Even for humidity-soaked boards, the culprit is barrel cracking.

Blowholes in through-hole joints are the result of barrel cracking in combination with outgassing humidity from the board material. Because of the higher process temperatures required for Pb-free soldering, the risk of barrel cracking increases.

During the soldering process, the board reaches high temperatures, and the areas directly adjacent to the through-hole joints come close to the melting temperature of the solder. This temperature of 217°C for SAC solder is well above the boiling temperature of water. So, if traces of humidity are present in the surrounding material, they may convert to steam with a locally high pressure.

In a well-soldered joint, the solder has wicked up at least to the upper edge of the top solder pad. This means that the temperature at this spot has been at least at the melting temperature of the solder alloy. The moment that the board leaves the solder wave, heat transfer from the wave to the board stops. The heat flow to the joint surrounding areas, however, will continue to increase for a while.

This is because the solidification energy from the solder in the joint will transfer its heat to the surrounding material. Only after the solder is completely solidified and the solidification energy dispersed will the temperature of the joint and its surroundings drop.

As a result of the large difference in thermal expansion between the board material (in most cases epoxy FR-4) and the copper barrel, there is a big tensile load on the copper barrel during this process. If the copper barrel plating is not able to withstand this load, it can crack or tear. Often, failures in the drilling or hole plating process are responsible for an improper barrel thickness. Such boards are more vulnerable for the barrel-cracking phenomenon.

If the barrel cracks, the encapsulated humidity in the base material may escape via this damaged barrel into the liquid solder in the joint. Note that this process will still go on when the board has left the wave because the solder in the joint is, at that point, changing from liquid to solid and still well above the boiling temperature of water; at the same time, heat is emitted to the joint surroundings, since the board material is outgassing.

During the solidification of a joint, the upper part normally solidifies first. This is the coldest part because the lead and component body will absorb the thermal energy rather quickly. Also, the surface at the underside of the joint exposed to ambient temperature may form a skin of solidified solder, while the remaining solder in between will solidify last.  

Often, the natural shrinking of holes is found in that part of the solder. These holes are a result of the 4% volume reduction that occurs when the solder goes from a molten state to a solidified state.

When outgassing is still active at the moment that the joint already has a solidified skin at the solder side, these escaping gases can create enough pressure to blow the skin open. This is clear evidence of the outgassing that creates such blowholes.

One must keep in mind that the cracked barrel is the root of this problem. Even when a board is soaked with humidity, the evaporated vapor can never enter a solder joint if the barrel is undamaged. Vapor will not pass through a closed metal shield such as a sound barrel.

Dirt or debris adhering to a lead – e.g., in the lead plating or skips of dirt in the joint gap – may also be a source for gas exposure, provided this dirt is encapsulated in the joint. This material may outgas or evaporate during soldering and cause a blowhole. More often, this may create a number of pores in the joint instead of a blowhole.

Flux cannot be the cause because flux cannot be encapsulated in a wave-soldered joint.

As soon as solder wets the component lead and hole wall (barrel), there is no flux on these parts. The rising solder on the lead and the hole wall will flush the flux from these surfaces once wetted by solder. As solder rises into the joint gap, this flux will float on the rising solder column and has no possibility of becoming encapsulated by the molten solder. The flux will move faster than the rising solder and moves in front of the solder column that fills the joint. As long as the solder rises in the joint, the solder is liquid and is not able to encapsulate the flux that has a much lower density.

Gert Schouten is a senior engineer at Vitronics Soltec (vitronicssoltec.com); gschouten@vsww.com.

• Prev
• Next