Ask the Experts
January 11, 2023 - Updated
December 9, 2020 - Originally Posted

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 to better understand what can be measured in the quality improvement realm.

Michael Sivigny
General Manager
CeTaQ Americas
As General Manager at CeTaQ Americas, Michael has grown the company from scratch in the Americas market since 2001. The CeTaQ global brand is known for 3rd party objective measurement solutions. Michael lives in Manchester, NH with his family.

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.

Paul Austen
Senior Project Engineer
Electronic Controls Design Inc
Paul been with Electronic Controls Design Inc. (ECD) in Milwaukie, Oregon for over 39 years as a Senior Project Engineer. He has seen and worked with the electronic manufacturing industry from many points of view, including: technician, engineer, manufacture, and customer. His focus has been the design and application of measurement tools used to improve manufacturing thermal processes and well as moisture sensitive component storage solutions.

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.

David Cormier
Engineering Manager
Circuit Technology Center, Inc.
Manufacturing Engineer of 20+ years. Involved in Industries relating to all sectors of defense, Commercial product Industries, RF - Microwave and Semiconductor industries. Vast knowledge and experience relating Mil-STD’s, IPC-STD’s, EAI-STD’s, GEIA-STD’s, J-STD’s and MIL-PRF-STD’s.

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.
  1. If the components are taking on moisture, then rapid expansion of moisture vapor from the component can cause cracking during reflow. This can be solved by drying the components prior to use in a suitable drying process. It would also be beneficial to ensure that components are stored in sealed packages and in a drying cabinet.
  2. Differences in coefficient of thermal expansion between the component bodies and the circuit board materials can cause expansion and contraction stresses, which may lead to cracks. This can be minimized by using a slower ramp up rate and a slower cooling rate in the reflow profile. Also the use of a soak type profile, rather than a linear ramp to peak profile may help.

Tony Lentz
Field Applications
FCT Assembly
Tony has worked in the electronics industry since 1994. He worked as a process engineer at a circuit board manufacturer for 5 years. Since 1999, Tony has worked for FCT Companies as a laboratory manager, facility manager, and most recently a field application engineer. He has extensive experience doing research and development, quality control, and technical service with products used to manufacture and assemble printed circuit boards. He holds B.S. and M.B.S. degrees in Chemistry.

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.

Swaroop Pawar
PCBA Industrialization
Schneider Electric
Have 18 years of experience in electronic Industrialization. Specialties in PCB Design & manufacturing process, PCBA Process Development and Continuous Improvement.

Pretty vague description of the issue. Some areas to look would be
  • moisture if the boards are going through a reflow process or if they are being heated in a manner that could cause absorbed moisture to expand quickly
  • stand up racks if the cracking is only happening to parts on the edge of the boards. We have pretty much eliminated all of the standing racks throughout due to this issue
  • other excessive strain such as de-panel or equipment clamping.
Not a lot to go on with the description, but these are some areas to look.

Tod Cummins
Director of Corporate Quality Assurance
Delta Group Electronics Inc.
Tod has been working in the Aerospace Electronics Industry for 25 years, beginning with 4+ years working for PCB fabricator ending as the Quality Manager and 20 years with Delta Group Electronics Inc. an AS9100 registered electronics contract manufacture. Currently position is Director of Corporate Quality Assurance.

A lot of thinks to be checked here as the information you have provided is not very detailed:
  • component prior to pick and place - check for physical damage, expiration and MSD level (if applicable)
  • pick and place - tool / machine, board supports/flatness
  • reflow profile, temperature and speed settings
  • time to cool down after reflow (should you wash the board) - thermal shock can create the physical damage
  • processes after smt: TH parts inserters (automatic or manual), wave soldering, selective or manual soldering)
  • handling

You can always send me more details and I will try to help

Georgian Simion
Engineering and Operations Management
Independent Consultant
Georgian Simion is an independent consultant with 20+ years in electronics manufacturing engineering and operations.
Contact me at

All of the experts have chimed in with good information on placement pressure, reflow profiles, etc. But I just wanted to point out that typically SOT-21 and SOT-23 SMT transistor packages, as well as many other plastic SOICs, etc., are prone to minor cracking along the mold release line where the leads exit the body, and in most cases are ACCEPTABLE per the MIL-STD applicable to the component type and may not require rework. The Quality or Components Engineer should have this info and should always be notified if component defects of this type are seen. Doing so can prevent many hours of rework and unnecessary component replacement, only to have to do it again later.

Richard D. Stadem
Advanced Engineer/Scientist
General Dynamics
Richard D. Stadem is an advanced engineer/scientist for General Dynamics and is also a consulting engineer for other companies. He has 38 years of engineering experience having worked for Honeywell, ADC, Pemstar (now Benchmark), Analog Technologies, and General Dynamics.
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