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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2011 Profiling Guide

The popular Profiling Guide is back with this second edition. I’ve added a new section on profiling for Wave with the help of two solder wave gurus Mike Young of Aligned Solutions and Dave Nixon of Ayrshire Electronics. Also a contributor is Ed Briggs of Indium who provides profiling solutions to common reflow defects such as voiding, tombstoning and solder balling. What you will certainly find particularly interesting about Ed’s work is he gives you real life examples on how you can modify your oven recipe to reduce or eliminate defects!

Also in this edition, I continue to work on what I started in the first edition the Six Sigma’s DMAIC approach to profiling. I’ve added Analyze and Control to Define-Measure-Analyze-Improve and Control. Being able to analyze and control your process are becoming the difference between whether or not a Contract Manufacturer secures a new customer (or keeps one) and a cornerstone to Black Belt manufacturing for OEMs.

You can pick and choose what is important from this guide, I purposely organized it to be read in segments and not necessarily from cover to cover. For example, perhaps a full DMAIC approach to profiling is too much to ask? You are struggling just to Define and Measure your process, am I right? Let’s face it, the majority of SMT and Wave processes worldwide are not even profiled and those that are profiled fail to go beyond the basics. Don’t feel bad, this guide will help you. You are not alone, the vast majority of process engineers still don’t realize how simple it is to improve their process. I made sure this guide helps to explain in a no nonsense easy to read fashion how this is possible, no theory, no boring one hour long technical seminars, no academic discussion, but what do I need to do to get actionable results.

Lastly, I wrap up the book with a chapter on Advanced Profiling Methodology. I focus in particular on non-destructive methods which even the novice can appreciate and implement. Included in the book you will find tools for creating offsets for components like BGAs, where drilling holes into your PCB for profiling I promise will become a thing of the past. I provide references to real science and free tools are even included. Furthermore, “virtual profiles” and other viable options to sacrificing an instrumented PCB are explored.

Just like last time I had a great time pulling this book together. I leave and breathe profiling and certainly no offense if you don’t share my passion. At the very least, I hope you have just as much fun reading it.

- Profiling Guru

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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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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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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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What is Reflow Process Inspection?

Reflow Process Inspection is catching on as the next advancement in-line inspection systems for SMT Reflow.   I have pulled together the various aspects of RPI to better explain how it works and what are its benefits.

How does RPI fit into the inspection processes in the SMT factory?

Unlike SPI and AOI that are defect inspection systems specifically designed for viewing solder deposition and component assembly respectively, RPI complements these systems by inspecting the performance of the thermal process IN-LINE.   RPI inspects the thermal process for any joint, including those that are not visible to the AOI system such as BGA components.

What is being measured?

RPI charts the thermal Process Yield and DPMO

What are the RPI benefits?

RPI provides information on the “health” of the thermal process over time.   The Yield and DPMO charts provide instant understanding of detrimental changes in the process.  The following format is easy to read and understand and often used by management as well as engineers.

dpmoyield1

For an Overview:

mbrpi

Awards for RPI:

2009 Innovation Award

KIC’s RPI Wins a 2009 NPI Award

Innovative Technology Center Award at Apex 2009

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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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Reading your Profile via Profile software

Let’s take a close look at the profiling graph and the specifications used to create the graph.

The Solder Paste Library of your profiling software provides several choices:

Maximum Slope Between Temperatures

Maximum Rising Slope (Ramp Rate)

Maximum Falling Slope (Cooling Rate)

Preheat

Soak

Time Above Liquidus (TAL)

Reflow

Peak

Maximum Exit Temperature

For some inputs, such as slope, preheat, soak and TAL you can define multiples of the same input.  For example, you might want to define more than one slope for your process.

profilemodel

Inputs and Segments of the Profile

These terms: specs, variables, segments, zones and inputs are thrown around and often used interchangeably. Unfortunately, some of these terms are used to describe completely different aspects of the reflow process, which leads to lots of confusion.  For this guide, we use two terms to describe slightly different meanings.  When I discuss setting up your profile, I use the term “input” to describe ramp, soak, slope, when I etc.  I use the term “segments” to describe these same terms in relation to the profile graph.

Inputs each have their own specifications. This section explains how they are measured and the defects associated with each of these segments of the profile. Interchangeable terms are also used across the industry to describe these inputs/segments. Here, I list them all. For instance, maximum rising slope and ramp are used interchangeably with the same meaning.

Maximum Slope Between Temperatures

Three important parameters make up this specification:

1.  First, at what point in the profile do you want to know the slope? Rather than looking at the whole profile, this specification will look at a specific temperature range, for example, what is the slope between 150°C and 200°C?

2.  Next what is the ideal slope range? From 0 to 4°C/sec?

3.  And third, how many seconds would you like to calculate the slope over? A default value is typically set to 20 seconds, more on this in a moment.

Slope is important for both component and solder paste tolerances.

Maximum Rising Slope (Ramp Rate)

The Maximum Rising Slope, or Ramp Rate, looks at the whole profile, specifically the steepest slope over the entire profile and does not just look at a specific region. At this stage of the reflow process, the temperature rise from ambient to the first heating zone is of most interest since the greatest potential for component damage and solder ball spatter from a high ramp rate exists. The parameters are measured in degrees per second as temperatures increase.

To calculate slope, you will need to input a specified “duration” of time. The typical default value is 20 seconds. The more data points you have, the more accurate the calculation since this increases your sample set, and in the end, the validity of your data.  However, not all processes will have a default value of 20 seconds. If the area in which you intend to calculate the slope over is small, your sample size will have to be measured in fewer seconds, perhaps adjusting the default value down to 10 seconds.

Maximum Falling Slope (Cooling Rate)

Properly cooling your product may be necessary for your process.  Some specs call for rapid cooling.  Depending on your profiling software, the maximum falling slope or cooling rate can be used to define the limit of the cooling rate or specify a certain decrease in degrees per second over a given time.

Soak

Preheat and Soak are typically listed as two separate inputs in most profiling software even though they call for, more or less, the same parameters. For some engineers the terms are distinguished by process type. Preheat being used for wave soldering and soak being used for reflow.  More commonly the initial ramp from ambient is called preheat and the relatively flat section from that initial ramp to the reflow spike is called soak.

Some solder paste manufacturers will request that the profile use preheat and some will call for a soak period. These are similar inputs, if not one and the same.  In some profiling software both terms are listed separately.   Based on a review of many solder paste specifications, the soak specification is normally for a longer duration and the preheat is a shorter duration with a higher ramp rate. Again, this is defined by the solder paste manufacturer, who determines the desired specification for the intended performance of their solder paste. Component manufacturers can also call for specifications of preheat or soak with respect to their components.

Time Above Liquidus (TAL)  (Reflow)

TAL and Reflow Process are both defined in terms of temperature over a period of time in seconds.  Generally, the temperatures are ~183°C  for eutectic solder and ~ 217°C for lead-free.

Of all the inputs, this is perhaps the most important since it can be the most troublesome, especially in the world of lead-free.  Look very closely at the different package types and density of a given area of the PCB since these factor into the required time to bring a given bond pad to the desired temperature specification.  Of course, we are talking about overall density, but not everything on your PCB is going to react the same to higher temperatures. While an exposure to the higher temperatures of TAL can be destructive over time, the duration necessary to achieve effective phase changes of the solder paste is, generally, not destructive.  The key is to get in and out as quickly as possible to get the job done, while limiting the exposure to these higher temperatures.  However, if the process is repeated several times, changes do occur in the PCB and destruction will begin.  Many PCB’s will undergo both top-side and bottom-side reflow. Occasionally, a third reflow will be required to attach specific components and, of course, selective, wave soldering and rework may factor into the equation for the same PCB.  This repeated combined exposure during the TAL segment can be destructive.

Peak

Why do we want to exceed the melting point of an alloy by a range of temperatures and duration of time?  Ask your QA department since cold solder joints are one of the most common defects associated with inadequate peak temperature.   The additional increase in temperature over liquidus guarantees that high density areas will have the opportunity to flow properly, ensuring a complete process. The solder paste manufacturer lists a peak spec but the component manufacturer’s specifications can be more important. The component manufacturer’s peak spec will be a “Do Not Exceed” value, in contrast to the solder paste manufacturer’s spec that calls for a peak range.  Your job in developing the spec is to find a peak value that does not violate your component manufacturer’s tolerance still completing reflow to the satisfaction of your QA department.

Maximum Exit Temperature

This parameter has little to do with the solder paste manufacturer’s specification and more to do with a requirement of your specific process.

Two values are listed: temperature and distance. Temperature is the desired exit temperature and distance is determined by the location of the product in the oven or at the exit. The product board sensor will aid in determining how this value is calculated.

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How to Establish the “First” Profile for a New Product

autofocus

Clever profiling software will calculate a profile based on the size of your PCB or from dummy profiles already loaded on the Reflow oven. It is part of the prediction utility of your profiling software or pre-installed on your reflow oven. This software mines prior work, including your own profiles developed from other products or, in the case of profiles that come pre-installed on the oven, known profiling outcomes based on the oven’s characteristics.  It looks at known sizes of PCBs and their respected weights (i.e. mass), and how they interact with a known process environment (i.e. your specific reflow oven).  What is exciting about this software is that it can give you a pretty good starting point when you are profiling a board for the first time. It is not foolproof, but it is quite common to get a process, in spec, after a few profiling runs while keeping your profiled PCB intact and sellable!  The alternative is to spend hundreds of hours profiling new boards, with starting points based on conjecture. In this case, many boards would be sacrificed in the process.  With products like KIC Auto-FocusTM and AUTOsetTM, your time and your PCBs are saved, saving you money!

autoset

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Configuration of the Graph | Reflow Process

profilegraph

The profile graph will tell you whether or not your process is within spec and that your reflow oven is creating the correct profile, nothing more. Double check your set-up again before diving into your profile. The information from the graph must be “true.”

There are dozens of software packages that mostly render the same information, with temperature on the Y axis and time across oven zones on the X axis. These software packages vary great with respect to how they manipulate this data or allow you to play around with the numbers.

Not all of your oven set points are going to match those of your profiling software. Profiling software can do this automatically but not all oven brands communicate with all profiling software brands. In the end, you might be left with a graph that appears to be in spec but, in reality, is meaningless due to differences in the set points in the oven and in the profiling software.

To be considered relevant, ALL profiles must follow these guidelines:

  • Profiles are generated by a specification from the solder paste manufacturer, component suppliers and substrate tolerances.
  • The set points of the oven heating zones are set.
  • The oven conveyor speed is set.
  • The profiler set points are the same as the oven heating zone set points.
  • The profiler oven conveyor speed is the same as the oven.
  • The reported number of heating zones are the same as the number of oven heating zones.
  • Any change in the set points of the oven MUST be changed to the profiler software.

Any violation of these guidelines will render your profile invalid and meaningless!

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