REFLOW SOLDER PROFILE DEVELOPMENT

by Ray Prasad

The solder profile, also known as thermal profile is one of the key variables in the manufacturing process that significantly impacts product yield. In this column we discuss the details of developing solder profiles for IR and vapor phase soldering (VPS). The term IR profile refers to the profiles of all three types of IR systems - radiant dominant systems, combination of convection and IR dominant systems, and convection dominant systems, although the convection dominant systems are the most common today.

Many people tend to think that convection ovens are not IR ovens. In reality, there is no such thing as perfect IR or perfect convection. To clear up the confusion, SMEMA (Surface Mount Equipment Manufacturers Association) established three classes of IR systems. They are: Radiant dominant systems (Class I IR), combination of convection and IR dominant systems (Class II IR), and convection dominant systems (Class III IR).

No matter which system is used, if one does not take the time to develop the appropriate solder profile, changing to a newer machine is not going to solve the yield problem. Also, even after one has developed the appropriate solder profile, if the source of the problem is poor design, poor solder paste, misregistration or misplacement of components, to name just a few, a convection oven or any oven or profile for that matter, is not going to eliminate the problems caused by the other variables.

Conveyor speed and panel temperatures are two variables in solder profile development. The solder profile is not only product specific, it is also flux dependent. Different pastes require different profiles for optimum performance. So it is important to consult the paste supplier before developing the solder profile. We should also note that panel temperatures for any setting will be very different from the temperature seen on the board and the component leads.

For developing the profile, we need the loaded board for which the profile is being developed. We can start with a given belt speed and monitor the top-side board temperature using thermocouples. Most new reflow ovens have built-in thermocouples and software packages to record the thermal profile. If not, commercial hardware and software packages, such as MOLE by ECD. in Portland, Oregon and KIC by EDI, Inc. in San Diego, California. are available that make thermal profile development an easy task.

Here are some guidelines for achieving the desired shape in each of the four zones of a solder profile.

Preheat Zone

The heating rate in the preheat zone should be 2C to 3C/second and temperature should be 100-125C. During preheat if the temperature ramp is too fast, the solder paste may explode and cause solder balls. Also, to avoid thermal shock to sensitive components such as ceramic chip resistors, the maximum heating rate should be controlled.

Soak Zone

The soak zone is intended to bring the temperature of the entire board up to a uniform temperature. The ramp rate in this zone is very low, almost flat. The temperature is raised to almost the melting point of solder (183C). The consequences of being too high in the soak zone are solder balls and solder splatter due to excessive oxidation of paste. The soak zone also acts as the flux activation zone for solder paste.

In vapor phase there is no soak zone. The preheat and soak zones are part of the in-line IR oven used just before the VPS machine. If an oven has many heating zones, a significant number of zones should be dedicated to soak zone.

Reflow Zone

After the preheat and soak zones, the board enters the reflow zone. In this zone if the temperature is too high, boards may char or burn. If the temperature is too low, cold and grainy solder joints will result. The peak temperature in this zone should be high enough for adequate flux action and to obtain good wetting. However, it should not be so high as to cause component or board damage or discoloration, or in worst case, charring of the board.

In VPS, the peak reflow temperature of the solder joint is determined by the boiling temperature of the primary fluid and cannot be controlled by the operator. The only controllable variable in VPS is the dwell time, which is controlled by the belt speed (in-line machines) or elevator speed (batch machines). The standard primary fluid used for surface mount assemblies has a boiling point of 215C at sea level.

Extended duration above the solder melting point will damage temperature sensitive components. It also results in excessive inter metallic growth which makes the solder joint brittle and reduces solder joint fatigue resistance.

Cooling Zone

The cooling rate of the solder joint after reflow is also important. The faster the cooling rate, the smaller the grain size of the solder, and hence the higher the fatigue resistance of the solder joint. So the cooling rate should be as fast as possible. However, from a practical standpoint, there is no control on the cooling rate other than making sure that the cooling fans at the end of the oven are operational. If the fans are non-operational, the cooling rate will be slow. This will increase grain size causing relatively weaker solder joints.

Bandwidth and Shape of Profile

In addition to meeting the shape of the profile, the oven settings must also show a narrow band width (5C to 10C). The band width is defined as the total temperature difference across the board in any given zone. The tighter the bandwidth, the more consistent is the yield. If the shape of the profile does not meet the desired shape or if the bandwidth is too wide (more than 10C), adjust the panel settings.

Generally two to four trial settings are required to meet the target shape and bandwidth. This could take 1-2 hours to complete. This is a significant improvement from the early days of only IR dominant systems with no built-in thermocouples (mid 1980-92s). Also, in those days, there were no commercially available profile development hardware and software packages with prediction features (after running the board at least one time through the oven) that are common today. Developing profiles with data loggers and plotting the data used to be not only a cumbersome task but also time consuming (usually a minimum of four hours for each board).

It should also be kept in mind that IR dominant systems require an added investment in engineering time to develop custom profiles because of potential problems with shadow effect and color sensitivity of the IR portion of the heating source. Because there is more uniformity of heating in a convection system, one can expect somewhat less time required to develop a solder profile. However, we should note that not all convection systems are created equal. Some ovens can achieve narrow band width for most products much easier than the others.

There is some misconception, mostly promoted by some suppliers, that if you buy their convection oven, there will be no need for developing a unique profile for each product. This is simply not true because each board has a different thermal mass and one may have different loading patterns (distance between boards as they are loaded in the oven). Even the same double sided board, depending upon component placement and distribution of copper planes on each side, may require different profiles for each side. And there is also a misconception that if you do need to change a profile, simply change the belt speed. Having to change only the belt speed is certainly easy, but it may not be the right approach. Changing the belt speed changes the temperature of the board in every IR zone.

Development of a soldering profile for VPS calls for the establishment of preheat temperature, elevator or conveyor speed, and dwell time in the primary zone. Like the IR systems, the vapor phase process also requires a custom solder profile for each product, but VPS profile development is much more straightforward because the fluid temperature is fixed. The only variable that requires adjustment in vapor phase is the conveyor speed. However, since IR is commonly used to preheat the board before it enters the VPS in the reflow zone, the requirements for preheat and soak zones for IR as discussed earlier are equally applicable.

In developing solder profile, it is important to rule out equipment dependent variables. Once the desired profile is achieved, run an actual production board with solder paste and components for reflow. After reflow, inspect the quality of the solder joints. A random problem only in a certain section of the board may be related to solderability but a consistent problem in a given section may be related to the solder profile due to non uniform heating (wide band width). Note too that consistent problems may also be related to paste quality and land pattern design.

Once the profile is found to give the desired results (assuming design and other material variables have been optimized), document the profile. After this point, no changes should be allowed in the profile.

Contact the Author:

Ray Prasad
Tel:503-297-5898
FAX 503-297-0330
Email:SMTSOLVER@aol.com