Solder joint reliability is often a pain point in the design of an electronic system. A wide variety of factors affect solder joint reliability, and any one of them can drastically affect joint lifetime in a negative way. Properly identifying and mitigating potential causes of solder joint failure during the design and manufacturing process can prevent costly and difficult to solve problems later in a product lifecycle. Some of the most commonly observed solder joint failure contributors to consider are described here.
1. Unintended Stresses from Potting, Underfills, and Conformal Coatings
Potting, coating, staking material, and other encapsulants are used regularly in the electronics industry to ward against environmental conditions that could otherwise damage an assembly. However, these polymeric materials can have thermal and mechanical properties that vary significantly. If the material properties of coatings and pottings are not understood fully during the design process, they can create complex loading conditions that negatively affect solder joint reliability. For example, if an assembly is dip coated, coating will flow underneath components like BGAs and QFNs. The coating will expand during thermal cycling and may “lift” the component off the board, adding tensile stresses to the solder joints.
Certain component mounting conditions and potting/coating application techniques can create tensile stresses like the one in this example or other unwanted stresses on component solder joints. Depending on the material properties of the potting/coating used, these stresses can be large enough to have drastic effects on solder joint fatigue life.
The most important material properties to consider when specifying a potting or coating are the glass transition temperature, as well as the modulus and coefficient of thermal expansion above and below the glass transition temperature. The glass transition temperature refers to the temperature at which a material transitions between hard/glass-like and soft/rubbery.
One common problem with potting is an unexpectedly high glass transition temperature in a material that was not fully understood during the product design process. In some polymers used in electronic potting, the elastic modulus can increase by a factor of up to 1000 when the material transitions below its glass transition temperature. If a thermal cycle extends below the Tg of such a material, the stresses and resulting creep strains experienced by the solder during the cold dwell will be much higher and more damaging. This effect can drastically reduce fatigue lifetime. The examples mentioned here are just a few of the complex and detrimental loading conditions that can result from not fully understanding the thermal and mechanical material properties of a potting, coating, or underfill.
2. Unexpected Temperature Cycling Extremes
Another common cause of unexpected solder joint failure is an incorrect characterization of the temperature cycling parameters of its environment. For example, on/off cycles, exposure to direct sunlight, travel between different climates (on mobile products), and several other sources can add unexpected temperature fluctuations to a PCBA or component. To generate the most accurate reliability metrics of an electronic system, detailed characterization of temperature cycling it will experience is necessary prior to FEA simulation or physical product qualification.
DfR Solutions has had success generating fatigue life predictions with the Blattau model. The model is a semi-empirical energy-based model that shows fatigue lifetime to be highly dependent on temperature range, dwell time, and temperature ramp rate. If the ramps, dwells, maximum temperature, and minimum temperature are not fully understood during the design or testing process, key factors that affect the reliability of the product may be overlooked. Furthermore, if the assembly contains potting or other polymers, the risk of experiencing the glass transition issues described above is increased if the maximum and minimum temperatures are not accurately determined.
3. Mechanical Overstress Events
Mechanical overstress failures occur when a solder joint experiences excessive loading during a mechanical event, such as shock, drop, in-circuit testing, board depanelization, connector insertion, or insertion of a PCBA into an assembly. Overstress failures can be difficult to prevent because they are often difficult to predict. Shock testing research suggests a random failure distribution for such failures.
Solder joint overstress failure typically manifests as a pad crater or a joint fracture along the intermetallic connection (IMC). A pad crater is a crater-shaped crack in the laminate layer beneath the copper pad of a solder joint. This type of failure is typically seen on finer pitch components (primarily BGAs) or when especially brittle laminates are used. Pad cratering is a serious issue because it often leads to trace fracture.
In contrast to fatigue cracks, which typically occur through the bulk of a solder joint, when mechanical overstress failures manifest as joint fractures, they generally occur along the IMC. The IMC is the region where the copper pad and solder combine to form Cu3Sn or Cu6Sn5. It is the most brittle region of the solder joint, which is why it is the area most susceptible to overstress.
Because mechanical event failures can be highly dependent on board boundary conditions and geometries, finite element analyses are typically recommended to predict mechanical overstress risk. Complex loading conditions or board shapes are difficult to predict with other methods. Additionally, FEA allows for strain and curvature quantification, as opposed to displacement only.
4. PCBA Over-Constraint Conditions
PCBA over-constraint conditions, including component mirroring, board mounting conditions, and attachment to housings, are another often overlooked design feature that can have significant effects on solder joint lifetime.
Mount points and other board constraints have a significant impact on board strain magnitude and location during thermal expansion, mechanical shock events, and vibration. Constraints reduce board compliance and create board strains that can cause premature solder joint failures on components that are positioned too closely. Additionally, the overall layout of the mount points will directly affect the likely mode shapes of the PCBA. If these mode shapes are not well understood, the board may be designed in a manner that places sensitive components in regions of high board strain. FEA is a strong mitigation tool for this issue, as it allows to the user to iterate different mounting conditions.
Component mirroring is another common over-constraint condition that can negatively affect solder joint lifetime. Mirroring refers to the position of two components in similar locations on either side of a PCBA.
Meifunas, M., et al. "Measurement and prediction of reliability for double-sided area array assemblies." Electronic Components and Technology Conference, 2003. Proceedings. 53rd. IEEE, 2003.
Mirroring reduces the package compliance of the component by constraining board motion, which creates extra stresses in solder joints. Research suggests component mirroring can reduce fatigue lifetime by a factor of 2-3.
5. Soldering Defects
All of the reasons for failure and mitigation strategies mentioned above will not prevent solder joint reliability issues when solder joint quality is poor. For this reason, it is imperative to construct PCBAs using a reputable manufacturer with a tightly controlled process. A wide variety of solder joint defects exists that can negatively affect reliability. Cross sections and visual inspection of solder joints should be conducted before PCBAs reach the field to ensure that manufacturing quality metrics are achieved. IPC, the Association Connecting Electronics Industries, provides manufacturing standards and acceptability criteria for all types of solder joints that are typically considered the industry standard for creating high quality solder joints.
DfR Solutions offers many services: reliability and accelerated life testing, design review and root cause analysis, that can enhance a company’s bottom line and increase customer satisfaction by producing more reliable, long lasting products. Contact us for a quote today.