Introduction
During a practical case we were asked how to optimize the process for a board that showed no complete hole fill for some joints. Whatever we tried, still these joints could not be filled with solder to the topside of the board.
Some samples where provided for further analyzing the problem.
Problem
Some components that were soldered in a selective pallet did not show a full rise of the solder to the component side of the board.
Provided that the board was well fluxed and the solderability of the components was in accordance with the demands; the problem is related to thermal aspects.
Related to thermal aspects are:
1. The layout of the board
2. The specific soldering distance
3. The preheat temperature of the board
4. The solder temperature
5. The dwell time in the wave
6. The accessibility of the joint area for the solder
Ad 1. The thermal layout of the solder joint is related to the component as well as to the board layout. If the component body or the hole barrel adsorbs too much heat during the process, the liquid solder will solidify before it could fill the hole completely. If the barrel ends in a massive copper clad it will be almost impossible to get a good hole filling.
See chapter 3 Fig. 3.30 Soldering in Electronics SE, by R.J. Klein Wassink.
Ad 2. To make a reliable solder joint within the specified process settings both the surface solderability and the thermal solderability must be in accordance with the process. For some components, e.g. can-capacitors this means that they should be mounted on a spacer to provide the correct specific soldering distance. If that distance is not respected, the lead may transfer so much thermal energy to the component body during the soldering process that the solder will solidify before the joint is completely filled with solder. This effect blocks the flow of the solder even with ideal surface solderability.
For the definition of the specific soldering distance we refer to chapter 3.4.1 of the book Soldering in Electronics SE, by R.J. Klein Wassink.
Ad 3. The necessary preheat temperature is in most cases depending on the directives for the applied flux. The flux supplier demands a certain preheat temperature for the flux for optimal functionality. For boards with a poor thermal layout topside heating is often recommended. However often the components with these poor filled holes will prevent this energy to come to the joints underneath the component bodies where this is needed most.
In the case where a shielding pallet is used, such as in this case, the preheat energy is mostly adsorbed in the pallet and not in the board.
Ad 4. When pallets are used one needs high wave pressures, thus high wave settings, to get the solder to the joints. As a result of that more dross will be generated. To reduce the dross often a low solder temperature is used. In the case of joints with a poor thermal design, such as holes ending in massive copper clad without thermal adaptation, one needs a high solder temperature to assist the solder flow as much as possible. One has to realize that the solder will stop flowing when the temperature in the joint is below 183°C, which is the solidification temperature of eutectic tin-lead solder. For SAC alloys the solidification starts already at 217°C
Ad 5. The dwell time in combination with the solder temperature is a process parameter that must be sufficient to give a sound joint, provided that the joint design is good. A longer dwell time will increase the thermal energy and may so assist in a better solder filling. A higher solder temperature will however provide a better solution to keep the solder in the joint liquid.
On the other hand longer dwell times and higher solder temperatures will exhaust the flux more rapidly. This might give an adverse effect on the solderability/wettability.
Ad 6. The accessibility of the joint area for the solder is restricted by the use of the shielding pallet. Due to the natural shape that the solder will have when flowing in a non-wettable small gap, the solder has a tendency to repel or withdraw from gaps that are too small for the solder to access the joints. Also due to the curved shape of the solder surface the edges of a gap will not be filled with solder unless a very high wave pressure is used. Even when these gaps are tapered with a wedge shaped aperture entry that is absolutely necessary, the solder has no optimal access to the joints inside such apertures
As a result of this mechanism, the contact time with the solder joints inside such a gap will be far more shorter than the theoretical calculated contact time. As a result of that the heat transfer from the solderwave will be far less or even insufficient to make a sound joint.
Conclusions
• The solution of the problem lies first in the design of the board. Larger holes and the correct thermal adaptation to prevent the heatsink effect of the massive copper clad will help in this respect.
• Also the specific soldering distance should be used for the components involved.
• Due to the pallet design some joints have a poor accessibility for the solderwave. This may enforce the effects that we already have as a result of the marginal board layout.
• The use of a selective soldering machine will give these joints the best accessibility.
• If the board layout can not be changed, only some improvement can be expected by using a higher solder temperature or a longer dwell time. Other joints will however increase in temperature as well. Care must be taken not to give a too high thermal load to sensitive components.