A quality solder fill on the topside of your PCB is a reward achieved by properly combining the necessary ingredients of the soldering operation.

To better understand the problem, let me first address “Prevailing Wisdom”. Prevailing wisdom is a one-liner that resonates with the masses. A classic example would be, “Get ‘er Done”! The prevailing wisdom for poor hole-fill is, “Heat ‘er Longer”! This is usually the wrong approach. To achieve proper hole fill, you must have the following: a solderable surface, a properly plated thru-hole with adequate copper thickness and no voiding, activated flux, virgin solder, a pin-to-pad ratio greater than 0.666 and, and a top side pad temperature 15–20 0C greater than the melt point of the solder..

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Top-side pad temperature is where prevailing wisdom — “Heat ‘er Longer” — enters the picture. You must supply enough heat to achieve the top-side temperature, but you must also stay within the guidelines of time-on-joint (discussed in other articles). Simply heating the joint longer than the recommended TOJ results in a series of catastrophic consequences.

Among those consequences are pad liftage, inner-layer separation, excess intermetallic formation at the heat entry point (bottom-side pad), flux starvation, and (perhaps) component destruction. If you do no achieve a good top-side solder fill within the recommended maximum time-on-joint, you have a discrepancy in one of the key ingredients listed above.

I’ll address each of these in future comments, but the best place to start, is to question the copper thickness in the barrel of the hole. Therefore, the correct wisdom” is, “Heat ’er Right”.

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