- Learn what might cause a solder joint to fail and how to prepare for it
- Discover how proper simulation can prepare a designer for solder joint reliability
- Understand what options there are in reliability simulation for soldering and beyond
A solder joint reliability simulation helps you determine whether these joints will fail at high temperatures or under repeated thermal cycling.
Any board that gets deployed in the field needs to have the longest possible lifetime. The last thing you need is to have your board fail prematurely due to poor design choices or incorrect assembly processes. Aside from component failures, solder joint failures are a known cause of product failure. If one solder joint on a critical circuit fails, many more are sure to follow.
Why do solder joints fail, and how can failure be prevented? There are several well-known reasons that solder joints failure during operation. Some of these simply relate to the incorrect choice of solder, operating at extreme temperatures, or thermo-mechanical failure. If you use a solder joint reliability simulation early in the design process, you can identify which solder joints are more prone to failure due to thermally-induced stress.
Causes of Solder Joint Failure
There are a number of common causes for solder joint failure, which can be mechanical or thermal in nature. Here are some of the known common causes of solder joint failure:
Weak Solder/Cold Joints
Solder joints may be formed at an incorrect temperature, leading to insufficient mixing of the liquidus and eutectic. As a result, the joint is simply not strong enough to handle even low stresses, and the joint eventually fractures. There are some other solder defects that can reduce the strength of a solder joint.
There are a number of sources of corrosion that can oxidize solder and nearby conductors in a PCB. Excessive humidity and condensation can induce an electrochemical reaction that drives oxide accumulation, causing the joint to have lower strength. Although solder flux is intended to combat oxidation, leftover residues from solder flux can drive corrosion during operation.
The dark film on the exposed conductors is an oxide film due to water damage and subsequent corrosion.
Repeated Mechanical Overstress
Just like any other mechanical element, excessive stress can induce fatigue and lead to mechanical failure. Mechanical cycling and shock can cause a solder joint to fail simply due to excessive deformation.
Fatigue Failure Under Thermal Cycling
When cycled between extreme temperature values, the solder joint and PCB substrate will expand, and stress will accumulate in the solder joint. Greater expansion will occur when the solder joints are brought above their glass transition temperature. Stress accumulation causes microscopic cracks to propagate in the solder. Eventually, these cracks can merge and lead to partial or total fracture. More ductile solder materials (i.e., with indium content) can withstand this type of failure.
Thermal Shock Failure
Have you ever seen a piece of glass shatter when it is heated up too quickly? This can also happen in weak solder joints when a joint is brought to an extreme temperature very quickly. When this occurs, the solder joint begins expanding so fast that it cannot maintain mechanical bonds and the solder joint fractures as a result.
Von Mises stress accumulation in BGA solder balls
Evaluating the last two points above require directly simulating the temperature of the joint over time and in the steady state. The temperature can be determined directly using a field solver, or it can be determined in a circuit simulation.
Solder Joint Reliability Simulation with Circuit Elements
There are two ways to determine the temperature of a solder joint in a circuit simulation:
Use the junction temperature of a component as a proxy for the solder joint temperature.
Include the solder joint in the simulation as a parasitic element.
The first method will nicely account for large heat dissipation in components like MOSFETs, high frequency processors, and components for power electronics. Nearly all the heat will be generated in the component itself, which will then conduct into the nearby solder joints. In the steady state, the solder joint temperature will be very close to the component temperature. This means you can determine the solder joint temperature using a set of verified component models.
No metal is perfectly conductive, and the same applies to solder joints. A solder joint effectively acts as a source of parasitic resistance, and the heat dissipated in the joint during operation increases its temperature. A solder joint on a component can also be included as a very small resistor on the input and output pins of a component. Standard DC sweeps or signal sources can be used to examine temperature rise in the joint itself, in addition to inferring the joint temperature from the component temperature.
The variation in the important thermal and electrical parameters make this approach difficult. Here are some important material properties for use in circuit simulations and later calculations:
Contact resistance: This is generally in the mOhm range, although this depends on the joint’s geometry and material composition.
Temperature coefficient: This defines the temperature change per W of heat dissipated across the solder joint. There is no single temperature coefficient value as this depends on the joint’s geometry, material composition, and surrounding substrate/component thermal conductivities.
Thermal expansion coefficient (CTE): This is not included directly in a simulation, but this is needed to determine joint expansion after the temperature rise is known. The exact value will depend on the solder joint composition and the material’s glass transition temperature.
A simple circuit simulation like this doesn’t consider the thermal conductivity of your PCB substrate, so heat transport away from solder balls can’t be considered in a circuit simulation. This is why a direct simulation of the package temperature is more useful for determining an upper limit on the temperature of your solder joints. The joint and component junction temperature calculated in these simulations can then be compared with the typical ~250 °F upper temperature limit for commercially-available solders.
More Tools for Solder Reliability Analysis
In a circuit simulation, the goal is to compare component temperature to a desired derating value, both in the time domain and when the steady-state is reached. Thermal shock and thermally induced stress are transient phenomena that can be examined with a SPICE simulation. This may take some manual programming over time, but these simulators can take parameter data directly from your component models for determining temperature changes.
In the steady-state, you’ll need to determine the equilibrium temperature of your solder joints and compare them to a desired derating value using smoke analysis. Determining the exact derating requires weighing a number of tradeoffs between component temperature, board temperature, and board thermal expansion.
Thermal expansion of the package and the board can occur when a board reaches high temperature, just as is the case during wave soldering.
A more powerful, yet computationally more intense, tool for solder joint reliability simulation is a 3D multiphysics solver. This allows you to examine heat and stress accumulation in solder joints directly, rather than inferring solder joint behavior from component temperatures. This is a more powerful solution, but it is not required in every design situation.
Solder joint reliability simulations can be a bit awkward if you aren’t using a field solver, but a suite of PCB design and analysis tools are ideal for evaluating temperature rise in circuit simulations. The design and simulation tools in PSpice Simulator and the full suite of analysis tools from Cadence are ideal for simulating all aspects of your board as it operates. You’ll have the features you need to evaluate reliability throughout your circuits and optimize various operating parameters.
If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.