thermal Dynamics of Proper Solder Ball Attachment
The formation of the bond between solder spheres and the lands of the BGA package is critical to the parts' thermal cycling performance once installed by the end user. It is known that the grain structure of solder and more importantly the intermetalic thickness of the bond layer with the copper land area has a direct influence on mean time before failure results in thermal cycling tests When molten Sn-Pb solder comes in contact with the copper pad surface, a layer of Cu-Sn intermetalics is formed adjacent to the solder ball and the copper land pad surface and it serves as the bonding material the solder joint. However due to its brittle nature and the thermal mismatch with the solder and the printed circuit board or the BGA substrate, excessive intermetalic compound formation at the solder-copper interface of the solder joint will potentially cause weakening in joint strength and eventually fatigue failure. This phenomenon can be controlled by reduced times above the liquidus temperature of the solder alloy.
Grain structure of the solder is controlled by the speed of cooling the solder from a liquidus state to a solid state. A tight solder grain structure from a rapidly cooled process will enhance the thermal cycling performance of the solder. The solder balls formed by this rapid cool down are also much shinier and do not exhibit a cratered appearance as is found with slower cool down rates. Coplanarity can be affected by solder spheres that have an irregular surface area as well.
With the potential for higher temperature solder sphere alloys clad upon copper cored spheres, both the intermetalic compound and the solder grain structure will play a significant role in the thermal cycling performance of BGA packages of this type. The original grain structure and intermetalics of the BGA will remain unchanged because they will not be reflowed when the parts are soldered onto the end users' printed circuit boards with lower melting (Sn 63) type solder alloys. Rework and re-soldering of this type of component allows for more flexibility to the end user, thus making this technology more industry friendly.
Conclusion
Now that we have gone through the design, thermal dynamics and function it is easy to see that a furnace can be compact with a very small footprint. The heat zone construction that is the heart of any furnace is accomplished with electrical cartridges maintaining temperature at plus\minus 2 degrees. Platens consist of cast iron and aluminum for even heat distribution. This compact design needs only 20 minutes for warm-up at shift start and consumes 11KW. Gas consumption is a nominal 350 cubic feet per hour -- Truly a Green concept.
