Electronic Supply Chain Solutions shares valuable information from Bob Willis on…
Disappearing Solder with a Lead-Free Process
Question? “Have you ever wondered why you see changing volumes in your lead-free solder joints?” Although lead-free materials do have a different density they are actually talking about the loss of solder to the component terminations during reflow. During lead-free introductions it is common, but not ideal, to use tin-lead coated component termination either because nothing else is available or they have been supplied without warning. Using tin/lead terminations with a lead-free paste has been shown, in some studies, to have an impact on joint reliability and also cause secondary reflow on wave soldering resulting in lifted leads. With a lead-free process solder paste is printed on to the surface of the pads, most commonly 0.006” of paste is used. Ideally one of the alternative lead-free printed board finishes like gold, tin, silver or copper OSP is specified. Lead-free solder leveled alternative like tin/copper/nickel may also be considered to replace tin/lead and is by far the most solder able coating depending of course on thickness. Component terminations are then automatically placed into the surface of the paste; the board then passes for reflow in the normal way. During reflow the board passes through convection or vapor phase soldering (VPS) process reaching and exceeding a normal reflow temperature of 210-220°C used for tin/lead assembly. In this situation if the component terminations are tin/lead plated or dipped they will move into a liquid state and are very wettable, probably more so than most alternative PCB finishes and the alternative component termination plating like tin and palladium.
Example of tin/lead “Wicking” This resulted in an open intermittent joint.
When the lead-free solder paste reaches reflow temperature at 217-220°C the solder is far more likely to wet to the component termination particularly on leaded parts like SOIC, QFP or PLCC. The board assembly will continue to rise to 235-245°C and hold until all the terminations are above reflow for a minimum of 30 seconds. In the case of parts with leads there is a large surface area to wet. The result is “Wicking” a process defect that can occur during normal tin/lead processes when the solder ability of the PCB is poor and the component very solder able. In the past it was very likely to occur with convection and IR ovens with mainly top heating and limited bottom heating. It could also occur with mesh belt systems as the belt could hold the board temperature down allowing the component termination temperature to rise much faster. VPS originally exhibited this type of defect more commonly on batch vapor phase systems with no or poor preheat. In this case the lead terminations would reach soldering temperature much more quickly than the printed board.
Example of tin/lead paste “Wicking” on copper OSP board finish after reflow. This was due to incorrect PCB cleaning which stripped the OSP surface protecting from the pads.
Examples of “Wicking” on gold pads on a pocket pager application with tin/lead paste when the solder ability of the pads was poor.
Example of a satisfactory lead-free joint but the increased volume of solder can be seen on the surface of the tin/lead pin away from the joint area.
In the case of lead-free reflow with tin/lead component terminations the wicking process is practically the same. However, it can result in the reported disappearing solder contributing to the component coating and not joint formation. Although many people will not have seen this type of defect before it has been encountered and fairly well understood. It’s the lead-free aspect that may catch people out during manufacture and final inspection.
Three images showing tin/silver/copper paste reflowing with tin/lead component terminations exhibiting the wicking effect.
Look to Electronic Supply Chain Solutions for industry information you can use.Visit www.mymilitarycomponents.com to learn more !!!!
Disappearing Solder with a Lead-Free Process
Question? “Have you ever wondered why you see changing volumes in your lead-free solder joints?” Although lead-free materials do have a different density they are actually talking about the loss of solder to the component terminations during reflow. During lead-free introductions it is common, but not ideal, to use tin-lead coated component termination either because nothing else is available or they have been supplied without warning. Using tin/lead terminations with a lead-free paste has been shown, in some studies, to have an impact on joint reliability and also cause secondary reflow on wave soldering resulting in lifted leads. With a lead-free process solder paste is printed on to the surface of the pads, most commonly 0.006” of paste is used. Ideally one of the alternative lead-free printed board finishes like gold, tin, silver or copper OSP is specified. Lead-free solder leveled alternative like tin/copper/nickel may also be considered to replace tin/lead and is by far the most solder able coating depending of course on thickness. Component terminations are then automatically placed into the surface of the paste; the board then passes for reflow in the normal way. During reflow the board passes through convection or vapor phase soldering (VPS) process reaching and exceeding a normal reflow temperature of 210-220°C used for tin/lead assembly. In this situation if the component terminations are tin/lead plated or dipped they will move into a liquid state and are very wettable, probably more so than most alternative PCB finishes and the alternative component termination plating like tin and palladium.
Example of tin/lead “Wicking” This resulted in an open intermittent joint.
When the lead-free solder paste reaches reflow temperature at 217-220°C the solder is far more likely to wet to the component termination particularly on leaded parts like SOIC, QFP or PLCC. The board assembly will continue to rise to 235-245°C and hold until all the terminations are above reflow for a minimum of 30 seconds. In the case of parts with leads there is a large surface area to wet. The result is “Wicking” a process defect that can occur during normal tin/lead processes when the solder ability of the PCB is poor and the component very solder able. In the past it was very likely to occur with convection and IR ovens with mainly top heating and limited bottom heating. It could also occur with mesh belt systems as the belt could hold the board temperature down allowing the component termination temperature to rise much faster. VPS originally exhibited this type of defect more commonly on batch vapor phase systems with no or poor preheat. In this case the lead terminations would reach soldering temperature much more quickly than the printed board.
Example of tin/lead paste “Wicking” on copper OSP board finish after reflow. This was due to incorrect PCB cleaning which stripped the OSP surface protecting from the pads.
Examples of “Wicking” on gold pads on a pocket pager application with tin/lead paste when the solder ability of the pads was poor.
Example of a satisfactory lead-free joint but the increased volume of solder can be seen on the surface of the tin/lead pin away from the joint area.
In the case of lead-free reflow with tin/lead component terminations the wicking process is practically the same. However, it can result in the reported disappearing solder contributing to the component coating and not joint formation. Although many people will not have seen this type of defect before it has been encountered and fairly well understood. It’s the lead-free aspect that may catch people out during manufacture and final inspection.
Three images showing tin/silver/copper paste reflowing with tin/lead component terminations exhibiting the wicking effect.
Look to Electronic Supply Chain Solutions for industry information you can use.Visit www.mymilitarycomponents.com to learn more !!!!
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