BGA Solder Ball Alloy Selection

A ball grid array package carries its own spheres, pre attached by the component supplier, and the assembly process joins those spheres to pads with paste. The alloy of the ball and the alloy of the paste are therefore two separate choices that meet at a single joint, and they have to be compatible in melting range and in mechanical behaviour.

The Two Alloys in One Joint

When the assembly reflows, the paste melts first and dissolves part of the ball, forming a joint that is a blend of the two compositions. The final joint is not the ball alloy and not the paste alloy but something between them, and its properties depend on how much mixing occurred during the time above liquidus.

Where the two alloys are close in composition the mixture is uniform, and where they differ the joint can retain a gradient with a core of one alloy and a shell of the other. That gradient affects the mechanical behaviour and the thermal fatigue life of the joint.

Melting Range and Process Window

A ball with a higher melting point than the paste lets the paste melt and bond while the ball remains solid, which keeps the package in position and prevents collapse. That is the principle behind the widely used combinations in lead free assembly, and it is what allows a controlled standoff after reflow.

A ball with a melting point close to the paste produces collapse, which changes the standoff and the joint shape. Collapse is not inherently bad, but it has to be accounted for in the design and in the profile, because the package settles onto the board as the alloy melts.

<img src="https://www.gopcba.com/wp-content/uploads/2021/05/section-3.jpg" alt="Cross section of a ball grid array joint showing the ball and paste regions” />

Ball and Paste Compatibility

Compatibility is more than melting range. The two alloys must wet each other, must not form brittle phases at the interface, and must not produce a joint whose composition falls into a pasty range during cooling. A joint that is partially liquid over a wide temperature band is prone to disturbed structure and to cracking.

The safest approach is to use a ball and paste pair that the supplier has characterised together, and to verify on a first article by cross section. A pair that is chosen independently may both be perfectly good materials and still produce a poor joint.

Thermal Fatigue Performance

Thermal fatigue is the dominant wear out mechanism for a ball grid array joint in service. The joint is constrained between a package that expands at one rate and a board that expands at another, and every temperature cycle imposes a strain. The alloy composition determines how the joint accommodates that strain and how quickly a crack propagates.

Alloys with a higher tin content are generally more resistant to fatigue, while those with a wide melting range can develop a coarse structure that cracks sooner. The selection should be supported by cycling data for the actual package size and the actual temperature range, because performance does not transfer between geometries.

Mixed Assembly and Backward Compatibility

Boards that combine leaded and lead free parts, or that use a leaded finish with a lead free ball, create additional constraints. A lower melting alloy in the joint limits the peak temperature that can be used, while a higher melting one may not fully reflow. The result is often a joint whose composition is acceptable but whose process window is narrow.

Where mixed assembly is unavoidable, the profile has to be verified on the actual assembly rather than taken from a datasheet, and the joints should be inspected more closely than usual. The measurements that establish whether the joints actually formed are described in our article on X-ray and AOI inspection.

Solder Sphere Size and Standoff

Sphere diameter and pitch are set by the package, not by the alloy, but the alloy affects how the sphere behaves during reflow. A collapsible ball produces a shorter standoff, which changes the thermal path from the die to the board and the mechanical strain in the joint.

Where the design depends on a specific standoff, for underfill flow or for thermal performance, the collapse behaviour has to be confirmed by cross section. Assuming the nominal sphere diameter survives reflow is a common source of a standoff that is shorter than the analysis assumed.

Reliability Testing

Qualification of a ball alloy usually involves thermal cycling, bend testing and, where relevant, drop testing. The first shows fatigue behaviour, the second shows resistance to board flexure and the third shows resistance to mechanical shock. Each addresses a different failure mode and no single test substitutes for the others.

Sample size and the definition of failure matter as much as the test. A crack that has not opened is still a failure in a cycling test, and detecting it requires either electrical monitoring during the test or sectioning afterwards. The design of the experiment is described alongside other reliability work in our article on thermal cycling test design.

Documenting the Selection

The component specification should record the ball alloy, the paste alloy, the peak temperature and the measured standoff, along with the qualification evidence. A package sourced from a second supplier may use a different ball alloy under the same part number, and without that record the difference surfaces only as a change in field behaviour.

Where a supplier change occurs, the assembly should be requalified rather than assumed equivalent. The cost of a cycling test on a sample is small compared with the cost of discovering a difference after a product has shipped.

Process Control and Verification

The measurements that matter are the repeatable ones: conductor width and spacing, annular ring, finished hole size, plating thickness and surface finish are all verifiable on a coupon that travels with the panel.

Related reading: our fabrication notes, board quality and design release notes cover the same ground.

FAQ

Can any paste be used with any ball? No. The pair has to be compatible in melting range and in the phases they form, and the combination should be qualified together.

Does a higher melting ball improve reliability? Not by itself. The joint composition after mixing is what determines the behaviour, so the paste matters as much as the ball.

Is cross sectioning necessary on production? Not routinely, but it is essential for a first article and for any change of supplier, alloy or profile.