http://www.circuitnet.com/articles/article_83073.shtml as a response to an Ask the Expert Question. We think it’s worth repeating here as well.

Subj: Unusual Component Lead Contamination

We suspect the issue visible on the attached image is due to contamination on this component lead. We only see this issue on one component type, and only on one side of the component.

Can you offer any comments? E.W.

REPLY FROM PAUL AUSTEN, OF ECD:

Here is one possible cause to check on before you apply the failure to the component.

As with most solder quality problems, it is best to make sure the solder thermal profile, as required for good soldering for you specific solder paste, is being met. Do not assume that a general thermal profile for this board is the same everywhere on the board.

Make sure the thermal profile on or very near each end of this component is as needed. I have heard of components as small as this stand up on one end and then lay back down again during the solder transition into the liquid state (AKA: liquidous, or liquidus) because one end of the part heated faster than the other by a few fractions of a second. By the time the component lays down again, it is too late for best wetting.

To look for this possible time delay in the heating of the component’s ends with your thermal profiling software, make sure the profile peak alignment tool in the profiling software is turned off so you can see instant by instant the temperatures measured at each end of the part through the liquidous point of the solder. If one end is hotter than the other during this time, this may be part of the problem.

The cause of the temperature difference may be because one end of the part was on a pad that had no (or poor) thermal relief compared to the other. Typically, you need both pads of a component to be thermally equivalent. It may be that the board design needs to changed, or it may be as simple as running the board through the oven process turned 90 or 180 degrees to the current orientation.

However, turning the board 90 to 180 degrees may introduce other production or thermal issues on other components. None the less it may be worth trying.

Paul Austen, Senior Project Engineer

Electronic Controls Design Inc

paul.austen@ecd.com

Paul Austen is a 30 year veteran Senior Project Engineer with ECD in Milwaukie, Oregon. Paul has seen and worked with the electronic manufacturing industry from many points of view, including: technician, designer, manufacture, and customer.

This month ECD announced availability of the new 2.20a version of M.O.L.E.® MAP software.  Introduced in 2007, MAP (Machine-Assembly-Process) received multiple innovation awards, and is now the software platform for ECD’s entire line of thermal profilers: SuperM.O.L.E.® Gold 2, MEGAM.O.L.E.® 20, V-M.O.L.E.®, SuperM.O.L.E.®, Gold and PTP® VP-8

This version release coincides with the new SuperM.O.L.E.® Gold 2 availability and implements inputs from our Software Advisory Board (yes, we have one!)  So without further ado, here are the top 5 new features and benefits of M.O.L.E.® MAP 2.20a.

1. AutoPlay

This new feature auto-detects your M.O.L.E.® type and quickly links your plugged-in M.O.L.E.® to perform these basic tasks:

• View the status of your M.O.L.E.®
• Setup your M.O.L.E.® to perform a data run
• Download your most recently recorded data
• Start M.O.L.E.® MAP

This instant USB access eases the learning curve for the novice and focuses the operator on the basic profiling tasks at hand, shielding them from the full feature set of the software.

2. Improved Navigation

When you do open MAP, the “Welcome” screen now displays links to recently used Directories and recently viewed Profiles.  Quickly resume your previous work session by clicking where you left off with this convenient new feature.

3. Bulk Import of Previous M.O.L.E.® Files

Speaking of Profiles, you will probably want to import your libraries of SuperM.O.L.E.® Gold profiles (from SMGSPC) into MAP, which converts the .mdm file into the new .xmg format.

This MAP version implements group importation of existing .mdm and collaborative .xmg profile data. With a simple click-shift and drag, you can now move the contents of old Workbooks (an SMGSPC term) into new Directories, M.O.L.E.® MAP’s term for the currently viewed data in the Spreadsheet Tab.

4. PDF Printing to File and Email

Another way to collaborate your process engineering work between EMS/OEM is to provide documents to operators in PDF format.  The new MAP integrates PDF printing with an improved Print Selection dialog to accomplish portrait or landscape orientation directly to Email or a File. Great when your customer demands hardcopy proof!

5. Free Self-Serve Web Authorization and Automatic Upgrade Notification

Last but not least, licensing fees and pay authorization have been replaced with free “Self-Authorization” through the ECD website.  We give you a 31-day window to go to the Help menu, select “Authorize” then click on “Web Authorize”.  After you fill out the web form and agree to standard terms, our site sends you an email with your software unlock key.

It’s as simple as that!  Plus, you will be notified of new releases in the future.  We always want you to have the advantages of our current release.  Thank you for reading this month’s What’s New!

Free MAP 2.20a download is available at ECD DOWNLOADS.  (Check out the Readme file for the entire list of Rev 2.20a M.O.L.E.® MAP improvements!)

Till next time,
Ray Pearce
ECD Sales Engineer
ray.pearce@ecd.com

Energy needs a conductor in order to flow

1) An energy difference between two objects. 

and 

2) There is a conductor to act as a bridge enabling the energy to flow. 

Energy always flows through a conductor from an object of high energy to an object of low energy. In this illustration, the high-energy object is a moving hammer, the low energy object is the table and the conductor is a block sitting on the table. 

When you hit the block with the hammer, the energy contained in the moving hammer is transferred to the block when it hits. Some is also conducted through the block and transferred to the table it is sitting on. However, because the block is not a perfect conductor, which is true for most things, some of the energy stays in block. That energy bounces between the molecules of the block like balls on a pool table. 

Because the molecules rub up against each other, and there is friction between them, some of the moving energy of the hammer is converted to heat energy, which causes a rise in the block’s temperature. It all comes down to molecular motion in an imperfect conductor creating friction that raises its temperature. Therefore, temperature increase is a way of observing energy flow, and energy flow that causes a temperature rise is called heat flow. 

What Affects Heat Flow? 

Heat "flows" from hot to cold

The amount of heat that flows is dependent on the same basic things as energy flow, only we sense its effects by measuring temperature difference. As with high-energy objects imparting their energy to low-energy objects through a conductor, heat flow happens when a hot object transfers its heat through a conductor to a cold object. The quality of the conductor determines how quickly, and how much of the heat is transferred. If no heat is lost to the outside, the objects and the conductor in the above illustration would settle to a temperature 1/2 the value of their difference above the cold object, or below the hot object. The quality of the conductor would then only affect the time it takes for the heat flow to even out the two temperatures. 

Heat flow, therefore, is affected by the temperature difference between the Hot and Cold object and the quality of the conductor. The higher the temperature difference, the faster the heat flow and the higher the conductivity of the conductor, the faster the heat flow. 

Methods of improving this conductor have been played with by oven manufactures for many years. Three common ways to conduct heat between the oven heater (the heat source) and your circuit boards (the heat sink) are: 

1) Infrared Radiation (IR) 

IR depends mainly on the “sun-like” conduction of heat via electromagnetic radiation. Although some conduction occurs due to heating of the atmosphere surrounding the circuit board, it plays a lesser role than the radiant energy. This is useful when the entire surface being heated will absorb the radiant energy in exactly the same way at exactly the same rate. The main drawback is that in order to make up for the inefficiencies of this method of heat transfer, the heat source is often set to temperatures much higher than the required melting temperature for the solder on your circuit board. In addition, the components on your circuit board each absorb radiant energy at different rates, aka emissivity. While one component may heat very little, another may be over-heated because it absorbs radiant energy very well. This uneven heat absorption makes it very difficult to choose a heat-source temperature that will meet the varying absorption rates of the variety of components on a typical circuit board. 

2) Vapor Phase (hot liquid) 

Vapor phase heat conduction is one of the best around because of its high conductivity and heat capacity, which is released as the vapor changes from gas (vapor) to liquid. Plus, it has the ability to conform to the irregular shape of your circuit board. This method of conducting heat in reflow soldering machines is currently enjoying a comeback. 

3) Convection (hot air) 

Convection has been very popular because it provides “vapor like” conformance to board shape, but does not require much more than electric power to function. Here hot air (or nitrogen) is used as the conductor, but with an extra kick: convection fans that move heated air from the heat source to the heat sink. This combination of an air conductor and air movement is called convection. The greatest asset of convection is that it allows you to lower the temperature of the heat source because the movement of the air creates a more efficient heat flow system. 

4) Direct conduction (metal plate) 

Using a direct conductive medium, like a metal plate, is efficient, but is only practical when the surface being heated is flat. 

What about my boards? 

In a convection oven, heat is "hammered" into your circuit board by moving air

So heat flow causes circuit boards to get hot within an oven. In this case, the oven is the heat source (like our hammers); moving air is the conductor that delivers the energized air (like moving the hammers) and pounds the heat into the circuit boards (the heat sink).

Because of the increased conductivity that the moving air creates, lower oven temperatures still allow a circuit board to reach the required temperature, and more evenly because conducted heat is not as dependent on a component’s ability to absorb radiant energy. The heat is conducted to the circuit board as fast as the moving air can deliver it. 

So, if you want to increase the heat flow, you just turn up the temperature, right? That is correct, but this creates a potential hazard for your circuit boards. Given enough time, your circuit boards will reach the heat source’s temperature. If that temperature is significantly higher than the specified limits for your components, you will damage them. Another way to increase the heat flow is to set the temperature at an acceptable level and only increase the airflow. In other words, add more hammers. This forces the heat into your circuit board components much more evenly and at a rate that meets your component specs. You can accomplish this at your desired process speed while lowering the risk of component damage due to over-temperature. 

How can I measure heat flow?

  

Heat flow is easy to understand, but difficult to measure. This is because the conductor of the heat flow (air) plays as big a role as the temperature in determining how much heat actually flows. For heat, everything is a conductor. There is no simple way to say where heat is going; it goes everywhere. So you cannot know how much of the energy consumed by your heater actually gets to your circuit boards, even though heaters are nearly 100% efficient at converting electrical energy to heat. Therefore, energy into an oven is not energy transferred to your circuit boards, because a lot of it is going into the room, up the exhaust, and some into you.

As a result, heat flow measurement has taken the form of very sensitive probes that measure the temperature difference between the surfaces of thin films. These measurements translate into the amount of energy in watts per square centimeter. Typical values for heat flow in a reflow solder machine from the top heating zones to the surface of a circuit board range from 0.1 to 0.7 watts/sq. cm. This value drops as it proceeds through the oven because circuit boards heat up and reduce the temperature difference between the heater and the circuit boards. This naturally reduces the heat flow because heat flows from hot to cold and if there is no more cold, there is no place for the heat to flow – no heat flow. 

Problem Solved 

Convection plays an important role in heat flow

Understanding that oven temperature alone “does not a temperature profile make” is vital toward making your oven consistently reproduce a good temperature profile. The temperature profile your circuit board experiences as a result of your oven settings is dependent on the temperature settings AND the heat flow capacity of your oven. To illustrate this fact, two thermal profiles were run on the same circuit board where the speed of convection air in the oven was the only parameter changed. As you can see, the thermal profile at low convection is dramatically lower then the same profile at high convection, while the oven’s zone temperature settings remained the same.

This does not mean that the heat flow capacity should be large, but it must be consistent hour-to-hour, day-to-day, and week-to-week. Small variations in heat flow can cause temperature variation on your circuit board, even though the oven temperature remains the same. A measuring device that can produce meaningful information relative to heat flow in addition to the temperature will give you a complete understanding of your oven and its performance over time. Of course I would not tell you this if I did not know of the tool that can measure your oven’s ability to “hammer heat” into your boards; that being the OvenRIDER. 

Conclusion

 

Controlling the reflow oven soldering process requires monitoring not only temperature but also heat flow. Since heat flow is what causes objects to increase in temperature, measuring your oven’s ability to increase the temperature of fixed thermal mass objects will allow you to detect variation in heat flow. By observing the temperature rise in the mass over a fixed amount of time, you can discover if the same amount of heat has been transferred into the mass. Such a measurement, combined with ambient temperature, yields a much more complete profile of a reflow oven. Only by knowing the heat flow capacity of an oven can you be assured that the oven will heat your circuit boards at the same rate and to the same temperature, every time.

Figure 1 – Vapor Phase Soldering and the condensation layer

When your room-temperature assemblies are immersed in the vapor cloud, the vapor quickly condenses onto the cold surfaces of all the parts, releasing the heat of vaporization, or the heat energy that was put into the liquid to evaporate it into a vapor. This energy release begins heating your assembly rapidly toward the 200+ºC vapor temperature.

To measure the thermal profile of the process to prove you are staying within the components’ thermal limits, (always a prudent step) you must attach a thermocouple to the component or solder joint you wish to profile. Here is where you must take care. (See figure 2)

Figure 2 – Typical thermocouple attachment to component lead

The thermocouple may be attached with solder or epoxy. However the thermocouple wires extend away from the point of measure and may be exposed to the vapor. The vapor will condense onto the thermocouple wires in the same way it condenses onto all the parts. Exposed bare wires will heat quickly because they are very small. These heated wires will conduct that heat toward the thermocouple bead, where it is sensing the temperature, and artificially heat the sensing point. This will actually show as a “false” temperature rise in the early part of the ramp up in temperature because the heat is conducted in to the part by the thermocouple wires. The insulated part of the wire will not heat as fast because the insulation slows the heat flow of the condensing vapor. .

So when connecting thermocouples to components to measure the thermal profile, it is very important to insulate any exposed thermocouple wire beyond the thermocouple sensing bead. (See figure 3)

Figure 3 – Insulated thermocouple wire

This is best done with a thin layer of epoxy covering the exposed bare wires of the thermocouple, up to and a bit past the insulation on the thermocouple wire, being careful not to cover the sensing bead of the thermocouple. The epoxy does not need to be very thick, just a thin covering. Using too much would have the “chilling” effect of preventing the heat from getting to the component or solder joint, giving an artificially cooler profile result.

You may also try Kapton® tapes or high-temp heat shrink around the end of the thermocouple. These may shrink back or come loose, however; and if you plan to profile more than once, these methods may not hold well over time.

References:
SMT Magazine, “Vapor Phase Soldering: The Comeback Kid” by Ray Prasad
Thanks to Kevin Syverson, Silicon Forest Electronics, Inc., for the use of their Vapor Phase soldering system.

Figure 1 Typical thermal profile with the four traditional parameters within spec

There are other ways to look at a profile which can be helpful in determining if the profile may threaten components and showing if it is consistent, both across solder joints, and over time.

In the profile example above, the Time Above Liquidus (TAL)on solder joints 1 and 3 are within 2 seconds, yet channel 3 (from the data; plot not shown for visual clarity) had more readings at higher temperatures. This means that although this part may have the same time above 183ºC, more readings were at temperatures higher than channel 1; higher risk of damage. Also note that the peak temperatures were not far apart; 222.2ºC vs. 223.5ºC.

So we added a new measurement to the MAP software to not only show Time Above Liquidus, but also consider the temperature values during the TAL portion of the profile. This new measurement has several names: “Total Heat,” ” Area Under the Curve,” or “Stress Integral.” It combines the time element of Time Above Liquidus with the temperature measurements during that time to give the Total Heat the component experienced, expressed in degree-seconds.

Figure 2 Total Heat measurements (component 1 only shown for clarity)

In this case, even though the Time Above Liquidus values are within 2 seconds and the peak temperature is less than 2 degrees apart, the Total Heat values are 2278º-sec and 2628 º-sec which differ by 350 º-sec! This clearly points out that component 3 had to withstand more Total Heat than component 1 and this simple parameter can now be examined in an instant, using the latest; version 2.18j of MAP software.

The cooling zone is where the quality of the solder joint is defined, with the cooling slope influencing the joint strength, and overall longevity. These two qualities are often at odds with each other because strength often comes from slower cooling rates, while longevity results from faster rates. Different cooling slopes have been tested to try to find what rate produces the best combination of strength and long life when subjected to accelerated thermal-cycling. These studies have concluded that slow cooling rates (1 to 2 °C/sec) allow too much time for intermetallic alloy growth, a strong but often brittle alloy prone to cracking when stressed. Faster cooling (5 to 7 ºC/sec) can form a softer solder joint with less overall strength, not to mention possible component damage. Cooling slopes between 3 and 4 °C/sec were found to be the best at producing a solder joint with both good strength and overall longevity.

So… don’t forget the cooling zone when developing the best thermal profile for your solder process.

References:

“Cooling Rates in Lead-free and Tin/lead Reflow”
SMT Magazine

by Denis Barbini, PhD.; Ursula Marquez

“Accelerated Thermal Fatigue of Lead-Free Solder Joints as a Function of Reflow Cooling Rate” Journal of Electronic Materials

by Qi Y; Zbrzezny A R; Agia M; Lam R; Et al

“Proceedings of 2005 International Conference on…” Asian Green Electronics
by Qiang Hu; Zhong-suo Lee; Zhi-li Zhao; Da-le Lee

This idea should be considered carefully, and here are some reasons why thermal profiling in vapor phase soldering is still a very good idea:

1. Although the boiling point of the vapor phase fluids is a physical attribute that limits the maximum temperature, the condensation of the fluid onto the components can impart a lot of heat, real fast. This can subject components to the old thermal shock problem, and unless this heating rate is carefully controlled by the vapor phase machine, you may well be shocking the components. Thermal profiling is the only way to show this is under control.

2. The maximum temperature is a function of the fluid type, and one needs to be sure the correct fluid is being use. There is a fluid whose boiling point is hot enough for lead free soldering, and not too hot for leaded soldering, about 230ºC. This “happy medium” is a good compromise, so one does not have to own two different vapor phase machines, or change fluids from one process to the other, but it is another reason why thermal profiling is a good idea: to prove that the process is meeting the need of the solder paste and the limits of the components.

3. A process undocumented is a process out of control. Unless you have some evidence that the thermal profile is meeting the requirement of the solder paste and the limits of the components, you cannot prove the process is in control statistically. You can’t make process control charts if you don’t measure the process. This is at the heart of a good Thermal Quality Management (ThQM) program.

4. Your customer still wants to know what the thermal profile looks like. No matter how you solder your customer’s boards, they still want to know what they were subjected to, thermally. This is your assurance to them that you have treated their product properly.

Calibration of electronic measurement instruments is a necessary process, even though most electronic equipment is very stable and somewhat “resistant” to the effects of environment and changes due to aging.

Q: So why calibrate if my MOLE is “in spec” every time I send it in for calibration?

Because calibration is not so much an adjustment process but rather a proofing process that shows, over time, that your MOLE has been in calibration and thus should remain in calibration, because you have a track record to prove it. Documented history of a MOLE’s performance is the only way to claim your MOLE is in calibration at any given instant.

Most good labs will tell you that when your MOLE is calibrated, it is compared to standards , typically standards that have traceability to NIST, and if it is shown to be measuring within its specified accuracy they will not make any attempt to “adjust” it. Only if it is “on the edge,” which usually means it is getting to the last 10% to 20% of the specified accuracy limit, will they make any adjustments. Your MOLE may still be “in spec” and thus “in calibration,” when the lab received it, but getting close, so they will adjust it closer to the middle of the spec. range.

If it is out-of-spec when received by the lab, then a red flag goes up and calls into question every measurement made since the last calibration! The lab will tell you how far out of spec it is, and you can decide if its measurements during that time affect the quality of the measurements made more than can be tolerated, or if they are “close enough” to still be acceptable.

Q: So, when should the MOLE be calibrated?

The number one best time to calibrate the MOLE is on a regular time-based interval, which is recommended once a year. However, there are other events which may cause you to want to seek calibration at other times of the year, such as:

1. When the MOLE is subjected to rough treatment like a fall to the floor,
2. When your MOLE is accidently “over heated” ,
3. When you are starting a new product introduction and you are characterizing an oven and new assembly to find the right recipe,
4. When a new customer’s contract stipulates you use equipment that has been recently calibrated,
5. When your in-house quality program requires a calibration interval.

Getting your MOLE calibrated is easy and we want to make sure you are always making the highest quality temperature measurements.

For example, component manufacturers would have you avoid certain limits in temperature or temperature change rate (slope) to avoid damaging their parts. And that’s ok, but that only tells you what to avoid, temperature wise. Most solder paste manufacturers would have you believe that their paste can take most any reflow thermal process so as not to be excluded from purchase. This too is understandable and in reality, most solder pastes are good and will solder your components to your circuit board. Many standards (like IPC standards) on the subject suggest what your product MUST withstand to be considered robust and not necessarily an ideal reflow process thermal profile. This makes sense, because there is no one reflow thermal profile that will solder every possible circuit board assembly, and standards must be general in their application. Then there are the public websites that are often peppered with bias toward a specific brand of profiler in their description of what’s important or how to view it.

Each year new talent enters the work force and training in the art of reflow soldering is limited or costly. Worse yet, some learning about the reflow process only occurs from failures caused by incorrect reflow process settings. And perhaps worst of all, many reflow solder machine are still running the same setting set generations ago because no one currently available has the skill to make them better. Just because many industry veterans understand the issues around reflow soldering and thermal profiling does not mean the new talent can hit the road running. And, since most every electronic assembly will pass through either reflow soldering, wave soldering, or sometimes both, I thought it important to take a look at the reflow solder process, dissect it and consider what’s important to measure and control. Click here for the more in-depth look.

1. “I’m from the Metrology lab and it’s time for the annual calibration of your reflow oven.” We’ve got an app for that.  After you’ve finished with the oven’s calibration procedure, you can run the MegaRIDER-20 with a Process Test Pallet to see if the machine is uniform across the conveyor width and has the same heating capacity as it did the last calibration or maintenance.

2. “I’m the Manufacturing Engineer and our QC Department wants me to show that this oven is in control.” We’ve got an app for that. You probably need more information than the once a year Metrology profile can provide. So weekly you can an OvenRIDER and see that every zone in the oven is performing the same using X-Bar R charts to prove it.

3. “I’m on the New Product introduction team and I need a good recipe to solder a new board without killing the parts.” We’ve got an app for that. The Super M.O.L.E.® Gold thermal profiler will let you connect T/Cs to the board to see exactly what’s going on, thermally, on the areas where you and the designer have the most concern. Use the Prediction tools in the new MAP software to lock in the perfect recipe.

4. “I’m a Line Technician and I have to know my reflow oven is ready to run product without all the wires and circuit board stuff.” We’ve got an app for that. OvenCHECKER is one pallet loaded with the most powerful profiler on the market today. It takes no more time to run than the first production board and it lets you know if the reflow oven is ready or not. No downloading, no comparing numbers on a chart, just Go, or No-Go.