Solder Limits: Updates for the Age of Surface Mount

Estimated reading time: 8 minutes

Solder Limits Defined

Solder limits are one of the fundamental parameters used when evaluating the PCB, solder resists, and metal-clad base materials for safety under the UL Recognition programme.  

Solder limits are designed to represent the soldering processes the PCB will be exposed to during component assembly operations and take into consideration any time that is spent over 100°C or the maximum operating temperature (MOT) of the PCB, whichever is greater. The only exception is hand soldering operations that do not need to be captured. Solder limits can be a single time and temperature, such as 288°C for 20 seconds, or multiple solder limits (MSL) that include multiple times and temperatures, including ambient periods to represent time between different soldering operations. Solder limits are used as part of the thermal shock procedure prior to many of the tests employed to evaluate the PCB for safety.

Interpreting Solder Limits

As previously mentioned, solder limits consider any time spent over 100°C or the maximum operating temperature (MOT) of the PCB, whichever is greater. To determine if the Recognized Solder Limits are being exceeded, we need to understand the soldering operations the PCB will be exposed to and the thermal profiles associated with these. We then use these thermal profiles to measure the time above the critical temperature.

Figure 1 shows a generic surface mount (SMT) type soldering profile; we can use this to determine the time spent over either 100°C or the MOT Recognized for the PCB. If the board has a Flame-Only Recognition then we determine any time spent over the 100°C line. For a Full Recognition PCB, it is any time spent over the MOT line.

Figure 1: Generic SMT profile showing how to measure the time for the solder limits parameter.

When you look at the total time above the line you can see why using multiple solder limits with different times at different temperatures becomes more appropriate for soldering profiles of this type, otherwise you would be looking at a long period of time at a high temperature.

The way we deem compliance with the solder limits can be seen in Figure 2; this provides an example for a Flame-Only Recognized PCB with solder limits of 288°C for 20 seconds. If the t₁ to t₂ period is greater than 20 seconds, then the solder limits have been exceeded and the UL Recognition of the PCB has been invalidated. In Figure 3, we can see the same evaluation being made but for a PCB Recognized with an MOT of 130°C.

Figure 2: Determining compliance of a Flame-Only Recognized PCB with solder limits of 288°C for 20 seconds.

Figure 3: Determining compliance of a Full Recognition PCB with a 130°C MOT and with solder limits of 288°C for 20 seconds.

If a PCB is Recognized with multiple solder limits (MSL), an example shown in Figure 4, then the PCB can be exposed to temperatures between 100°C and the temperatures detailed for the time shown; these are in addition to each other and not alternatives to one another. For the example shown, this means 10800 seconds between >100°C and ≤180°C plus 80 seconds between >100°C and ≤230°C plus 10 seconds between >100°C and ≤260°C plus a minimum of 300 seconds at ambient plus 10 seconds between >100°C and ≤260°C.  

Figure 4: Example of a Flame-Only Recognized PCB with multiple solder limits.

Why are the Solder Limits So Important?

The IPC D-32 Thermal Stress task group have conducted research that has shown that PCBs that pass a solder float test can fail during surface mount assembly soldering operations, which is not surprising to those of us in the PCB industry. We have been aware for a long time that the more severe the soldering operations are the greater the degradation of the PCB. The increased degradation impacts not just the reliability properties of the PCB but also the properties we evaluate for safety.

For UL to conduct an accurate safety assessment of the PCB we must use solder limits that are representative for the actual soldering processes the PCB will see during assembly operations; if we use inadequate solder limits during the testing than are intended for use in production then it invalidates the safety testing that has been conducted on that PCB, as we cannot be confident that the PCB will behave the same after being exposed to these more severe conditions.

What UL is Doing to Help the Industry

One of the main things we get told at UL, when it comes to the PCB manufacturer selecting the solder limits they want to use for Recognizing their PCB in combination with, is that they do not know what soldering profiles their customers will use. Coming to UL from a Tier 1 automotive electronics manufacturer, I completely appreciate this point. I never told my PCB suppliers what soldering profiles I was using on my boards and I was just one of many customers to that PCB company. What UL is doing is offering some standardised soldering profiles for the PCB manufacturers to use for Recognition purposes.

We are taking the IPC-TM650 2.6.27 T230 and T260 soldering profiles (Figures 5 and 6) and offering these as an option for the PCB manufacturers to use. These will only ever be optional and we will not force anyone to go down a specific route. We are also happy to use any other reflow profile a PCB manufacturer requests to be used.

UL is recommending a minimum of three reflow cycles, but the PCB manufacturer needs to understand the maximum number of cycles their customers may need. Recent suggestions of six cycles have been made by one contract assembler!

We are endeavouring to add these IPC T230 and T260 soldering profiles to UL 796—the standard used to assess PCBs—but this is not as a requirement for them to be used by manufacturers; it would only be as a more accessible guideline/option. UL does not control what goes into the UL standards; this is done through a consensus process where UL has only a single vote on the Standards Technical Panel (STP); we have been trying to add these profiles to the standard for some time but hope that the STP will see the value in doing this now and that they will enter the standard shortly.

UL is very open to having other standardised reflow profiles added to the standard and are happy to receive suggestions on this. It is not intended that UL will create its own reflow profiles.

Figure 5: IPC TM-650 2.6.27 T230 Soldering Profile

Figure 6: IPC TM-650 2.6.27 T260 Soldering Profile

How to Update Solder Limits

The solder limits are used as part of the test procedure for the three primary tests of bond strength, delamination, and flammability and, as such, increasing the severity of the solder limits for an existing PCB type will involve testing using these revised limits. They are used in some other construction-specific type tests too, such as conductive paste adhesion, so these would also need to be considered, but for the vast majority of boards Recognized with UL it will be the three primary tests that need to be considered.

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