Tips on Cooking Both a Perfect Thanksgiving Turkey and a PCB

Why does it take half a day to cook a Thanksgiving turkey?  The answer is simple ― you have 20 lb of bird that simply cannot just be nuked in a microwave like last night’s dinner.  If not properly thawed, prepared and monitored, you either have an overcooked, dried-out bird or worse: Salmonella. Strangely enough, as you will see in a moment, PCBs are not that much different.

Let’s say you skip the thawing process and in your haste stick a frozen bird in the oven.  What happens?  The bird may look properly cooked on the outside, but as soon as you try your skill with the carving knife, you either hit bedrock or the inside is completely raw. OK, I will admit I speak from personal experience on this one (please do not bring this up with my wife).  Are PCBs any different?  Well, your reflow profile has a preheat phase, with the purpose of bringing your PCB to temperature. In other words, the entire mass of the board with all its components is gradually brought to equilibrium. If you do not do this, you run the risk of thermally shocking your components when they hit reflow and peak.  Thawing your bird and preheating your PCB ― you have the same objective in mind.

So, for the vast majority of us, we really have no idea when the turkey is fully cooked until getting an internal reading. A PCB is no different. On the surface, both might look great, but upon closer inspection, you discover some components have defects due to improper reflow or, for that matter, when you cut into a turkey that is still pink it really hits home that you aren’t cooking a TV dinner.

turkey-in-Spec_SM01

Because of this, as we all know, a 20 lb turkey requires a thermometer. I will concede that some of you use the old “poke the bird and check for pink until done” trick. Let’s assume you are not as skilled, like me, for example. Would you seriously cook a turkey by relying solely on the oven’s temperature reading on your stovetop?  Of course not, but why do some of you profile your PCB by relying on your reflow oven’s reported readings? Are either situation that much different?  Actually, yes. Your nice self-contained turkey cooking oven is more of a steady state, but there remains a large difference between what is reported by the oven and the internal temperature of your turkey. In contrast, your PCB is exposed to anything but a steady state environment because it rides on a conveyor through different heated zones with blowers, extraction systems and both ends of the oven even open to the elements!  For this reason, any oven manufacturer will adamantly tell you to profile and with regularity. Alright, you may have learned how to cook a turkey in your Mama’s kitchen and, in fact, be skilled at not using a thermometer; however, I doubt any serious SMT manufacturer would take a similar approach, checking your PCBs regularly for “doneness” in your reflow process.

What about placing the fate of your Thanksgiving feast on the cheap-o plastic pop-up indicator that likely came with the turkey? Do not laugh. How many of us use the trailing wires that came with the reflow oven?  Now to be fair, both work in principal; otherwise, you would have the likes of Purdue Farms with food poisoning lawsuits on their hands, but they only give you ballpark readings in many cases. By design, the turkey is going to be a little overdone and dried out.  Your PCB, on the other hand, cannot afford to be a little overdone or it is simply OUT of spec.  You can get by with eating the overcooked turkey … the gravy and mashed potatoes are there to make up for less than a perfectly cooked bird. But your PCB will not be as forgiving.  Trailing wires, never mind being cumbersome to use, have a tendency to kink and stretch, which compromise their readings.  They also are susceptible to 50 or 60 cycle noise from some reflow oven environments, further questioning their accuracy in some cases.

So you want to cook the perfect bird. Who doesn’t? So you pony up for a stainless steel large-dial meat thermometer to accurately read the internal temperature of your 20 lb bird. You also pony up for a KIC Explorer with Navigator because you want to create the perfect deep-in spec reflow profile. It will not only tell you the specific temperature of the joints of your $500 BGAs, but it also will find a balance that does not overcook them or any of your other temperature-sensitive components on the PCB.  No pop-up indicator profiler needs to apply since the KIC Explorer with Navigator will go the extra mile and tell you not only if you are in-spec but how DEEP in-spec your profile is, along with what can you do to improve the profile in minutes, if not seconds.  Now do you know of any turkey thermometers that can do that?

So when you prepare your Thanksgiving turkey, and as you pause to give thanks, consider applying the same care and consideration that you have given to your family’s feast as you do to your PCBs.

Happy Thanksgiving – Profilingguru

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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!!!

To view the complete video series (click here).

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

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SMT related Links to know

RSS feeds, Tweets, blogs and newsletters, how do you keep up?   Well here is the latest on what’s available in the SMT industry.   I subscribe to all of these newsletters and regularly pick out areas of interest related to profiling for you.   I also comb the blogs though I only know of two, not including profilingguru, which is quite remarkable considering other industries have hundreds if not thousands.   The SMTA group forum on LinkedIn yields on occasion a nugget, but you need to build a profile to join.  SMTnet has always been a jewel.  Lastly, Twitter is a new phenomenon for many of us.   I am still trying to get the knack of it myself but it does have some value no doubt and will continue to grow.

On-line Newsletters:

Circuitnet

Electronics Production World

EMS Now

GlobalSMT

PCB Update

SMT Week

Blogs:

Circuits Assembly

Forums:

SMTA on LinkedIn

SMTnet

Twitter:

Circuit Assembly

Global SMT

SMT Magazine

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Four Ways to Reduce your Reflow Oven’s Power Consumption

What are you paying annually in electricity to run your reflow oven?  Not taking into account indirect costs, surcharges, taxes and added wear and tear of running your oven hotter and harder, you might be paying anywhere from $6-8K per line.   This number is based off a study conducted at Flextronics Poland, where they pay close to the US national average of $.072 kWh.

Pop Quiz: Can you rank the following in order of impact on lowering your utility bill for your reflow oven?

  • Taking Oven Control Measures
  • Peak-time Power Up Minimization
  • Off-Peak Savings
  • Profiling for Energy Savings

Well if you are savvy with your utility bill, you probably identified Peak-time Power up Surcharges as the biggest money drain.  You probably did not guess Profiling for Energy Savings as the #2 energy savings technique.

Before I take you through all four techniques, keep in mind there are dozens of variables that come into play.  The numbers I use for one municipality and/or manufacturer may be vary by location, but the point should not be lost that you can save money and not sacrifice quality production in the process.  As an added bonus many of these techniques may also prolong the life of your oven and have other hidden benefits that may impact your operation.

#1:  Peak time Power Up Minimization

The following represents a fairly typical energy ramp up of a reflow oven from a dead cold state.   Many manufacturers will use the default start up to quickly get your reflow oven up to temperature and stabilized for production.  Thanks to BTU for providing the following data.

Peak Power Up 1

Now compare this to an energy savings ramp up mode for the same oven.

Peak Power Up 2

By extending your oven warm up time by only ~15 mins, there is a 15 KW difference in the peak energy output.   Many municipalities will charge a monthly surcharge based off of whatever happen to be your peak electricity use over typically a 5-15 min period.   So if you happen to turn on all your reflow ovens at the same time, AC, coffee machine, PCs, etc., you are in for a big added surcharge on your utility bill that month.

Potential Savings:

Let’s say you are in South Carolina, Duke Energy charges $13.16  KW as a peak surcharge.  Your monthly savings would be  $198 per month.  Of course if you have more than one oven this savings will be even more significant.

Bonus:

Many smaller manufacturers that perhaps have a single reflow oven, may be close to maxing out on their service.  I’ve seen more than one case of a 100 amp facility paying anywhere from $15K – 25K to upgrade to 200 amps.  As an example, a 9 zone Heller oven will run at 100 amps at full throttle when heating up, but you can set the oven to heat up in an energy savings mode, knocking your power down to about 63 amps.  Suddenly you don’t have to go out and install more service by just making a software change.  I know that all the major oven manufacturers that sell to about 80% of the US market (BTU, Heller, Speedline, Vitronics Soltec) have this feature, so check it out.

#2:  Profiling for Energy Savings

After 5 years,  evidence is pretty conclusive that smart profiling optimization tools can reduce reflow oven energy consumption by as much as 15%.  The following three studies demonstrate where power meters were used to measure  a “before” profile to an optimized “after” profile, using KIC Navigator-Power or KIC Auto-Focus Power.

There are basically three steps that should not take more than 15 mins to complete:

Step 1: Audit your SMT line speed.  You want to determine where is your bottleneck.  It is not uncommon to find the reflow oven running faster by 20% or more to the slowest system on your line such as the pick and place or screen printer.

Audit

John VanMeter of DG Marketing timing the line

Step 2: Run a profile

Profile

KIC Explorer 7 CH

Step 3: Run KIC’s power optimization feature in KIC Navigator.   As an process engineer I would set up your minimum allowable conveyor speed in the software above your bottleneck speed.   For example, if your current line speed is 30 in/min and an audit reveals your screen printer is running at 20 in/min, set your tolerance in the software to 23 inches.  You don’t need to make your reflow oven a possible bottleneck!   Lastly, you have the freedom to set the maximum allowable process window index (PWI).  In other words, if you know your oven can handle using up to 70% of your available spec, without any drift/variability causing you to go at times out of spec, you know your limit.   It really depends on the personality of your reflow oven.

Optimization

Potential Savings:

Based off the Flextronics Poland study cited above which was conducted on a Heller 1912 EXL  manufactured in 2005 and using a kWh rate of $.076 which is practically dead on to the US national average, results in $1062 in annual savings.  Which depending on the state of manufacturing can be as high as $2472 annually per oven.   15% savings which was the case at Flex Poland, is not unusual as you will see similar results in the Delta study in Arkansas to be released in October’s issue of  Global SMT.

Bonus:

Added features to having KIC’s optimization software Navigator-Power or Auto-Focus-Power are the additional tools you now have for decreasing defects.   It is hard for me to know what it costs you each time you send a PCB to rework, the cost of time spent profiling when you should be making on-time deliveries and the stress and aggravation of trying to produce a run of a 100 boards when your customer wants all 100 back!  Auto-Focus power allows you to make a very good first guess profile of new board before you even profile!  You can find discussions on these tools throughout this blog.

#3:  Off-Peak

Off-peak hours vary widely per locale.   Also depending on the time of year it can vary.   Nevertheless, if it is possible to run even a portion of reflow production in off-peak hours your costs kWh can sometimes be half of on-peak prices.   I like to use the same rate chart example give above for S. Carolina where Duke Energy charges between 2pm – 6 am, $.0297 kWh vs. $.0563 kWh.  Many of us logistically may not have in place a night shift, but most of us can certaintly take advantage of production after 2pm.   This is more an issue of smart planning, an exercise in management.

Potential Savings:

If you can schedule a quarter of your production off-peak, and by doing so are able to reduce your rate per kWh by half  which is possible in some municipalities your savings could be on the order of $62-74 per month per reflow oven.  I came up with this number by again using the Flextronics study as a guide, where they are paying a kWh rate similar to the US national average and shelling out between $5.8K – 7K per year per reflow oven.

#4:  Oven Control Measures

By buddy Bob Powledge of DG Marketing out of San Antonio, Texas likes to say, “sure the heck cheaper to blow air than to heat it up!”  I agree and there are studies to prove it.   Basic physics comes into play.  It you can move more heated air over a surface, it will heat up more efficiently and faster.   This is why squirrel cages have by and large gotten bigger over the years and other technologies such as static pressure have come about.   In one study conducted by BTU who plays around with the idea of static pressure another approach at improving heat transfer rates, the same set-points could increase temperatures by as much as 5C  by only changing static pressure.   Take this to the next step in our discussion, you can thus REDUCE your oven set-points by that same amount thus reducing electricity usage.  Just a word of caution.  If you use blowers, you don’t want to crank them up too much unless you like moving components across your PCBs.  Many ovens have precision controls for this reason while others offer this as an add on option.

Static Pressure

Potential Savings:

I have to take a wild guess in what this translates into dollars since there has not been a study specifically addressing what this means in terms of electricity savings.  Considering we have so far been able to build cost models from the profiling studies we can extrapolate some reasonable numbers.   In the Delta study, the cumulative setpoint change across their 8 zone Vitronics Soltec oven was 198 C.  If you run through each zone, some zones like Z1 there was no change, but when you get to Z5 the delta was 50C!  So how do you compare both?  If you achieve a 5C reduction across 8 zones or cummulatively 40C and you compare this to our 198C study, this would represent 20% difference.  So take our numbers from our profiling study and cut them down to 20%.  Remember in the national average example, you could expect $88 in mountly savings per reflow oven, therefore for this example we might see about 20% of that number or $17 per month per reflow oven.   I please welcome any oven manufacturer to share the results of a study that questions these assumptions since some guesswork is involved.

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Increasing Silicon Solar Efficiency Manufacturing

Global Solar Technology printed an article on Sept 16, 2009 highlighting an exciting ground breaking study that shows by optimizing the profile during the wafer firing process, a significant gain of .51% is achievable.  .51% is HUGE, which can easily translate into hundreds of thousands of dollars in increased revenues per solar manufacturing line.  That’s even in today’s depressed silicon market.

(Click here to view full article)

The thermal process of the wafer is one of the keys to achieving improved efficiencies. Drying steps are expected to remove most of the solvent used in the pastes before entering the firing zones. Solar cell metallization generally follows a spike profile type. Wafers only see peak temperature for approximately 1-4 seconds based on wafer and metallization chemistries. The most important steps include the clean burnout of the organics in the paste followed by etching through the silicon nitride (or other) passivation/ARC layer and, ultimately, the formation of good ohmic contact between the sintered silver and the very top layer of n-type silicon. These all lead to low contribution from series resistance and recombination resulting from the formation of the contacts. Control of this profile will become more crucial as the emitter depth decreases with increasing sheet resistance. Both uniformity of diffusion and furnace will be necessary to achieve the desired efficiency improvements.

The article walks you step by step through the study, here is an extend excerpt from the article related to profiling:

The base line profile on these wafers had been developed prior to the project based on extensive knowledge of the paste chemistry and years of practical experience with the metallization process. The base line profile can be seen in dark blue in Figure 1. For the base line test, as with all the subsequent process improvement tests, the wafers were processed at the same time and fired under the same conditions. Ten wafers were run through the furnace within a short period of time, and all were subjected to the same profile. After firing, we measured the cell efficiency in our continuous lamp tester. The average efficiency for the base line profile was 15.53 percent, as can be seen in Figure 2 (η Cell). Based on the type of wafer that was selected for this study, and the fact that a continuous lamp tester was used rather than a flash tester, this efficiency number was considered good. Now we wanted to make it better.

kic_article_1

Figure 1: The wafer profiles for each group

It is important to acknowledge that what we were trying to accomplish was not to find a single “golden” profile for the wafers, but rather the optimal thermal process window. The Heraeus paste SOL9235H is a very robust paste that can perform well throughout a range of profiles. Establishing a thermal process window will set the upper and lower limits for the wafer’s peak temperature, time above certain temperature levels, etc. within which the cell efficiencies will be highest.

Figure 2: Cell efficieny testing

Figure 2: Cell efficieny testing

Figure 3: Boxplot of cell efficiencies for base wafer profile

Figure 3: Boxplot of cell efficiencies for base wafer profile

Since we did not yet know the upper and lower limits to our process window, we used the base line profile as a starting point, and we initially set relatively wide process limits around it as shown in Figure 4. The profiler software always measures how well the profile fits the chosen process window with a single number called Process Window Index (PWI). The PWI number is 100 percent when the profile is at the edge of the process window. The lower the number, the closer the profile is to the center of the process window. A PWI of 0 percent represents a profile at the very center of the process window.

Figure 4: Original Process Window

Figure 4: Original Process Window

Our KIC profiler also has profile simulation software that allowed us to change the furnace zone temperatures or conveyor speed in the software, and to immediately predict the resulting wafer profile. For the first process improvement step, we suspected that a higher peak temperature would benefit the metallization. We tried a few zone temperature changes in the software and studied the software simulation of the corresponding profile before settling on a 10°C increase in the furnace peak zones (Zone 5 and 6). Once the furnace stabilized on the new settings, we ran a set of 10 wafers for our Group 2 test. The average cell efficiency increased from 0.40 to 15.93 percent. For Group 3, we increased the peak temperatures settings in zones 5 and 6 another 10°C, but the average cell efficiency of the 10 wafers dropped by 0.12 percent.

For the Group 4 test, we set the zones back to the Group 2 level and reduced the furnace conveyor speed. The prediction software showed the impact on the wafer profile both in terms of peak temperature changes and, in particular, in terms of time above the various temperature levels shown in Figure 4. Due to this, we reduced the conveyor speed from 200 to 190″/min. The average cell efficiencies increased yet another 0.11 percent above the Group 2 numbers to a cell efficiency of 16.04 percent. Our final test for Group 5 kept the temperatures stable but increased the conveyor speed from 190 to 210″/min. That dropped the average cell efficiency by 0.16 percent.

Figure 5: e-Clispe TC attachment fixture

Figure 5: KIC's e-Clispe TC attachment fixture

Conclusion

By systematically changing certain key profile dimensions, such as peak temperature and time above 500°C, we were able to identify the “sweet spot” in the metallization process. The PWI index and the profiler’s simulation software allowed us to quickly identify the appropriate furnace settings for profiles below, above and in the middle of the optimal settings. This sweet spot yielded an average cell efficiency of 0.51 percent higher than previous experiments had allowed.

The Heraeus SOL 9235H silver paste’s properties allow for high-efficiency processing in a range of profiles, hence a process window can be established around the “ideal” profile identified above. Heraeus now advices its clients to the appropriate process window for each application.

With modern profilers, solar cell manufacturers can adjust their furnace setup until the wafer profile is positioned within the suggested process window. Over time, the thermal process will drift due to a number of variables such as heating lamps changing as they get older, wear and tear in the furnace, conveyor speed drifts, exhaust changes, and more. It then is a simple task for the manufacturing engineer to run another profile, and to use the profiler process optimization software to identify the furnace settings that will yield the appropriate profile.

This method for process optimization depends on accurate and repeatable profile readings. One excessive noise in the profile readings historically has been caused by the attachment method for the TCs. Both cemented and dummy wafer TCs tend to measure the material used to secure the TCs in place, rather than to measure the surface of the wafer. Pinning the TC to the wafer with a weight suffers from non-repeatability. The fixture with flattened TC beads has worked well for us.

Finally, process optimization must be quick and easy enough to be useful for volume production lines, as opposed to only the laboratory line. There is little use in perfecting the process in the laboratory just to see the transfer to the production lines fail because the furnace properties are different. Once the correct process window is established, the high-volume furnaces can be adjusted within minutes, keeping production downtime to a bare minimum. This task must not only be performed during transfer from the lab to the production line, but it also must be performed periodically due to the drift in the thermal process that is a fact of life in any production line. The few minutes it takes to adjust the production furnaces for peak performance is richly rewarded by the ability to consistently produce higher efficiency cells.

Future Studies

The temperature readings taken by the e-Clipse TC attachment fixture are higher than historic readings taken by older TC attachment methods. A future study will focus on quantifying the accuracy and repeatability of the new profiling method as it relates to the theoretical true wafer surface temperatures.

More information: Bjorn Dahle, president of KIC, +1-619-300-5586.

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Save 15% in 15 minutes on your reflow’s energy consumption

Topic: 15% electricity savings in 15 minutes

Save Electricity in 15 mins. R0909B DRAFT

Date: Friday, September 25, 2009
Time: 1:00 pm, Pacific Daylight Time (GMT -07:00, San Francisco)
Meeting Number: 336 789 255
Meeting Password: (This meeting does not require a password.)

To join the online meeting
1. Go to https://kicthermal.webex.com/kicthermal/j.php?ED=107946437&UID=1013415122
2. Enter your name and email address.
3. Enter the meeting password: (This meeting does not require a password.)
4. Click “Join Now”.
5. Follow the instructions that appear on your screen.

Topic: 15% electricity savings in 15 minutes
Date: Friday, September 25, 2009
Time: 1:00 pm, Pacific Daylight Time (GMT -07:00, San Francisco)
Meeting Number: 336 789 255
Meeting Password: (This meeting does not require a password.)

——————————————————-
To join the online meeting

——————————————————-
1. Go to https://kicthermal.webex.com/kicthermal/j.php?ED=107946437&UID=1013415122
2. Enter your name and email address.
3. Enter the meeting password: (This meeting does not require a password.)
4. Click “Join Now”.
5. Follow the instructions that appear on your screen.

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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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Energy Consumption Reduction for Reflow | Better Thermal Management

Is your reflow oven throwing money away with a poor thermal management?

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

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