BGA and CSP Voids

By Akber Roy - Chief Technical Officer at RUSH PCB

Ball Grid Array (BGA) and Chip Scale Package (CSP) Voids

Plaguing the printed circuit board assembly industry (PCBA) for decades, voids in the solder attaching BGAs and CSPs to the printed circuit boards are one of the more serious problems.

What are BGA & CSP Solder Voids?

Reducing the electrical current carrying area and reducing the mechanical strength of the solder, voids are bubbles. Basically they are holes in the solder holding the components to the PCBs. They can be completely inside the solder joint, or on the surface of the solder.

Voids aren’t usually filled with breathable air. More often it’s a combination of:
* Boiled off gasses from the solder flux
* Outgassed products from either the PCB or component
* Gasses from the chemical reactions of the flux and metal oxides
* Solid residues left over from the soldering process
* Vacuum from when the solder solidifies before the gasses condense out

Size of the Solder Joint Matters

On the circuit board assembly, when there are voids in the solder, the smaller the solder joint, the bigger the problem. Because of their smaller sized solder joints, BGA (Ball Grid Array), PBGA (Plastic Ball Grid Array), CSP (Chip Scale Packages), and HDI (High Density Interconnect) components are greatly affected.

What Causes Voids?

In the soldering process, while the solder is still molten, bubbles form, grow larger, sometimes pop to release the gasses, and once the solder is cool enough to solidify, the bubbles that haven’t popped yet are frozen into the solder, creating a space or void.

Voids get into solder joints:
Before reflow : - Inside a BGA solder ball (having been manufactured into the solder ball)
During reflow : - Before the bubble can escape as the solder solidifies

Size and Quantity of Voids

Within the solder joints of the assembly, the size and number of voids determine whether they are a problem. The large ones of course, push the molten solder out of the joint as the solder solidifies, but even the smaller voids, if there are enough of them, also push the solder out.

As the molten solder is pushed out of the joint by these voids, the resistance of the solder joint increases, and its mechanical strength of the circuit assembly decreases.

Types of Voids

Voids are classified by what caused the void in the first place.

* Inclusion/Macrovioids : – Inherent in the solder melting or reflow process, these are a result of bubbles forming during the flux-breaking-down-oxide reactions. During solder flow, boiling flux trapped in the edges of microvias (dead corners) inside the solder pad can also create these bubbles.
* Planar Microvoids : – These voids are at the common plane, or border between the solder and the solder landing pad of the PCB assembly.
* Shrinkage Voids : – These voids form in the regions of the solder joint that are the last to solidify. As the solder cools and starts solidifying it pulls the still molten solder creating a void. Lead based solders have less of this shrinkage than non-lead solders.
* Micro-via Voids : – These are bubbles formed from the outgassing of micro-vias inside a PCB solder pad. These are usually a consequence of reflow combined with the characteristics of the micro-via.
* Intermetallic Microvoids : – These are voids that form within the boundary region, or Intermetallic Compound (IMC) region, which is between the base metal for the component or PCB solder pad, and the solder. These voids do not form immediately during the soldering process, but grow after extended thermal aging. As this region becomes brittle, IMC Microvoids can reduce the long term mechanical strength and reliability of the connection. Sometimes, after years, a few components will just fall off the circuit board assembly.
* Pinhole Microvoids : – During the reflow soldering process, gasses squeezing out through pinholes in the copper soldering pads cause these voids.

Problems that Voids Cause

* Mechanical : - Voids reduce the mechanical strength of the solder joint. This is not just a short term problem, but as the board is repeatedly heated and cooled over the lifetime of the printed circuit board assembly, flexing of the joint occurs and the mechanical reliability is decreased.
* Size of Joint : - The amount of solder stays the same with or without voids, however, the volume of the solder in the joint increases as the number of voids increase inside the solder. This becomes critical as the pitch (number of connections in a specific area) increases.
* Electrical Resistance : - The resistance of the solder joint increases as the size and number of voids increase.

BGA and CSP Voids

Inside solder joints of a printed circuit board assembly, voids are bubbles and they are hard to totally prevent.

However, the type of flux, the soldering temperatures, the materials used for the components and solder pad, the design of PCB, and the manufacturing process of the bare printed circuit board all influence the overall reliability of the finished printed circuit board assembly.

Contact RUSH PCB

For further information on reducing voids in solder connections, contact the engineers at RUSH PCB  .