|Ask the Experts|
December 14, 2020
SOT (Small Outline Transistor) Body Cracks
We have been facing an issue with SOT body cracks in our PCBA assembly line. What could be the reason behind these cracks? How we can simulate the conditions to cause the cracks?
|Expert Panel Responses|
An often overlooked but very important quality parameter in the placement process is placement force. Too much force during touch down can cause cracks in any component, but mainly smaller and more delicate body types. Too much force can be caused by any number of factors. Some are: head/nozzle/spindle spring constants incorrect for application, improperly assembled springs after PM event, force specification for machine not statistically proven, force control (if equipped) not calibrated correctly, wrong nozzle type for component size, among others.
Conditions can be simulated with a realistic force measurement test using a 3rd party tool that simulates a PCB running through machine. Placements are done like in production with tool measuring the peak placement force, and duration at 4800Hz. This many samples per second allows the user to see a very defined curve and exactly how much force is exerted by each spindle.
In addition to the force applied during placement, another factor can contribute to cracks, more specifically micro-cracks, is the amount of energy that is dissipated into the component upon initial impact. A steep impact curve causes high energy release into the component. Less impact speed creates better conditions for successful placement. The daunting problem is that micro-cracked components still present a successfully tested product at ICT and final tests, but inevitably fail in the field after an unknown time frame due to product thermal cycling. Best is the solve the problem at the line before anything leaves the factory.
This can be done with regular measurement of accuracy and force to validate machine specifications during placement and other process steps. If you don't measure, you don't know. For more information on relevant measurement applications for nearly every SMT process step, visit cetaq.com to better understand what can be measured in the quality improvement realm.
There are several unanswered questions, the main one being:
At what point in the assembly line is the cracking noticed?
If before reflow/soldering, it could be pick and place mechanical damage. Look to the machines that handle the SOT as it is being placed. It could also be cracked from the factory, although less likely.
If after reflow/solder, it could be moisture absorption causing cracking as the heated moisture expands in the solder process cracking the component's body. Make sure these parts are stored and baked (if needed) according to J-Std-033, which covers the handling and storage of Moisture Sensitive components.
If still later in the process such as in test or after additional assembly, it could be due to mechanical stress or flexing of the PCB or damaged during test such as exceeding the components electrical specs.
Senior Project Engineer
Electronic Controls Design Inc
Not knowing the process and where the cracking occurs. Check your pick and place force if using an automated machine. The pick nozzle should have some give (spring) to absorb shock to the parts and a rubber tipped end.
If post reflow oven, this could be related to moisture within the package and cracking/blowing out the package. Pre-bake the devices to the proper MSL rating and if rated at level 1, go with level 3 for moisture bake out to be safe.
Pre-inspect the parts before processing and mimic the process without mechanical contact and see if it cracks on it own.
Circuit Technology Center, Inc.
Cracking in component bodies usually occurs due to stresses that are applied to the components during the SMT process. With that said, it would be a good idea to look at the components as received to ensure they are not cracked before being used. Then I recommend inspecting the placed components (before reflow) to see if they are cracked at that point. Finally inspect the components after reflow to determine if cracking is taking place there.
If the components are not cracked after receipt but are cracked during placement, then the placement equipment should be adjusted to minimize forces applied to the component.
If the components are cracked during reflow, which is likely, then there are a couple of causes to investigate.
Cracks are mostly caused by excessive stress. Suggest to check the component thickness between different SOT vendors for any difference in height, as well check the position of backup pins if any provided and Z values mentioned in pick & place software, as it is possible Z-axis travel too far than needed to crack the SOT.
Pretty vague description of the issue. Some areas to look would be
Director of Corporate Quality Assurance
Delta Group Electronics Inc.
A lot of thinks to be checked here as the information you have provided is not very detailed:
You can always send me more details and I will try to help firstname.lastname@example.org
Engineering and Operations Management
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