What is the likely cause of this problem and are there ways we can overcome it?

Answer: Most times this is related to the components floating on the molten solder and the equipment having some vibrations, which could impact the locational position of those components. The vibration in a reflow oven may not be felt by the people in the area, but could be within the machine, or caused by a truck going by outside the building or a piece of equipment moving around within the building. Although the components are moving from their placed position, there was no mention as to whether they moved to a position where they were not acceptable, such as defined in J-STD-001 for Class 2 and 3 assemblies. If the movement did not cause them to be rejectable, then there is nothing to worry about. If the movement results in a defect condition, then further steps need to be taken to address this process problem.

The other issues to consider are the amount of solder paste deposited on the pads and the size of the pads or land areas. If the pads or land areas are too large, then excess solder will be deposited, which will allow the component to float and move around, so these condition need to be checked. The paste deposition should be around 6 mils or less for small chip component and the pads just slightly larger than the component termination.

There are lots of unknowns in the question, so I’ve answered it as generally as can be answer with the information provided.

Have you ever wondered if you have created the perfect solder joint, or for that matter one that is strong enough for the particular application it is being used in? Sure we can pull test and stress the solder joint, but these things don’t fail that way. They fail by many years of fatigue, or more importantly, by stress fractures occurring over years of cycling in the environment. So after all these calculations, are we even close to getting an idea as to whether or not, we created the right joint for the design and use? Take a moment to spend some time with us as we review some of the fundamentals and formulas that help us at least get in a comfortable ballpark for these predictions.

Topics we will be reviewing are:

• Solder Joint Creation 101 for Through Hole Components.
• The three structural components of the THT solder joint.
• Applicable formulas for THT designs.
• Examples of these formulas in action.

Part 2 in this series, has everything to do with Surface Mount Technology, or more importantly surface mount components. In the previous discussion we explained that the majority of these solder joints fail by many years of fatigue, or more importantly, by stress fractures occurring over years of cycling in the environment. We continue with this series by reviewing the soldering process as it relates to SMT assemblies and how we can best interpret formulas and calculations that lead us in a direction to better understand, or predict, how a solder joint will fair after many years of service and exposure in their operating environment. Join us again as we finish this two part series and discuss how this all relates to surface mount component assemblies.

Topics we will be reviewing are:

• Solder Joint Creation 101 for Surface Mount Components.
• The three structural components of the SMT solder joint.
• Applicable formulas for SMT designs.
• Examples of these formulas in action.

Answer: I’ve not heard of this condition, but I would suspect the initial plating on the component terminations. If this coating is oxidized by going through the surface mount process, it could change the color of the terminations.

A question, however; is whether or not it is just the component termination turning color or the total solder joint including the component termination and the solder which is turning color. If it is the entire joint, I would look at the activity of the flux and whether or not the fluxes have changed and the vapors from the flux during the reflow operation are tarnishing the metal surfaces.

To fully answer this question, we would need an investigation of the component termination metallization, the type of solder alloy being used, the manufacturer of the solder and the type of flux base material being used in the solder paste. This could provide some information to proceed in the solving of that problem.

Answer: If you check J-STD-002, you will see the methods to check for solderability of those components. Basically they have to be introduced to the solder pot at an angle and then removed from the solder with a sweeping motion to prevent icicles from forming on the termination pad. This can be a very time consuming process if there is a large quantity of components.

I would suggest finding a company that tins components as their business as they would automate this process and provide you with the results you are looking for on those bottom only terminations and castellations.