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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Thermocouple Attachment Results are in!

The Rochester Institute of Technology under the guidance of Dr. S. Manian Ramkumar Ph.D. just conducted (October 2009) the most comprehensive study to date on thermocouple attachment methods.  Part I of II was to determine the most accurate and reliable method of thermocouple attachment.  Part II that has yet to be released is to determine the best attachment methods for BGAs, with the goal of seeing if there are reliable non-destructive methodologies, so stay tuned.

Results in a nutshell:

Aluminum Tape out performed all materials even Kapton! In an ideal word, the best attachment method of a thermocouple to a component is what I like to call a naked TC.  Aluminum double sided conductive tape was the closest thing to having nothing at all to attach the thermocouple.  Kapton tape is less responsive (deflecting and insulating heat), never mind if you have ever seen a saw-tooth TC plotted on a profile you know it has a very hard time staying in place on your PCB.   Additionally, High Temperature Solder which I have always considered the gold standard, is the least accurate or responsive.  When you get to the critical peak temperature of your profile, high temperature solder is sluggish to respond to the rapid change in temperatures, thus distorting your readings. As Phil Zarrow and Jim Hall discuss in Board Talk, “mass” on your thermocouple is not your friend.  Phil Zarrow:

any measurement method, the key element is to get the thermocouple in good contact with what you are trying to measure and to do it in a way that does not modify the area with a lot of extra mass or material that is going to give you an inaccurate reading….

Bingo!  This is actually what this study shows, now with the numbers to back it up.

Study Methodology:

The study looked at:

  1. Aluminum Tape
  2. Kapton Tape
  3. Chemtronics – CircuitWorks CW2400- Two Part Epoxy
  4. High Temperature Solder
  5. Loctite – 382 Instant Adhesive

The study used a KIC Explorer with standard type K thermocouples.  Multiple runs of a substrate coupon (62 mils thick plain copper coated with silver) was routed into 12 uniform 0.24″ isolated sections.

Test bank2Three identical test coupons were used and run multiple times.  KIC’s air-TC was utilized as the control to which each thermocouple was measured as the coupon traveled through all heated zones.

A total of three boards were used, running each board through twice, allowing the internal temperature of the KIC device to drop below 40 degrees C before rerunning the profile.

The tape attach methods were measured uniformly for each RTD connection, using a dial caliper, while the high temperature solder and epoxy quantities for attach were found to be visually uniform.

Mean

This graph indicates the mean temperature differential that was noticed within the oven for the various attach methods. The readings are based upon the complete profile starting at room temperature and ending at the peak temperature. The data from the cool down zone was eliminated from the analysis.

The graph shows the mean differential and the 95% confidence interval for each attach method. The Aluminum tape had the least differential (-0.48) followed by Kapton Tape, Loctite Adhesive, CW-2400 and then HT-Solder. The Confidence intervals among most of the attach methods do not overlap except Kapton and Loctite, indicating that the means of the attach methods are significant. Significant differences exist between the methods except between Kapton and Loctite as there is overlap. Clearly Aluminum tape outperforms all of the other methods.

Zone Differences

The thermocouples seem to behave similarly within each of the zones of the oven. Zone 6, where the soldering takes place or the peak temperature is reached, the thermocouple attach methods show a much higher temperature than the air temperature, indicating that the PCBs have attained much higher temperatures than the air. A closer examination of ZONE 6 reinforces the selection of Loctite or Aluminum Tape for Phase III of this project.

Conclusion:

When considering accuracy, repeatability and responsiveness, Aluminum Tape is a winner.   There are of course advantages and disadvantages to each material.  For example one can argue you can re profile a PCB set up with high temperature solder, but considering that the mass of the solder distorts your readings, this study even brings into question this bedrock of thermocouple attachment.  Never mind high temp solder destroys your PCB as well as there is little control over the size of the blog from TC to TC and board to board.   Also don’t forget every time you profile the same board again it loses some mass, which will be the focus of more blogs to come.

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Why are you replacing BGAs?

There is a great post today in Circuitnet titled “BGA Replacement Limits,” that can be found under Circuitmart.  Panelists answer the following question:

How many times can a BGA component be replaced at the same location on the same PCB and retain reliability?

Mark McMeen of STI Electronics suggests that the answer may be as little as two times!

…most companies err on the cautious side and only replace twice at the same location after the initial build which is normally 2 thermal cycles for top and bottomside reflow thermal cycles.

I think a broader question needs to be asked, why are you replacing BGAs in the first place?  In my experience, often the answer is due to poor reflow profiling.  Often there is nothing wrong with the oven, PCB or BGA.   Why is it so hard to properly profile a BGA?  I believe the reason is most folks don’t have the option of placing a thermocouple underneath the BGA nor sacrificing a board in drilling a hole on the underside for TC placement.   In the old days, you could get away with snaking a TCs under the BGA, but with micro BGAs this is just not an option.  So what do people do?  They stick a TC on top of the BGA or along side it.  Many do nothing at all which is kind of scary and wind up asking question like how many times can I redo my board.

To go to show how hot of topic this is, I held a series of webinars a couple months ago with a turnout in the hundreds.  I shared some ideas, here is an abridged 8 min version of the session for those of you that missed it. Part of the answer is proper TC attachment which by the way is currently under study at RIT to see the most reliable method as well as determine if there is a non destructive methods that is both valid and repeatable.

The other part of the equation is profiling your PCB not only for your BGAs but also those components that cannot tolerate as high of temperatures. I’ve seen plenty of manufacturers so focused on a $500 BGA, ignoring pretty much what else is going on with other components on their PCB.   Certainly having the ability to define separate specifications, for example a peak temp for a DIP while addressing the special needs of your BGAs will lead to fewer BGAs having to be reworked in the first place.

After all, which is better, to treat the symthoms or the root cause?

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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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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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Thermocouple Attachment Discussion

Phil Zarrow and Jim Hall of ITM Consulting have a very good piece on TC attachment on Board Talk hosted by Circuitmart.com.

In their first session, they talk about permanent TC attachment, such as high temp solder and epoxy (click here for a link to their recording).  Yours truly left a comment with the boys:

We are a big fan of conductive Aluminum tape, used along with Kapton for strain relief like you mention in your podcast. We talk about high temp solder and epoxy which can work also, but like you said you got to be careful of mass. Lot of times we see unequal amounts applied per TC that can throw your readings. What is your take on aluminum tape, realizing of course it is a non permanent solution?

Well, they came back with a terrific response (click here for a link to their recording), where they make a clear distinction between destructive vs. non destructive methods.  Non destructive methods are often the only option, since customers cannot sacrifice a board for profiling.

Phil goes on to say:

any measurement method, the key element is to get the thermocouple in good contact with what you are trying to measure and to do it in a way that does not modify the area with a lot of extra mass or material that is going to give you an inaccurate reading….

Phil talks about using for example Kapton as a strain relief to ensure there are no stresses on the point of TC attachment.  I’ve been saying for years to use techniques such as window paning where you apply Kapton around the boarder of your aluminum tape to help keep your TC secure if profiling more than once the same PCB.  Make sure not to put Kapton over the bead since Kapton can behave as an insulator.

I think Phil makes a great point on emphasizing the “size” of the tape you are using.  Again you don’t want the material’s mass to become an issue.  So the name of the game is don’t go overboard.  Personally I prefer a 1/4″ square piece of aluminum tape along with 1/4″ Kapton.

Jim Hall makes also an excellent point that the same goes for “destructive” methods when using high temp solder and epoxy.  You don’t want to overdo it, or the mass can effect your readings.   I would add further that you need to be very careful that the mass be equal from TC to TC.   It has been my long held belief that the blob of epoxy or solder if of unequal amounts TC to TC, PCB set up to PCB set up will add variability into your process.  Just keep your materials to a minimum to get the job done.

Many of these assertions are currently under review by an RIT study.  Hope to have results as early as the end of this month.  KIC conducted a study 10 years ago on all the materials mentioned (click here for the report).  Since a decade has past, one could assume materials have improved therefore warranting a second look.  Stay tuned!

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Non Destructive BGA Profiling Test #1

I am currently investigating a non destructive method of BGA profiling that is reliable.  Here are the results of my first test.

Set Up:

Four thermocouples are attached to the same BGA (TOP, SIDE, INSIDE and BOTTOM surface), as pictured below.  Conductive aluminium double sided tape is used along with Kapton.  A KIC Explorer is the profiler.

To see more on Thermocouple attachment visit my post:  http://profilingguru.com/tcs/thermocouple-attachment/

A hole was drilled out to attach the INSIDE TC.

pic1

pic2

Results:

Two tests were run, the first was running the board on the belt followed by running the same board on the chain/tab conveyor.

sample1

As you can see the delta for ramp and peak is the greatest, while soak is minimal.  The inside TC runs the hottest and the underside bottom TC follows fairly closely the behavior of the inside TC.

sample2

This second profile was run on the belt with the same board but for a different BGA.   Again we see similar behavior, where the INSIDE and BOTTOM TCs exhibit similar behavior.

sample3

This third profile was running the same board and same BGA as in the second example but this time on the chain/tab.   Interestingly, all TCs were a good predictor of the INSIDE TC except when getting to the cooling zone.  The BOTTOM TC was only a good predictor of the INSIDE TC.

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Profiling BGA Webinar

Profiling BGA Webinar Supplemental (July 1, 2009):

Component Specific Specs

We discussed the need to define BGA specs separate from other components that have different reflow requirements.   BGAs typically require more heat to reflow properly but typically there are many other “smaller” components that also populate a PCB that will overheat if you develop your process solely around the BGA.   The following 2:40 min video reviews how you can bring both your BGAs and other temperature sensitive components into spec, striking a thermal balance that results in quality products.

Thermocouple Attachment

The following 1 min video shows one of the most reliable direct methods of TC attachment for BGA profiling.

…..but, who can always sacrifice a PCB in the process?   We talked about some indirect/non-destructive methods for profiling BGAs that are suggestive, but inconclusive.   In the fall I hope to have some results of a study that will help our industry come up with solutions that one can reasonably predict the temperature/profile of a BGA without destroying the PCB in the process or worse the BGA!

BGA Inspection

First there was SPI (solder paste inspection), then there was AOI, now there is RPI (Reflow Process Inspection)

rpi-smt-linerev-11

You can see a prior blog posting discussing RPI at:   http://profilingguru.com/reflow/what-is-reflow-process-inspection/

RPI works in the world of continuous reflow monitoring, where a profile is created for each and every production board.

In order to automate reflow profiling, a baseline/virtual profile is first established, where one runs a traditional profile with PCB, TC attachment and profiler while the on-board system of 30 thermocouples gathers the same profiling data and reconstructs and converts the traditional profile to a virtual representation. Once a virtual profile has been established, profiles can be collected for all production boards.  SPC charting, cPk, traceability and process control are all possible.

So rather then the reflow process being a black box, where anything and everthing can go wrong…..

illustration_5….alternatively, do you not only know what is going on continuously, but your BGAs using the techniques above are being monitored on a continuous basis.

reflow-yield_3in_nk

Your Questions:

Q: Doesn’t the thermocouples utilized by the oven itself (assuming that they are calibrated and verified) provide the same basic information as the secondary set of TCs you are referring to?

ANSWER:  No, the oven thermocouples and the secondary KIC  TCs have completely different and separate functions.  The oven TCs are typically located close to the heaters since their job is to turn the heaters on and off as the temperature drifts from the set points.  The KIC 24/7 (or KIC Vision) TCs, located along the conveyor, help to automatically measure the profile that each PCB experiences as it is processed through the reflow oven or wave solder machine.  This function is called Virtual Profiling.

Virtual Profiling (VP) provides process traceability as it logs the profile for each PCB, along with information on how this profile fits the established process window.  The VP works in real time and offers instant alarm when the process (profile) drifts out of spec.   Because it provides basic SPC charting, it acts as an early warning system for trouble ahead.  Think of the KIC24/7 or KIC Vision as an automatic profiling system in real time.

Q:  I encountered wetting issue with CSP and BGA, how do I solve them?   /   Q: How about wetting issue?

Answer; In some cases, but of course not all cases, wetting issues are a result of incomplete flux activation in the solder paste and an overall low temp soak, where the components did not reach sufficient energy levels before entering the reflow, TAL stage of the process. Many of these issues are related to Pb – free solder pastes, mixed RoHS components or a number of other variables.

I suggest that the best answer is to research the publications available on the Web for the most relevant solution. The following is a link that closely resembles the issue, but again, you will need to research the most relevant to your situation.

http://www.emsnow.com/cnt/files/White%20Papers/Henkel_Leadfree_Designing_Reliability.pdf

Q:  How do you take measurements on each board without TCs?

Answer: KIC software algorithms compare what was observed at the time of the Baseline Profile to what is present within the oven during production. Using the 30 thermocouples in the oven, this data is communicated to the eTPU and the output is the PWI based on the specific process and the specification of that process.

Q:  How well does the DPMO relate to the actual defect where there could be placement defects interacting with reflow?

Answer: DPMO is a parameter of only the thermal reflow process. If issues exist in placement or screen printing, it will not be reflected in the DPMO, since KIC is only monitoring the thermal process. Given that all other aspects of the SMT line is functioning properly, DPMO will give an assessment of the thermal defects assuming that the proper solder paste and placement is present at the time the product enters the oven.

Q:  What about paste formulations?

Answer: KIC works with any solder paste manufactures to build the solder paste library that is present in the KIC software. This library is updated periodically and verified by the solder paste manufactures in most instances. The library however does not at any one time contain all information about all possible solder pastes. We try our best to be certain the information is present, but changes in formulation and engineering at the solder pate manufactures sometimes causes gaps that are beyond our control.

Q: How important is it to drill into the BGA ball and put the TC in it, vs. putting on the package, slip under the package, and on the bottom side of the board?

Answer: There are many variables in PCB design and component placement that directly and indirectly affect other components, in this case BGA. The best possible answer to this question is in the amount of data that is collected, how it is collected and how this information is applied to the specific PCB and BGA directly. Gathering as much information as possible, charting this info and drawing data driven values is the best possible formula for successful BGA reflow. Using all available data collection methods and positions aids in successfully reflowing this package.

As indicated during the webinar, we are currently commissioning a study to see if non destructive methods can be used in place of drill a hole.

Q: Does your software always choose an extended peak recipe?

Answer: No. Based on the type of recipe and profiles that are part of your normal production determines what path the KIC Navigator (auto-prediction) directs the profile. If your profiles are mainly RTP, the software looks at the values of the library data and suggests set points that will lead to a RTS profile. If your profiles are largely RSS, then the suggested set points will tend towards a RSS profile.

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Soldering and Profiling Discussion Panel at Apex 2009

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/

apex2009

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