Brittle Fracture in Lead Free Solder Joints

A brittle fracture in a solder joint is a separation that happens without the deformation a ductile failure would show. The fracture surface is flat and granular rather than torn, and the failure occurs at a load lower than the joint would normally survive. Lead free alloys are more prone to this behaviour than leaded ones, and the conditions that cause it are specific enough to be controlled.

What Makes a Fracture Brittle

A ductile material absorbs energy by deforming before it breaks, so a ductile joint stretches under overload and gives a warning. A brittle material does not deform, so the load is carried until the material separates. The difference in a joint is mainly in the intermetallic layer at the interface and in the microstructure of the alloy.

The intermetallic compounds that form between the alloy and the copper or nickel are hard and brittle by nature. A thin layer is necessary for a good joint, but a thick layer becomes the weakest link, and a crack that starts in it propagates without the alloy deforming. The layer thickness therefore sets the brittleness of the interface.

Intermetallic Growth and Its Drivers

The layer grows with time and temperature. A joint that is exposed to a high temperature for a long time, or that goes through multiple reflow cycles, develops a thicker layer. The growth rate depends on the metals involved, so the choice of surface finish changes the rate as well as the composition of the compound.

A high peak temperature and a long time above liquidus both accelerate the growth, which is why the profile matters for reliability and not only for wetting. The relationship between the profile and the joint structure is the same as the one described in reflow oven profile verification, and the joint quality criteria that follow are covered in solder joint acceptance criteria.

Impact Loading and Strain Rate

Brittle behaviour is more likely at a high strain rate. A slow load gives the material time to deform, while a fast impact does not, so a joint that survives a static test can fail in a drop or a shock. This is why a product that passes a bend test can still fail in a drop test, and why the two tests are not interchangeable.

The loading direction also matters. A joint is strongest in shear and weakest in tension perpendicular to the interface, and impact loading often includes that tensile component. Board flexure during a drop applies exactly that load to the joints at the board edge and near heavy components, which are the positions where failures are usually found.

Pad, Finish and Design Effects

The pad size and the joint geometry determine where the stress concentrates. A small pad with a thin joint concentrates the load, while a larger pad distributes it. The mask opening and the resist definition can also create a step that becomes a stress riser at the pad edge.

The surface finish affects the interface composition. A finish that forms a fast growing intermetallic, or one that leaves a residue that weakens the bond, produces a more brittle interface. The finish should be selected with the alloy and the profile rather than independently, and the interaction is described in the context of PCB quality assessment.

Design for Shock and Vibration

The design can reduce the load that reaches the joints. Supporting a heavy component with an adhesive or with a bracket removes the load from the terminations, and stiffening the board reduces the flexure that applies the load in the first place. Adding vias or a thicker copper layer under a heavy part spreads the load over more area.

Where the product will see repeated vibration, a joint with a compliant geometry survives better than a rigid one. A lead that can flex absorbs the movement, while a lead that is rigid transfers it to the interface. The mechanical design of the component and the mounting should therefore be considered together with the joint.

Testing to Reveal the Risk

A drop test or a shock test with the assembly in its enclosure is the most direct evaluation, because it reproduces the loading that the product will see. A bend test on a board with the same joints provides a simpler comparison between two designs, and a ball shear or pull test provides a material level comparison between two alloys or finishes.

The tests should be run after the assembly has seen its full thermal history, including any rework, because the intermetallic layer grows with each exposure. A joint tested immediately after assembly has a thinner layer than the same joint after a second reflow and a repair, and the difference can change the result from a pass to a failure.

Analysis of a Brittle Failure

A brittle failure is diagnosed by examining the fracture surface. A ductile failure shows deformation, a rough torn surface and often evidence of the alloy stretching before separation, while a brittle failure shows a flat, granular surface with the crack path through the intermetallic layer or through the bulk alloy without deformation.

The examination should also determine where the crack started. A crack that begins at the pad edge indicates a stress concentration, one that begins at the interface indicates an intermetallic problem, and one that begins in the bulk alloy indicates a microstructure problem. That information decides whether the corrective action is a design change, a finish change or a profile change, and it is the reason a failure should never be dispositioned from the electrical test result alone. The analysis method follows the same approach as other solder defect investigations.

Process Controls and Documentation

The controls are a profile that limits the peak and the time above liquidus, a finish that is compatible with the alloy, a joint geometry that avoids stress concentration and a thermal history that is documented. Where a product is reworked, the additional thermal exposure should be recorded so that the accumulated time at temperature is known.

The gopcb assembly group records the number of reflow exposures for every assembly and treats a second exposure as a change that requires a review of the joint reliability. That record is what makes it possible to identify a product whose field failures come from accumulated intermetallic growth rather than from a defect in the original assembly.

FAQ

Are all lead free joints brittle? No. A joint with a thin intermetallic layer and a fine microstructure behaves acceptably. The risk increases with layer thickness and with the strain rate.

Does a second reflow increase the risk? It increases the intermetallic thickness, so it does increase the risk, particularly on a joint that is already marginal.

Can a brittle joint be detected by inspection? Not visually. A cross section shows the layer thickness, and a mechanical test shows the behaviour, but the finished joint looks normal.