Blowholes in Pb-free wave soldering: 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 Outgassing (sometimes called "Offgassing," particularly when in reference to indoor air quality) is the slow release of a gas that was trapped, frozen, absorbed or adsorbed in some material. 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 Melting temperature may refer to:
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 thermal expansion Increase in volume of a material as its temperature is increased, usually expressed as a fractional change in dimensions per unit temperature change. between the board material (in most cases epoxy epoxy Any of a class of thermosetting polymers, polyethers built up from monomers with an ether group that takes the form of a three-membered epoxide ring. The familiar two-part epoxy adhesives consist of a resin with epoxide rings at the ends of its molecules and a curing 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 thermal energy Internal energy of a system in thermodynamic equilibrium (see thermodynamics) by virtue of its temperature. A hot body has more thermal energy than a similar cold body, but a large tub of cold water may have more thermal energy than a cup of boiling rather quickly. Also, the surface at the underside of the joint exposed to ambient temperature Outside temperature at any given altitude, preferably expressed in degrees centigrade. 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 evaporated reduced in volume by evaporation; concentrated to a denser form. 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 out·gas v. out·gassed, out·gas·sing, out·gas·ses v.tr. To remove embedded gas from (a solid), as by heating or reducing the pressure. v.intr. To lose gas, as from a solid. or evaporate e·vap·o·rate v. 1. To convert or change into a vapor; volatilize. 2. To produce vapor. 3. To draw or pass off in the form of vapor. 4. during soldering and cause a blowhole blowhole the anterior nares of whales and dolphins. . 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 en·cap·su·late v. 1. To form a capsule or sheath around. 2. To become encapsulated. en·cap the flux that has a much lower density. Gert Schouten is a senior engineer at Vitronics Soltec (vitronicssoltec.com); gschouten@vsww.com. [ILLUSTRATION OMITTED] |
|
||||||||||||||||
Printer friendly
Cite/link
Email
Feedback
Reader Opinion