|Ask the Experts|
July 17, 2019
Suggested Limit for PCBA Heat Cycles
What is the suggested limit for the total number heating cycles that can be done on one PCB assembly? Our PCBs contain BGAs, leadless components and large fine pitch components.
|Expert Panel Responses|
Most enterprise and telecom companies require the ability to withstand six thermal cycles (two reflows, wave, and three reworks). Note, this does not take into account copper dissolution.
CEO & Managing Partner
I would "suggest" no more than two heating cycles, to solder reflow temperature. Most assemblies can be soldered in one pass through reflow or wave soldering. Some require two, if components are applied to both sides and you do not wish to "glue" the parts on the bottom side. And this reserves a couple of cycles which may need to be done locally on the board, should it need to be reworked.
Each time you heat the assembly you stress ALL the components: the board, the solder joints and all the components. Check the specifications of ALL the components of your assembly, the small passives as well as the large active components, to see what time above temperature limit they recommend, AKA: the Process Sensitivity Level (PSL), see IPC J-STD-075.
Remember, an entire assembly can fail (or be compromised) when any single component is damaged, no matter how small or large. Some simple passive components may have a lower time over temperature exposure limit then some large active components. So take all possible measures to limit the number of cycles. And also, take the time to measure your soldering processes using a thermal profiler to prove you have not exceeded the component's limits.
Senior Project Engineer
Electronic Controls Design Inc
There cannot be a unconditional defined number of heating cycle that be be permitted for rework on PCB. This has to be based on qualification of the PCB for rework cycle. This test involved physical soldering and de-soldering and ensuring that a minimum number of re-work cycle can be permitted.
If I am not wrong this should be greater than five and largely depends on pad size. Larger the pad more cycles and smaller the pad it is less and there is likely chance of delamination at fewer cycle. Conclusion, experiment and come out what is right for your board.
Astra Microwave Products, Hyderabad, AP India
The answer is "it depends." We can think about rework cycles as taking up some portion of the useful thermo-mechanical life of the PWB. The amount of the total available that's consumed in a single rework cycle can vary widely due to PWB design and materials. Thick, high-layer-count PWBs with high aspect ratio through vias can fail after only a few soldering cycles if the materials of construction are not robust.
In general, for PWBs that are well designed with appropriate material specifications, we typically allow three reworks that re-heat the same area of the PWB. This is for aerospace applications, and we have validated that the amount of life reduction is acceptable for our designs under these circumstances. Keep in mind that a single rework operation often involves two heating cycles of the PWB (component removal, component replacement). We count the two cycles combined as one rework.
When assessing what is appropriate for your specific circumstances, you also need to take into account the number of soldering cycles that the PWB sees during normal assembly. This can range from two to four or even more, depending on process flow.
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