February 3, 2010 - Updated
February 1, 2010 - Originally Posted
Long Term Concerns with Disturbed Solder Joints
What are the short term and long term defects that may result from a disturbed solder joint? I know that multiple reheating of a solder joint can degrade the solder joint reliability. Might we be better off leaving disturbed solder joints as they are, or should they be reworked?
E. D.
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This policy, is pending the PCB Solder finish. Eutectic Solder, itself, will not be affected by heat cycling performed during rework. This would apply to 63Sn/37Pb solder. Grain structure, upon cooling rate might differ, but solder should be reliable. RoHS Solder Alloys are non-eutectic, and will have different Alpha/Beta composition during reflow. This coupled with unstable solder finish like OSP, White Tin or ENIG can be affected by heat cycling and change the disposition, or mechanical properties of the solder joint system. This is due to the Solderfinish/Solder Joint interface and the metallurgical changes which are occuring at this interface. Other considerations in the solder joint heatcycling and reliability would be laminate CTE and the method/control of heatcycling, and reflow temperature required per the alloy used. I would suggest, based upon the systems employed that some HALT testing be performed based on different heat cycles performed.
Rodney Miller
Capital Equipment Operations Manager, Specialty Coating Systems
Rodney is currently Operations manager at SCS coatings, Global Leader in Parylene and Liquid Coating equipment. Rodney applies his BS in Computer Integrated Manufacturing from Purdue University, along with 20+ years of Electronic manufacturing and Equipment Assembly, to direct the Equipment business at SCS Coatings. "We provide unique, value added coating equipment solutions for our customers". Including conformal, spin and Parylene coating expertise.
Definition of a disturbed joint is one which has moved during solidification. This produces a distorted structure with lines of dislocation within it. Are you sure this is what you have since they are not that common? The non-homogenous microstructure would, in my experience, have a detrimental effect on reliability by providing pathways for solder joint cracks to propogate under thermal or mechanical stresses. These would definitely require touching up and you should find the cause and eliminate it. If the joints in question are "grainy" as opposed to disturbed the picture is different since grainieness is less of a concern. Slow cooling during solidification will cause grain growth and this may be design related in terms of the thermal mass of the component and/or assembly.
Bryan Kerr
Principal Engineer - CMA Lab, BAE Systems
Bryan Kerr has 35 years experience in providing technical support to PEC assembly manufacturing. His experience ranges from analysis of materials and components to troubleshooting and optimizing, selecting reflow, cleaning, coating and other associated processes.
The answer depends on the type of component, the severity of the disturbed joint, and the potential impact to adjacent solder joints. There are two potential issues with disturbed solder joints.The first is related to the number of heating cycles.Prolonged exposure to elevated temperature will affect solder joint quality primarily due to growth of intermetallic compounds at the joint boundary. Increased intermetallics will weaken the joint and eventually render the contact insolderable. Exposure with rapidheating/cooling also can alter the grain structure of the joint, causing embrittlement. The board and components may show signs of degradation including weakening of the substrate and loss of adhesion on traces and pads. The second issue is the stress that may be introduced into solder joints. There is far less concern with introducing stress into simple discrete components. However, area-array components such as BGA, CSP, and flip shipare another story. When performing rework, it is very important to avoid secondary reflow of these types of components. If secondary reflow cannot be avoided, it is far better to take the entire component through reflow. Partial reflow of an area-array component can result in high levels of stress in the solder joints with either immediate failure or significantly shortened life. So if the disturbed joint is in an area array-device, rework is highly recommended (keeping in mind the need to protect adjacent components). If the disturbed joint is in a discrete device, the decision may come down to what impact the rework will have on the rest of the assembly. Either way, keep in mind the general guidelines for maximum number of rework cycles on your product type.
Don Naugler
General Manager, VJ Technologies, Inc.
Don is the General Manager of VJ Technologies, Inc., a leading manufacturer of X-ray Inspection and Rework equipment for the electronics manufacturing industry. He has more than 20 years experience in development, manufacturing, and support of a wide range of capital equipment.
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