What is a baseline profile?

Baseline profiles are used purely for set-up purposes. They are manually-fed into the reflow oven and are run in tandem with a monitoring system in order to create what is called a “virtual profile.”   Baseline profiles, thus, become the standard by which all automated profiles are compared.  If there are any deviations between the baseline profile and the automated profile, this information is recorded and can trigger alarms and stop production temporarily. Baseline profiles also have the added advantage of becoming a true “gold standard” for all of your profiles, since the same profiling board is not being used repeatedly or subject to degradation of the PCB, thermocouples or operator error.

The set-up of a baseline profile is performed by running a normal profile, per recipe.  Meanwhile, 1-2 fixed mounted probes (typical an array of 15-30 thermocouples) collect their own set of data as the PCB runs the length of the Reflow oven with a profiler. This information is then compared and processed by the monitoring system.  This array of internally-mounted thermocouples is able to reconstruct a virtual model of a profile. When a production board enters, without the profiler, the same TC readings that were present when your oven was profiled are now re-mapped onto each and every PCB.

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Oven vs. Process Monitoring, what’s the Difference?

Thermal monitoring systems can often help you troubleshoot your oven by getting to the root causes of many process-related problems. Information on oven and process changes can be investigated. There is a very important distinction to be made between oven monitoring and process monitoring.  Changes or no noticeable changes to your “oven” may or may not impact your process.  At best, you can only infer if changes to the “oven” are impacting your process.  Changes that occur to your “process” can be tied directly to the inputs that are causing the change.

troubleshooting

We see changes to the oven, which is only half of  the picture. A sudden spike in oven temperatures strongly suggests problems, but what happens if it is only momentary? Can you say then that your product is out of spec?  If so, for how long, how many PCBs do you need to chase down the line for rework?

This is where Process Monitoring comes into play. If I look at each and every PCB profile, then I know whether or not a given product is within spec.  Typically, engineers will run their process from SPC Charts tied to Cpk.  Only when the system alarms on Cpk or there is an out-of-spec profile will they look at the data presented above to try to troubleshoot the root cause of the problem.

cpk

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Getting your Product Deeper in Spec | Profiling Software

Q: What happens when defects occur when the thermal load on the oven increases?  Do you slow down production?  Do you change the oven set points by cranking up the heat to compensate for the increased load?

A: The answer is to establish a NEW profile that is deeper in spec., a profile that is able to better stand up to the daily variations of a reflow process.

Today, profiling software allows you to establish these new deep-in-spec profiles with relative ease.  You can precisely define your specifications and run various predictive scenarios.  For example, you know that you can’t slow down your conveyor speed, but you can change your oven set points.  The profiling software can give you a predictive result that puts you as deep in spec as possible before ever having to run a profile.

optimizepwi

In practice, how much work needs to be done to take this out-of-spec process and bring it within spec depends on a lot of factors. How far out of spec are you to start with? What inputs can be changed? How tight are your specs?

If your process is already taking up most of your process window or not far out of spec, then only minor changes will most likely be needed to bring your profile much deeper into spec.  In this case, only one additional profile is likely required to bring your profile very deep within spec.  In my experience, this profiling process takes about 30 minutes, most of which is waiting for the oven to cool. If your profile is far out-of-spec., you may need up to an hour to bring it within spec.

Each time you re-profile, it is an opportunity to further improve your profile, bringing it further into spec with each effort.   Profiling software will tell you a possible scenario for improvement each time, which takes your excellent deep-in-spec profile still deeper within spec.  Each one of these changes, on average, takes about 30 minutes.

A word of caution: having a profile in the center of the spec or at 0% PWI, is not always the optimal improvement. While “0%” PWI is statistically desirable, there are other factors to consider. For example, though 30% PWI indicates that you are only utilizing 30% of the allowable process window of your solder paste, in practice, when you find that a PWI of 65% produces a physically better connection, which is better?  Specs are just that: specs. They have a range for a reason. In this case, at the upper end of the spec (opposed to the center of the range), a joint may solder better. Advantageous about most profiling software is that you can go back and re-define your specs to see what your new profile will look like without having to rerun the profile. The allowed range can be further narrowed to a particular spec, which results in a better joint.  In essence, you are now re-defining your spec, and all future profiles will only consider this new range.

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How to Establish the “First” Profile for a New Product

autofocus

Clever profiling software will calculate a profile based on the size of your PCB or from dummy profiles already loaded on the Reflow oven. It is part of the prediction utility of your profiling software or pre-installed on your reflow oven. This software mines prior work, including your own profiles developed from other products or, in the case of profiles that come pre-installed on the oven, known profiling outcomes based on the oven’s characteristics.  It looks at known sizes of PCBs and their respected weights (i.e. mass), and how they interact with a known process environment (i.e. your specific reflow oven).  What is exciting about this software is that it can give you a pretty good starting point when you are profiling a board for the first time. It is not foolproof, but it is quite common to get a process, in spec, after a few profiling runs while keeping your profiled PCB intact and sellable!  The alternative is to spend hundreds of hours profiling new boards, with starting points based on conjecture. In this case, many boards would be sacrificed in the process.  With products like KIC Auto-FocusTM and AUTOsetTM, your time and your PCBs are saved, saving you money!

autoset

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Configuration of the Graph | Reflow Process

profilegraph

The profile graph will tell you whether or not your process is within spec and that your reflow oven is creating the correct profile, nothing more. Double check your set-up again before diving into your profile. The information from the graph must be “true.”

There are dozens of software packages that mostly render the same information, with temperature on the Y axis and time across oven zones on the X axis. These software packages vary great with respect to how they manipulate this data or allow you to play around with the numbers.

Not all of your oven set points are going to match those of your profiling software. Profiling software can do this automatically but not all oven brands communicate with all profiling software brands. In the end, you might be left with a graph that appears to be in spec but, in reality, is meaningless due to differences in the set points in the oven and in the profiling software.

To be considered relevant, ALL profiles must follow these guidelines:

  • Profiles are generated by a specification from the solder paste manufacturer, component suppliers and substrate tolerances.
  • The set points of the oven heating zones are set.
  • The oven conveyor speed is set.
  • The profiler set points are the same as the oven heating zone set points.
  • The profiler oven conveyor speed is the same as the oven.
  • The reported number of heating zones are the same as the number of oven heating zones.
  • Any change in the set points of the oven MUST be changed to the profiler software.

Any violation of these guidelines will render your profile invalid and meaningless!

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Low-Long –Spike

lsp-profile

Not a common profile, the LSP is intended to eliminate or reduce the occurrences of voiding. The slope is similar to the RSS profile but with the soak segment at a lower temperature and for a longer duration. It may be successfully used in an R&D environment.

I have used this profile when there is limited data on a product or on a first pass of a new product for which the process is not well understood. It is a good profile to use when characterizing a new process. The LSP profile will provide an indication of interaction amongst oven, PCB and components with the solder paste.

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Ramp to Spike

rts-profile

RAMP

The Ramp to Spike Profile is, more or less, a linear graph, starting at the entrance of the process and ending at the peak segment, with a greater Δ t in the cooling segment. In general terms, the RTS profile is a long, slow ramp to the peak temperature, with a rapid cool down in comparison to the initial ramp.  Setting your oven’s set points and conveyor speed can be much easier when the characteristics of the oven and profile are well understood.

Similar to the RSS profile, the beginning of the profile is the slope of the ramp from ambient temperature through the first heating zone.  The slope is not as steep as for the RSS profile. While the RSS allows for ~4°C/sec, the requirements of the RTS is ~1-2°C/sec. Again, this is dependent on the solder paste specification and you need to double check your component’s spec.

The RTS soak period is part of the ramp and is not as easily distinguishable as in the case of the RSS. The soak is controlled primarily by the conveyor speed settings.

PEAK

The peak of the RTS profile is the endpoint of the linear ramp to the peak segment of the profile. The same considerations that lead to defects in an RSS profile apply to an RTS profile.

COOLING

When the PCB enters the cooling segment, the negative slope is generally steeper than the rising slope.

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Ramp Soak Spike

rss-profile

RAMP

Ramp is defined as a change in temperature over time, expressed in degrees per second.  A common value is 4°C/sec, though many component and solder paste manufacturers insist on 2°C/sec.  Who ever said reflow was easy?

Change in temperature (Δ t) with Respect to your Components

Many components have a specification where the rise in temperature should not exceed a specified temperature per second, such as in the 2°C per second example already given. The solder paste manufacturer will require comparable upper limits.  Tinned components (containing solder) and solder, in this case, behave in a similar ways.  Rapid evaporation of the flux contained in the solder paste can lead to defects, such as, but not limited too, lead lift, tombstoning and solder balls.

Possible Moisture in your Components

Rapid heat introduction to a component can lead to steam generation within the component if the moisture content is too high.  Microcracks will likely appear under these conditions.

SOAK

In the soak segment of the profile, the solder paste will approach a phase change.  The amount of energy introduced to both the component and the PCB will approach equilibrium.  In this stage, most of the flux should be evaporated out of your solder paste, but this can also depend on the type of paste.

The duration of the soak varies by solder paste manufacturer’s specs.  The mass of your PCB also factors into the required duration of the soak. Again, a balance of heat transfer and flux evaporation must be achieved. Too rapid of heat transfer can cause solder splattering and the production of solder balls, bridging and other defects. If the heat transfer is too slow, the flux concentration will remain too high and adversely impact the wetability of the bond pads, resulting in cold solder joints, voids and incomplete reflow.

PEAK

After the soak segment, the profile enters the ramp to peak segment of the profile, which is a given temperature range and duration of time that exceeds the melting temperature of the alloy, as determined by the solder paste spec.  A successful profile will typically range in temperature up to 30°C higher than liquidus, which is ~183°C for eutectic and ~ 217°C for lead-free. It is VERY important throughout the peak segment to consider the temperature tolerance of your components.

COOLING

The final area of the RSS Profile is the cooling section. A typical specification for the cool down is not to exceed -6°C/sec (falling slope).  Check the spec of your paste, but don’t forget to also check the specs of your components.  Certain packages may be sensitive to rapid cooling.

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