soldering

Process Window Index

May 11, 2009 By Bjorn Dahle Leave a Comment

Each thermal profile is ranked on how it fits in a process window (the specification or tolerance limit). Raw temperature values are normalized in terms of a percentage, relative to both the process mean and the window limits. The center of the process window is defined as zero and the extreme edges of the process window are ±99%. A PWI greater than, or equal to, 100% indicates that the profile does not process the product within specification. A PWI of 99% indicates that the profile processes the product within specification, but that it runs at the edge of the process window. For example, if the process mean is set at 200 °C, with the process window calibrated at 180 °C and 220 °C, respectively; then a measured value of 188 °C translates to a process window index of -60%.

By using PWI values, manufacturers can determine how much of the process window a particular thermal profile uses. A lower PWI value indicates a more robust profile. For maximum efficiency, separate PWI values are computed for peak, slope, reflow, and soak processes of a thermal profile.

To avoid the possibility of thermal shock affecting production, the steepest slope in the thermal profile is determined and leveled. Manufacturers use custom-built software to accurately determine and decrease the steepness of the slope. In addition, the software also automatically recalibrates the PWI values for the peak, slope, reflow, and soak processes. By setting PWI values, engineers can ensure that the reflow soldering work does not overheat or cool too quickly.

Example of a Process Window Index for peak, soak, and slope values

The Process Window Index is calculated as the worst case (i.e. highest number) in the set of thermal profile data. For example, a thermal profile with three thermocouples, with four profile statistics logged for each thermocouple, would have a set of twelve statistics for that thermal profile. In this case, the PWI would be the highest value among the twelve values, expressed as a percentage. The formula to calculate PWI is:

where:

i = 1 to N (number of thermocouples)

j = 1 to M (number of statistics per thermocouple)

measured value [i, j] = the [i, j]^th statistic’s value

average limits [i, j] = the average of the [i, j']^th high and low limits of the statistic

range [i, j] = the [i, j]^th high limit minus the low limit of the statistic

Thus, the PWI is the worst case profile statistic that is the maximum, or highest percentage of the process window used.

^{Source: Wikipedia: http://en.wikipedia.org/wiki/Process_Window_Index}

Filed Under: Process Window, Reflow Tagged With: cost-based tools, defect rates, efficiency, electronics manufacturing services, Go no go, Process Window, profiling, Reflow, reflow oven, reflow profiling, SMT and standards, soldering, surface mount technology, thermal process, thermal profile, thermal profiling

Soldering and Profiling Discussion Panel at Apex 2009

April 30, 2009 By Bjorn Dahle Leave a Comment

Panelists at APEX discuss misconceptions about the reflow process and how to Minimize Delta Ts, etc.

Mike Buetow of Circuits Assembly magazine moderates a discussion panel on soldering and thermal profiling at APEX 2009. Panelists include Keith Howell of Nihon Superior, Fred Dimock of BTU and Michael Limberg from KIC.

Much of the 30 min discussion hits upon how customers often confuse an oven’s recipe with a PCB’s profile/recipe. Factors such as density, delta Ts, belt speed, different components and extraction are used as examples as to why the oven’s set points don’t always match the temperatures on the PCB. All panelists agree that a fair amount of customers do not understand these important concepts.

Fred Dimock of BTU cites an interesting study he conducted to highlight the difference mass has on the peak temperatures a board experiences without changing the oven set points. The example he gives is a 100gram board that achieves a 231 C peak when compared with a 230gram board only reaching a 225C peak with everything else being equal. Panelists agree that customers often expect to see the same profile at a given oven set up, when obviously factors such as mass play such a critical role!

All panelists talked at length about how to minimize delta Ts as an important factor in producing quality PCBs. The PCB design and layout of components was discussed by Keith and Mike.

Fred cited a study that higher convection rates also yield a lower delta T, taking into account the need to maintain a stable environment early on in the reflow process before components have had a chance to take hold. Starting at low convection allowing the flux to become tacky (thus keeping components in position) and eventually raising convection in the peak zone can minimize large deltas.

Fred also shared a profiling trick with Ramp Soak Spike profiles he likes to use when trying to minimize the delta Ts at peak. In RSS profiles, one would run as close to the edge of the top of the spec of soak and get as high as you can in temp early before you hit the spike, but you need a quality profiler and good ThermoCouple attachment to pull this off, Fred added.

The session also covered briefly upon topics such as:

Vapor Phase profiling: Keith & Mike
Nihon’s SN100C paste: Keith
How to Profile Expensive Components: Mike
Importance of Cool Down and considerations, such as the roll of large BGAs: Fred and Keith

To watch a video of the session, click here: http://blip.tv/file/1969267/

Filed Under: Featured, Reflow, TC's Tagged With: BGA, cost-based tools, defect rates, efficiency, electronics manufacturing services, profiling, Reflow, reflow oven, reflow process, reflow profiling, SMT and standards, soldering, SPI, surface mount technology, thermal process, thermal profiling, thermocouple attachment

Is Go-No-Go profiling good enough?

April 18, 2009 By Bjorn Dahle Leave a Comment

What is go-no-go profiling? Well basically you get a pass/fail, green light/red light indication at the end of your profile letting you know whether you are in spec or not.

How often are most of us, when we get the “go” or green light at the end of the profile indicating that we finally got the profile in spec, ready to button it up and turn the thermal process over to production?

If I have and in-spec profile does this mean I’m ready for production?

Well, if you think about it, if you are just relying on a go-no-go indication for your profile results you are never sure if your process is at the very limits or somewhere deep within spec. In the case of the former, depending on the demands of your production on your thermal process you could be operating at an out of spec condition during production, even though your profile gave you a green light!

If you are like most production facilities being in spec during production not only matters but having an indication of how far within spec is your process is just as important since depth of being in spec is the only way to ensure continuous quality production. A common tool used for not only determining whether you are in or out of spec as well as how far in or how far out (depth) is using PWI (process window index).

See the blog entry PWI.

http://profilingguru.com/process-window-index-pwi/

Filed Under: Define, Reflow Tagged With: cost-based tools, defect rates, efficiency, electronics manufacturing services, Go no go, Process Window, profiling, Reflow, reflow oven, reflow process, reflow profiling, SMT and standards, soldering, surface mount technology, thermal process, thermal profile, thermal profiling

Thermocoupling BGA

March 28, 2009 By Bjorn Dahle Leave a Comment

Perhaps the most challenging components to thermocouple are BGAs, since the area of reflow is hidden underneath the component. The most accurate methodologies are destructive. The simplest way to thermocouple a BGA may be to drill a hole on the underside of the BGA and thread the TC bead into the drill-out hole that allows access to the target area without having to remove and re-attach the BGA.

Another method is to use a very thin gauge TC wire (40 AWG) and separate the two dissimilar wires, as shown below. Then attach the TC and place the BGA on top of the TCs. Again, the point of this exercise is to achieve an accurate “direct” reading.

Special thanks to Scott Nelson at Harris Corporation for providing this example.

A non-destructive method for thermocoupling BGAs is to simply mount the TC on top of the BGA and, perhaps, to the underside of the PCB directly below the BGA, and develop an offset. There is no right or wrong answer; much depends on your production tolerances and a whole host of other variables that have been discussed in this guide. The point is that only you know your process and limitations with respect to product and tools.

Note: This is an area of particular interest to me since it is a concern that just about everyone has an opinion on and no true right or wrong way has been developed. Stay tuned more to come soon.

Filed Under: Reflow, TC's Tagged With: BGA, BGA Profiling, cost-based tools, defect rates, efficiency, profiling, Reflow, reflow oven, reflow ovens, reflow process, reflow profiling, SMT and standards, soldering, surface mount technology, thermal process, thermal profile, thermal profiling, thermocouple attachment

What is a baseline profile?

March 28, 2009 By Bjorn Dahle 2 Comments

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.

Filed Under: Automation, Reflow, TC's, Types of Profiles Tagged With: BGA, cost-based tools, defect rates, efficiency, profiling, Reflow, reflow oven, reflow process, reflow profiling, SMT, SMT and standards, soldering, surface mount technology, thermal process, thermal profiling

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

March 28, 2009 By Bjorn Dahle Leave a Comment

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

Filed Under: Automation, Process Window, Reflow, Types of Profiles Tagged With: cost-based tools, defect rates, efficiency, Process monitoring, Process Window, profiling, Reflow, reflow oven, reflow profiling, SMT and standards, soldering, SPI, surface mount technology, thermal process, thermal profiling

Minimize Energy Consumption through profiling software

March 28, 2009 By Bjorn Dahle Leave a Comment

The days of cheap energy are gone and in this economic environment, we are all looking to consume less and cut costs! You can start looking for energy savings in your solder paste specification and the component layout of your PCBs. Using simpler designed products with fewer components is an excellent way to begin. This, of course, is not always an option, especially for contract manufacturers.

What if your profiling software can reduce your oven’s energy consumption, while maintaining a process that’s in spec? I can do this by defining the minimum allowable conveyor speed for the reflow oven. Many engineers will set the conveyor to be a little faster than the bottleneck on their SMT line. Also, the engineer will define how much of their process window can be used or how close to the limits of their specification are they willing to run their process.

optimizeenergy

In a few seconds, your profiling software searches billions of combinations. It will settle on a profile that uses the lowest oven set points.

Filed Under: Improve, Process Window, Reflow Tagged With: cost-based tools, defect rates, efficiency, Process Window, profiling, profiling software, Reflow, reflow oven, reflow profiling, SMT, SMT and standards, soldering, surface mount technology, thermal process, thermal profiling

Minimize Product Changeover through Profiling Software

March 28, 2009 By Bjorn Dahle Leave a Comment

Since it is not always possible to run all products under a single profile, most factories require product change over time in order to reset the oven to a new set of temperatures, per product class or board. Depending on the variations of PCBs, changeover time can add up to hours, days or weeks of lost production time.

So, what can you do to minimize this dead period? Determine, in advance, the best grouping of products that have shared profiles and then schedule product runs over available equipment that minimizes changeover. It is worth every minute of company effort and time to review daily production in order to run products that require small set point changes.

It is entirely possible, in an organized methodical way to consolidate groups of PCBs under the umbrella of one or more Thermal profile, I like to think of putting them into small, medium and large buckets. Testing can be done to prove to yourself and/or your customers that the profile is both producing a product within specification and, quantifiably, how deep within spec. Yes, it is possible to have your cake and eat it too!

Profiling software can do some of the heavy lifting for you. You can reload past profiles to see whether or not they can be produced at given oven set points. Every time you ask the profiling software to run a prediction analysis, it searches billions of possible combinations. If you want the deepest in spec process, it will pick the one closest to the center of all variable specs. If you want to maximize conveyor speed, it will search for that one profile that gives you the highest throughput without violating any of your parameters. As it is crunching through these possible combinations, there are millions of other profiles in spec that you don’t see because they are not the optimal profile for a given request. This is not to say that they are any worse. When you reload a recorded profile to determined fixed oven set points, there may very well be profiles that are still deep within spec that can be run under your new Reflow oven configuration. Perhaps, instead of changing your oven over 4-5 times a day, it now only needs to be changed 1-2 times.

Filed Under: Improve, Reflow Tagged With: cost-based tools, defect rates, efficiency, Process Window, profiling, profiling software, Reflow, reflow oven, reflow process, reflow profiling, SMT and standards, soldering, surface mount technology, thermal process, thermal profile, thermal profiling

Maximize Throughput | Profiling Software

March 28, 2009 By Bjorn Dahle Leave a Comment

Many of you will have an issue with “bottlenecks” in your process. This can happen at any point in the SMT Reflow process. Depending on the product that you are manufacturing, it is also likely that the “bottleneck” will jump from equipment set to equipment set. For our purposes, let’s look at the reflow process when it is identified as your bottleneck.

I have seen several methods that address a reflow bottleneck. The obvious solution is to increase the conveyor speed of the reflow oven. This is a task that requires a bit of skill. Your profiler becomes the single most effective tool to improve the Throughput Time (TPT) of the reflow process.

Let’s look at the fundamental changes of your profile with an increase in conveyor speed. First, the PCB will spend less time in each zone. Also, your process will move toward the shorter end of your spec as defined in seconds. For example, if soak time is defined as 30 to 90 seconds, your actual process will be perhaps in the 30-50 second range as opposed to a comfortable 50-70 second range that was established at slower conveyor speeds.

The longer the oven, the more wiggle room you have for increasing conveyor speed without having to make significant changes to your profile. Having a longer oven suggests that the PCB stays in the reflow process for a longer period of time, but keep in mind that it really comes down to how many products per minute exit the oven. Whether the oven is 10 feet or 30 feet in length, a higher conveyor speed will increase the number of products that exit the oven per minute.

Work In Process (WIP) is determined from the time the lot enters the SMT process to the time it is ready for shipment as a finished product. This duration is stated normally in hours and can add up to a few days to weeks, depending on the product. Having a longer oven does not mean increased TPT nor does it violate your WIP objectives.

It is tricky working with shorter ovens with fewer zones since they do require higher temperatures per zone to reach the desired specifications, as compared to longer ovens. I have found that using solder paste that uses a Ramp to Spike (RTS) profile works better in ovens with fewer zones. The RTS pays less attention to the soak and more to the overall length of the profile (the soak, of course, is often not listed in the solder spec for an RTS profile). Also, shorter ovens impact the slope in the RTS profile. For example, if you have a 5 zone oven, the first zones will need to be set at a fairly high set point in order to process the rest of the profile. At this point, the slope becomes very steep as the PCB moves from ambient to 160°C. 160°C could be the set point of the first zone! Also, in a distance of just a few feet, the product will need to rise in temperature to greater than 217°C in PB Free and above 183°C in eutectic solder. Many specifications will call for a peak of 200 to 240°C, which puts further demands on your shorter oven. In this instance, it is desirable to calculate the slope over the entire profile, setting it from 130°C to 180°C over a shorter period of time. Again, you need to look at how long (in seconds) the PCB stays in each area of the profile when designing your spec.

Now that we have looked at oven length when considering an increased throughput, how do we develop a profile that will remain within specification at higher conveyor speeds? Some profiling software will allow you to make a desired change to conveyor speed and then return a predicted profile. This typically can be completed in seconds, before even running your first profile.

A clever feature of some profiling software is that you can set a range of allowable conveyor speeds while maintaining acceptable limits to your process window. For example, when factoring the variability (drift) in your oven, you can comfortably run your process using only up to 70% of your available process window. In practice, anything over 70% is risky due to drift that can push your process out of spec. To eliminate this concern, run a “what if” scenario, where you define your minimum and maximum range for conveyor speed and maximum allowable PWI (in this case, set to 70%).

optimizeconveyor

The profiling software will literally search billions of possible combinations, giving you the maximum possible conveyor speed without violating your 70% PWI threshold. Of course, you may find the conveyor speed to be too slow still. What can you do? Bump up your allowable process window or buy a new oven. In either case, you are in control of your predictive modeling (this sure beats hundreds of hours of trial and error and possible board destruction). This process is similar to the prior section on Getting your Product Deeper in Spec and it will take as long as it takes to heat up or cool down your oven and re-profile if verification of your new predicted profile is required. In practice, I find that it takes less than one hour.

Filed Under: Define, Improve, Reflow Tagged With: cost-based tools, defect rates, efficiency, maximize throughput, Process Window, profiling, profiling software, Reflow, reflow oven, reflow process, reflow profiling, SMT, SMT and standards, SMT Reflow, soldering, SPI, surface mount technology, thermal process, thermal profiling

Getting your Product Deeper in Spec | Profiling Software

March 28, 2009 By Bjorn Dahle Leave a Comment

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.

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.

Filed Under: Define, Improve, Reflow, Types of Profiles Tagged With: BGA, cost-based tools, defect rates, efficiency, Process Window, profiling, profiling software, Reflow, reflow oven, reflow ovens, reflow process, reflow profiling, SMT and standards, soldering, surface mount technology, thermal process, thermal profile, thermal profiling

Panelists at APEX discuss misconceptions about the reflow process and how to Minimize Delta Ts, etc.

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