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.
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.
