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.

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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

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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!

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Configuration of the Graph | Reflow Process

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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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Setting up your Reflow Oven

All modern profilers include an oven set-up tool. Your profiler must be mapped to the environment of your reflow oven in order to provide an accurate profile. If you have any built in offsets for problematic zones or any other variables that make your oven truly unique, your profiler has no idea how many zones it is looking at. In my experience, I have yet to see any two reflow ovens create EXACTLY the same profile due to the variability of exhaust, oven controls, TCs, etc…reflowoven

Some profiling software packages attempt to make your life easier with a pre-loaded oven library, but for the reasons discussed in prior blogs, it is still likely you will need to make further modifications.  Some profiling software packages will automate the process of mapping the profiler to the oven. For example, the KIC system will sacrifice channel one of the profiler to act as an on/off trigger as well as a sensor for determining its position relative to oven zones.  This eliminates the need to manually trigger the profile run and take physical measurements of each discrete heating zone making your job even easier! Easier still is that setting up your oven only needs to be done once unless, of course, you want to make changes to your original settings.

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Components

Technician fitting a processor

Paste specs will typically exceed those of components, therefore, much thought is typically not given to component specs until they are literally flying off your PCBs.  Like PCBs, the material of the components plays a role.  I’ve seen more time and effort spent on profiling ceramic packages than I care to remember. When you have a capacitor close to a large ceramic BGA, how do you get the BGA to reflow and still keep the capacitor from looking like popcorn?  One common technique is to isolate each component with its own individual process spec and then run this new setting through the profiling software prediction, as discussed in this prior.  The profiling software will tell you if it is possible to achieve what you want with your given board, paste and oven. Solutions to shield sensitive components and a complete re-design may also be in order. Additionally, the solution might be that you need new equipment that can follow the new process recommendations, allowing for tighter controls and/or more zones.  Your profiler will calculate the possibilities.

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Factoring in Length, Width and Mass

lengthwidthmassWhile most individuals understand that larger boards will require additional temperature in the oven recipe, several other parts play an equal, but less obvious part, in the reflow process. The number of components and materials used, including connective material all factor into the profile. While it takes more energy to raise a dense part one degree, it also takes more energy to preheat the surrounding area to a temperature that will allow the solder joint (the area of concern) to reach the desired temperature.

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Setting up your Solder Spec

Many profiling software packages will have a pre-loaded library of solder specs. You should still refer to the solder paste manufacturer’s data sheet to confirm accuracy, but these libraries are becoming ever more accurate and robust. Providers, such as KIC, offer regular updates online for download.


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Solder Paste Library from KIC® 2000 software.

Once you input your solder paste spec, you can use the default setting or further refine your solder spec to reflect limitations to component temperatures and change other parameters as well.

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You can further refine any of your paste specs to compensate for component tolerances and other inputs specific to your process.

For example, your paste may allow a certain maximum peak temperature, but knowing that one of your components cannot tolerate such a high temperature, you can manually change a peak of 245°C to the component’s tolerance of 243°C.

Many profiling software packages also allow you to define specific segments of your profile.  You may, for example, have more than one rising slope.

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Many inputs can be defined for your profile.

Lastly, you may need to define component or density-specific regions of your PCB.  You might first define a general profile for the entire PCB and then pull out focused areas on the PCB with tighter or expanded specs.  For example, you may need to tighten your temperature tolerance for ceramic packages. Tighten the specs around a troubled area, while loosening them elsewhere.  After all, your goal is to produce a PCB in spec.  Some profiling software packages can help you target these areas, even TC-specific.

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Each TC can have its own individual spec, which is common when you have a component which high heat tolerance issues.

You can eliminate wasted time and effort by simply running “what if” scenarios.  In the case of PWI-driven software, a single value is assigned to your user-defined inputs that indicates if a process and/or component are within spec. By tracking these PWI values alone, instead of trying to interpret hundreds of data points, a multi-variant statistical relationship is created for all inputs. A single value, or PWI, is outputted. The bottom line is that you will get your process within spec, with the least amount of effort!

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