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BGA and QFN Failure Mitigation: Underfilling, Edge Bonds and Corner Staking

Posted by Craig Hillman on Oct 5, 2016 9:30:00 AM

BGA-QFN-Failure-Mitigation.jpgDespite presenting a greater risk of solder joint failure due to thermal cycling, vibration, mechanical shock and a less robust board connection than their leaded counterparts, Ball Grid Array (BGA) and quad flat-pack no lead (QFN) are among the most prevalent packages for integrated circuits. 

Why the popularity? BGAs and QFNs are more powerful and less expensive than lead packages, leading to widespread incorporation into today’s technologies. In fact, BGAs generate almost two-thirds of semiconductor packaging revenue, and QFNs are the most popular IC packaging by volume – statistics that are only growing.

Clearly, BGAs and QFNs aren’t going anywhere anytime soon, so designing them out or not using them altogether are not options in mitigating the substantial risks these packages present. Therefore, many are turning to one of two methods to address potential failure: using underfilling, edge bonds and corner staking to reduce solder joint strain, or using Physics of Failure (PoF) to change the design.

Both solutions are comprehensive, so we’ll break our discussion into two blogs. This post delves into how to keep the design and minimize failure with underfilling, edge bonds and corner staking – particularly in BGAs, since QFNS do not offer enough space for underfilling to be effective. Part two  of the blog series talks about using PoF to change BGA and QFN design.

A “band aid” fix?

As a risk mitigation measure, underfilling, edge bonds and corner staking can be cost effective, as compared to the varying engineering costs associated with redesign. However, solving a failure challenge after some or all of the aspects of components and design are determined – as is often the case with this option – is a “band aid” fix.

Now, sometimes the underfilling “band aid” fix is exactly what’s needed to mitigate risk without incurring package re-design expense. Underfilling, although an expensive material and time-consuming process, could be the solution for small quantity runs and small BGAs. In other scenarios where high throughput and lower costs are targeted, edge bonds and corner staking are better choices.

No universal solution

Underfilling, edge bonds and corner staking are not interchangeable, universal mitigation solutions. To select the right one, you must know exactly what you’re mitigating for – temperature, shock or vibration – and understand that the application of general trends and known equations is necessary for optimal outcomes. To do otherwise could result in adverse thermal cycling and mechanical cycling issues:

Thermal cycling

Unlike lead packages, temperature impacts optimal solder joint and mitigation performance in lead-free options:

  • Underfilling requires the proper balance between the Glass Transition Temperature (Tg), Coefficient of Thermal Expansion (CTE) and Elastic Modulus (E) or the degree of mitigation is compromised.
  • Corner staking uses the least amount of material, has the highest throughput and presents the lowest expense and risk, but loses its improvements in high or low temperatures.
  • Edge bonds are similar to corner staking, but provide improved time to failure.
Mechanical cycling

As with thermal cycling, underfilling, edge bonds and corner staking present mechanical cycling challenges:

  • In shock testing, all failures were solder joint failures.
  • In vibration testing, behaviors varied but all mitigation techniques provided worse failure in cold testing than applying no mitigation whatsoever. 

BGAs and QFNs are increasingly the packaging of choice for most integrated circuits, but they are also subject to greater failure risk. Learn more about mitigation techniques and practices in Selecting the Right Mitigation for BGAs and QFNs. Click the button below for your free download.

Select the Right Mitigation for BGAs and QFNs webinar

Topics: Design for Reliability

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