Reducing Reflow Product Changeover Time

2009 Presentation at SMT Long Island on how to reduce the changeover time from one reflow profile recipe to another.  If you ever opened up your reflow oven to dump all its heat to lessen downtime, this 4 min video is for you!!!

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Getting your Profiler Deeper within Specification

2009 Presentation at SMTAI San Diego on how and why you need to drive your reflow profile deep within specification.  After you watch this 8 min video you will never take profiling for granted again!

To view the complete video series (click here).

To subscribe to my Podcast for iTunes (click here).

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What to do with Zig-Zagging TC Readings?

What’s wrong with this picture?

Profile Lose TC

Well if you have ever used Kapton tape to attach a thermocouple, you have certainly seen your share of profiles like this!

So what, it is a perfectly good profile, right?  Yes, but no.  I had a customer who was using KIC’s Navigation (auto prediction) to help create a better “deep in-spec” profile.  The only problem, they were trying to optimize on a TC reading that was bouncing literally all over their PCB.   Navigator is an awesome tool, but it can only work with what you feed it.  If you feed it garbage, it will give you garbage.  In their case, it was trying to find them a new solution where literally every time the board was run the bouncing TC that was attached (or I should say was not very well attached) with Kapton was giving false readings.    Navigator would give a different solution based on what the TC was reading at that given time.   It is like try to put post-it notes on the ocean.

Solution is very simple, eliminate the TC reading from your graph.  You can easily do this with the profile you just ran.  Look what happens, you go from a far out-of-spec of 126% PWI to a far in-spec of 48% PWI.

Profile Lose TC2

So you saved your hard work this time, but you are after all one thermocouple reading short.  You added that TC to your profile for a reason.  Next go around, do yourself a favor and use a better material for attachment, such a conductive double side aluminum tape, which by the way, a recent study from RIT proves it a superior attachment method aside from sticking to your PCB much better.

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Running lead free and eutectic PCBs simultaneously on the same reflow oven

Surface Mount Technology ran a piece titled Parallel Processes: Simultaneous Lead and Lead-free Soldering with a Single Reflow System written by Hans Bell of Rehm Thermal Systems GmbH.  Hans details a study where by controlling conveyor speed of each lane of a dual-lane system, it is possible to run both a lead and lead free product simultaneously.

The devil of course is always in the details:

Definition of the process window must always be based on the “weakest link,” namely the component with least amount of thermal stability during the soldering process. If two different processes are to be set up next to each other in the same reflow system, and if thermally sensitive components are included on the PCB, great flexibility is required for parameters configuration.

The ability to develop process windows for each product leaving enough room for each to call upon the same oven zone set points is key and of course taking into account special temperature tolerant components on each board.  Hans’ idea is intriguing.  Based on my experience in a world were many PCBs manufacturers struggle to profile or perhaps do not profile at all,  this is certainly a tall order.  Nevertheless his idea is do’able for perhaps many processes, since changing just the conveyor speed to reduce product changeover on a single lane oven is being done today (click here for an excellent application note using KIC product’s to achieve this end).  Why this couldn’t be adopted to a dual lane system running both lead and lead free simultaneously has its merits.

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Plugging the Hole in the SMT Reflow Inspection Process

MB (Marybeth) Allen, General Manager of KIC Europe in an interview with Globalsmt.net makes a terrific case for RPI (Reflow Process Inspection). MB_Allen

Here are some excerpts:

Q. 2009 saw the introduction of your RPI In-Line Process Inspection System for SMT reflow ovens.  For manufacturers currently relying on AOI and X-Ray systems to carry out inspection functions, can you explain how this system works and why RPI should be the choice for this process?

Automated inspection systems have become critical in controlling quality throughout the manufacturing process.  SPI (solder paste inspection) and AOI (automated optical inspection) are excellent defect detection tools, within the limitations of their design.  The RPI (reflow process inspection) inspects the reflow process for each and every manufactured PCB.

The quality of a solder joint is not only a function of whether there was adequate solder, accuracy of placement, missing components etc., but that the solder was processed correctly.  For example, the peak temperature needs to be high enough, but not too high to damage the component; the time above liquidous must be within the required range etc.  The AOI machine is not designed to check for these critical events.

KIC’s RPI verifies that the PCBs have been manufactured within the required thermal process window.  Perhaps the best example of where RPI complements AOI is in the soldering of BGAs and other Area Array Packages, where the AOI machine cannot see the solder joints as they are hidden from view by the component body.  RPI even complements X-Ray machines as these inspection systems cannot tell whether the solder joints were processed in accordance with the required profile specs.

Q. So KIC RPI offers both oven and product data in one solution, this obviously enables the operator to harness this key data and use the yield charts to refine the process. What type of data do they receive and how easy is this to understand?

RPI automatically generates both Yield and DPMO (Defects Per Million Opportunities) production charts.  There’s really nothing for the customer to do as the information on all boards produced is captured automatically.  You’ve seen these charts in many factories showing product data for many steps in the manufacturing process.  However, previously data from the reflow process was missing.  Only reflow oven machine data was available.  KIC’s RPI now provides this missing key product process data, providing another key link to product quality.

Q. This product offers a timely solution for manufacturers in this tough climate and I understand it has already received awards for its innovation. What has been your feedback so far?

Yes, RPI has already received several awards around the world.    People are looking for a solution to save money and ensure continued quality control.  When I visit customers and prospective customers their initial questions or requests can be taken care of by using RPI.  It’s wonderful to be able to say “Yes, RPI can help you with that” to most of their requests.  We have plugged the hole in the inspection process.

For the full interview go to: http://www.globalsmt.net/content/view/7583/70/

Awards:

2009 EMAsia Innovation Award in the category of Process Control Software for its RPI in-line inspection system.

2009 NPI Award in the category of Process Control Tools for its RPI in-line inspection system.

Innovative Technology Center Award at Apex 2009

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Process Window Index

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:

pwiformula

where:

i = 1 to N (number of thermocouples)

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

measured value [ij] = the [ij]th statistic’s value

average limits [ij] = the average of the [ij']th high and low limits of the statistic

range [ij] = the [ij]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

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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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Minimize Energy Consumption through profiling software

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.

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Process Window Index – by Vitor Barros

The term process window is used to define your reflow spec in relation to your process.  Vitor Barros of KIC Europe has used the following analogy to describe this relationship.

The following example describes when you have a reflow process that does not fit your product’s specification.  You cannot change your process but you change change your spec (get a bigger picture frame).

pwi1The other extreme is where your spec is fixed and you need to modify your process (the picture canvass).

pwi2

When both fit, you have what would be expected, a picture that fits perfectly to its frame.

pwi

I discuss with Vitor Barros of KIC Europe the Process Window.  vitorsim

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