High mix circuits have always been a challenge and as the alloy temperatures increase the challenges increase. Trying to manage rates of thermal conduction when taking into account the mass, conductive medium of the reflow system [gas] and the conduction rates of the parts on the circuit, the problem is significant.

The simplest way around such challenges is to employ a Vapor Phase process. Its Thermal transfer rates exceed gaseous systems by a factor of 7+, the peak temperature is a physical constant and cannot be exceeded by any component or PCB feature.

Thermal ramp rates are controlled between 1 deg and 3.5 de C /Second, added many systems today also offer the secondary but significant benefit of being O2 free through the whole process, pre-heat and reflow.

Profiling a high-mix board can be challenging, but these pointers can help you through it.

It is critical to pinpoint the hottest and coolest locations on the board before any serious profile optimization should take place. Although you may intuitively have a good idea where these locations are, it is wise to verify this information by thermocoupling 3-4 presumed “cool spots” and 3-4 presumed “hot spots”. Once you zero in on the actual hottest and coolest points, the profiling optimization can begin.

The first key for profiling a high-mix board is to insure that the coolest point on the board reached the necessary peak temperature and time above liquidus in order to make a reliable solder joint. Generally, for Sn63Pb37 soldering applications, this can be as low as 203C with a time above liquidus as low as 25-30 seconds.

Once your profile is set up around this minimum requirement, you then have to see just how hot the hot spots are reaching in this process. If they are getting too hot (creating either some dewetting behavior in the solder or possibly even reaching the maximum temperature for which the component is rated), you will clearly need to tweak the oven settings somewhat in order to obtain a more robust process.

The goal at this point should really be to minimize the Delta T, which is the difference in temperature from the hottest point to the coolest point throughout the process. The Delta T will naturally be high when you are heating rapidly, either just as the board enters the over or when the board temperature is increasing rapidly as it transitions from the soak zone to the reflow zone.

The best bet to lower the Delta T is to put in a soak zone that is as long as is necessary to lower the Delta T to <5C just prior to reaching the liquidus phase of the solder. The practical way to go about his is to set the final 2-3 zones at a temperature just below the liquidus temperature of the solder, or, for example, 180C for traditional Sn63Pb37 applications.

By soaking for enough time to insure that the hot spots and cool spots converge at a common temperature, this will minimize the Delta T that will arise during the rapid heating that will take place when the board moves into the reflow stage. If you are unable to lower the Delta T sufficiently through this process, you can add an additional zone at 180C or simply slow down the conveyor speed incrementally until you have reached your goal.

Once the Delta T is minimized at ~180C, this should also help minimize any thermal differences that you may see at the peak temperature, allowing you more flexibility to narrow the time above liquidus and/or the peak temperatures across the board. This will enable you to have the optimal soldering performance across a high-mix board.

If you had been trying to employ a “tent-style” profile (no soak) for a high-mix board, you may become disappointed with the inability to minimize Delta T and really optimize the process around the pointers described above. A “ramp-soak-spike” profile is really a must if the board is truly high-mix and has high Delta T.

Optimize is a complicated word.

From a statistical viewpoint optimize could mean obtaining the highest CpK value on each step of a thermal profile (low variability and centered in the spec.), or the lowest Process Window Index (PWI).

From a production viewpoint it could mean operating at the lowest power consumption, the highest through put, or the best belt speed to match the other steps of the process.

From a quality view it is a recipe that produces the lowest failure rate. In any case optimization requires a stable oven and repeatable profiling techniques.

Stable ovens have the ability to maintain zone temperatures, belt speeds, and convection rates over varying loads and time. Sophisticated control algorisms found on modern reflow ovens ensure that set points are maintained and variability is at a minimum.

Additionally, the control range of the oven needs to be sufficient to meet the profile requirements. For example when we began processing lead free solder some people found that their reflow ovens cold not obtain the higher temperatures. In other cases they could barely get to the higher temperature but lacked the ability to control in the new range.

Many excellent papers have been written about profiling techniques but in my opinion the most important step to repeatable data is to firmly attach the TCs to the components or board. Loose TCs or TCs with varying amounts of epoxy defeat any attempt at repeatability. Companies such as KIC and ECD have developed equipment to make thermal data acquisition and analysis easy.

In some cases their software will even assist in setting the oven set points and belt speeds. The trick to obtaining meaningful profile data is to make sure the TCs are attached to the critical components on your board.

Thus optimization is both simple and complicated. The simplicity comes with stability of the oven, repeatability of techniques, and accurate measurements. The complication comes in the ability of the engineer to decide what to measure on the board and what the target of optimization is.

My name is Bjorn Dahle and I am with KIC, a company that makes profilers and thermal optimization software.

The first step in optimizing your profile is to define your process window, which lies at the intersection of the components, solder paste and substrate tolerances. Your solder paste supplier will provide you with their spec limits, and some of the modern thermal profilers already include a library of these pastes with associated tolerances.

You also need to identify the most temperature-sensitive components on your board. Examples of these include chip capacitors, oscillator crystals, some sensitive ICs and more. Your component supplier should have the temperature/time tolerances for those, or they should at least provide you with information on where to find them. Finally, the substrate tolerances must be identified. Your overall process window is then the “worst case” if you adhere to the three elements listed above.

At this point, you want to attach thermocouples (TCs) to the critical areas of the PCB, i.e. the hottest and coldest areas, as well as the most sensitive components, and then run a profile. This profile is compared to the previously established process window.

In some cases, there may be unique process windows associated with individual components. If you are lucky, all the TCs will show that you are in spec. Most likely, however, you need to change your reflow oven settings in order to optimize your profile.

To optimize the profile, you can use a manual “trial and error” method. Because a modern reflow oven has millions of alternative setups, the manual method may be very time consuming, especially for complex boards or for lead-free applications. Alternatively, you can use a profiler with prediction capabilities or even process optimization software.

Such capabilities tend to act as the “Google” of your oven recipes. They race through all alternative combinations of zone temperatures and conveyor speeds, simulates what the profile would look like at those settings and how the resulting profile fits the established process window. It then selects the single best profile based on your criteria.

You can select the profile positioned towards the center of the process window, find an in-spec profile with the fastest conveyor speed, identify oven setups that yield an in-spec profile for a variety of PCBs, or even select an oven recipe that uses the least amount of electricity while providing an in spec profile. All this is done automatically.