BGA Solder Void Causes And Their Control
A solder void is a bubble of gas trapped inside a joint. In a ball grid array joint it appears on an X-ray image as a dark circle inside the bright disc of the ball, and it is one of the most frequently discussed defects in surface mount assembly. Voids form because something in the joint generated gas while the solder was molten, and since gas cannot leave a joint that is enclosed by a package on one side and a pad on the other, it stays as a cavity when the solder solidifies.
This article explains where the gas comes from, which process settings make it worse, how much voiding matters, how it is measured, and what actually reduces it.
What A Void Is And Is Not
A void is a volume of gas enclosed by solder. It is not the same as a crack, which is a separation with no material removed, and not the same as a void in the plated barrel of a through hole, although the two are often confused because both appear as dark areas on an image. The mechanical effect of a void depends on its size and position: a small bubble near the pad interface is far more significant than a larger one in the middle of the ball.
The volume of a joint is also relevant. A large ball with a void of 10 percent of its area still has a substantial cross section of sound solder, while a small joint with the same percentage may be reduced to a shell. That is why the acceptance criteria are written in terms of the projected area of the void at the interface, and why the same percentage is treated differently on different package types.
Where The Gas Comes From
The largest source is the flux in the solder paste. The flux contains solvents and activators that must volatilise and be driven off before the solder melts, and the amount that remains trapped depends on how gently the joint was heated. A profile that brings the assembly to the melting point quickly leaves the flux no time to escape, and the volatiles are released into molten solder instead.
Other sources add to it. Water absorbed in the laminate or in the package is released as steam; the plating on the pad can carry organic co-deposition that decomposes; and oxygen from the atmosphere is entrained in the paste during printing or in the solder during the wave. The oxidised powder in a paste that has been left open on the stencil is a particularly common contributor, because the oxide layer on the powder is what the flux has to reduce, and an excess consumes the activator and produces more residue.
Process Causes And Their Signatures
The reflow profile is the first thing to examine. A long soak at a temperature below the melting point allows the flux to work and the volatiles to leave, and a slow ramp to the peak allows the remaining gas to escape while the solder is still solid or just beginning to melt. A short profile with a fast ramp and a high peak produces more and larger voids, and the effect is strongest on the largest joints, which are the ones with the most flux under them.
The paste and the stencil contribute as well. A paste with a high solvent content or a high metal load behaves differently from a low residue type, and a stencil aperture that is too small for the pad leaves a gap where flux can collect and vaporise. Printing a thin deposit and reflowing it twice produces joints that void differently from a single pass, which matters on a board that is assembled on both sides.
When Voids Matter
The electrical effect of a small void is negligible. A void does not significantly raise the resistance of a joint and it does not open a connection. Its influence is mechanical and thermal: it reduces the area that carries the load, it removes a path for heat to leave the package, and it creates a stress concentration where a crack can start when the assembly is thermally cycled.
The consequence depends on the application. A joint that carries a heavy static load, such as a large inductor or a connector, is more sensitive than a signal ball on a fine pitch package, and a joint that has to conduct heat out of a power device is more sensitive than either. That is why a single acceptance limit applied to every joint on a board misleads, and why the requirement is normally written per package type and per joint function.
Measuring And Accepting Voiding
Voiding is measured by X-ray inspection, normally with a system that can rotate the view so that the interface between the ball and the pad can be seen rather than the whole ball projected onto one plane. The software reports the percentage of the joint area occupied by voiding, and the figure is compared with a limit taken from the product specification or from a standard such as IPC-7095.
The measurement has uncertainty. The threshold used to define a void, the grey level of the solder and the geometry of the joint all affect the reported percentage, so two machines can report different values for the same joint. Comparing results between suppliers without agreeing the threshold first is a common source of argument, and it is avoided by writing the method into the inspection instruction. Related inspection choices are covered under SMT component shift causes and the sequence as a whole under PCBA development process.
What Reduces It
The most effective changes are in the profile. A soak that holds the assembly just below the melting point, followed by a controlled ramp to the peak, gives the flux time to release its volatiles before the solder becomes a liquid. Extending the time above liquidus slightly allows the remaining bubbles to rise and escape, provided the peak temperature and the component ratings allow it.
The second lever is the paste and the print. A paste with a lower solvent content, a stencil that matches the pad, a print that is clean and a stencil that is wiped rather than smeared all reduce the amount of material that can become gas. Where a package is known to be difficult, some lines use a paste with a modified flux chemistry or apply the profile that the package manufacturer recommends for that body size. The choice of alloy and its melting behaviour also interacts with the profile, as described under lead free versus leaded solder.
Package And Pad Effects
The package itself sets the ceiling on how easily the gas can escape. A large body with a wide standoff leaves room around the ball and allows flux residue to vent sideways, while a fine pitch package with a low standoff encloses the joint and traps the volatiles under the body. A package with a heatsink or a stiffener on top behaves like a lid, and a joint under the centre of such a package voids more than one at the corner, which is why the distribution of voiding across a package is as informative as its average.
The pad finish and the solder mask also play a part. A pad that is smaller than the ball leaves the solder unsupported at the edge, a mask defined opening changes the shape of the joint, and a finish that wets slowly holds the solder molten for longer at the interface. None of these is a voiding mechanism on its own, but each changes the path the gas has to take, and the combination explains why two assemblies built from the same paste and profile can void differently.
FAQ
How much voiding is acceptable? It depends on the joint. Limits of 25 percent of the projected area are common for a general purpose ball, with tighter figures for thermal pads and for joints that carry mechanical load, and the requirement should come from the product specification rather than from a habit.
Does a void mean the joint will fail? Not on its own. A void reduces the margin the joint has, and it becomes significant when it is combined with a thermal cycle, a mechanical load or a second reflow.
Can voiding be reduced by reflowing twice? A second reflow can sometimes reduce the measured voiding, because the joint is molten again and the gas has another opportunity to escape, but the second cycle also grows the intermetallic layer and consumes more of the flux.
