2005 Articles

Optimized thermal design lies in board layout and placement.

To achieve good hole-fill on through-hole solder joints with protruding leads, surface and thermal solderability – and the specific soldering distance – must be within process specifications.

Board design often has a major effect on the thermal solderability of certain joints. If the correct design rules regarding this variable are not observed, it will not be possible for solder fill to reach the top of the board surface with the given process settings. Thus, a second soldering pass will generally not improve the results.

Reasons for insufficient hole-fill are usually design-related, and thus limited to a number of specific solder joints. Occasionally, poor solderability can be responsible, but in such instances poor hole-filling would be more widespread across the assembly.

Insufficient fluxing or flux activity is the most common cause of poor hole-filling. If this problem is evident, most joints on a board will show this effect. Insufficient contact with the solder wave may also cause poor hole-fill.

Once a soldered board is discovered to have insufficiently filled holes, it is difficult to correct the problem by re-soldering for many reasons.

First, re-soldering will not improve flux insufficiency unless extra flux is applied to the top-side of the solder joint. This reduces the oxides that inhibited good solder flow during the first soldering operation.

Second, if the incomplete hole fill was due to lack of thermal solderability, then re-soldering will not help unless a higher process temperature is employed. Most of the heat required to melt a small volume of solder goes not toward raising the solder temperature to its melting point but rather to apply its latent heat of fusion to get that solder fully molten again. Because solder that filled the joint during the first soldering operation will transfer the applied heat directly to the connecting metal parts, it will take more local energy to re-melt the joint than was required during the original joint formation.

Only if the solder in the joint has completely re-melted, and new flux and solder been added to facilitate wetting, can the solder wick to the top of the board during a second-pass or re-soldering operation. However, the process is generally not recommended as it usually applies a higher thermal load than is permitted or advisable for the components and board.

Check the settings. How to solve the problem? Check the soldering process settings and correct them if necessary. If a higher thermal load is permissible, change the process settings so that the joints will solder properly and possibly fulfill the solder rise quality requirements within these new process boundaries on the first pass. Do it right the first time.

The best solution when dealing with a poor thermal design lies in the optimization of the board design and the correct mounting of components.

With the advent of Pb-free solders and their associated higher melting points, new rules for determining solderability aspects and specific soldering distance must be redefined for individual applications.

Gerjan Diepstraten is a senior process engineer with Vitronics Soltec BV (vitronics-soltec.com); gdiepstraten@nl.vitronics-soltec.com. His column appears monthly.

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