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Underfill – Reliability Booster

There’s growing interest in underfill, and rightly so since component and IC packaging is continually shrinking to meet the demands of today’s handheld and portable electronics systems. But still the OEM customer wants the highest reliability possible, and this is where underfill plays a key role.

Challenges to PCB-level reliability are introduced as a result of the decreasing pitch between balls in a fine pitch micro BGA, for instance. Growing numbers of OEMs are coming to us with 0.3 mm and even 0.25 mm pitch requirements. With the pitch decreasing, ball size is also being reduced.

That means the standoff height between the PCB and chip package is decreased. The shorter standoff height reduces PCB-level reliability for a package. In turn, fine pitch micro CSPs and micro BGAs incur difficulty meeting mechanical shock and substrate flexing tests for portable electronics applications.

Figure 3_Using top layers (1)Underfill takes care of those challenges, as shown in Fig. 1 showing a micro BGA. It provides extra rigidity with strong mechanical bonding between chip and the PCB’s corresponding connection, thus protecting solder joints from mechanical stress. It also helps transfer heat from highly complex, high-power chips and sometimes acts as a heat sink.

While the underfill process may sound relative easy to implement at the PCB design and assembly stages, there are special pre-cautionary steps that need to be considered. Check out our article to get more details. Meanwhile, here are some tips and hints that’ll give you a basis for moving forward.

  • Assure that the PCB designer and assembling engineering work hand in glove to assure underfill and the correct amount and type are used in your PCB project.
  • Understand that there are two types of underfill material. Don’t let an inexperienced PCB designer use the wrong one on your project.
  • Find out how much your PCB designer knows about glass transition temperature (Tg) and generic low ionic requirements when it comes to underfill. Those are vital when selecting an underfill.
  • Experience with successful implementation of micro BGA and micro CSP devices is at a premium. Without it, the result can lead to an unreliable product that incurs intermittent or latent problems in the field.
  • Get a thorough understanding from your CM or EMS Provider on the assembly method they’ll use for the underfill process.
  • Find out what tradeoffs are involved at assembly to assure the highest possible reliability for your product.

We’re seeing a number of customer boards highly populated with micro CSPs and micro BGAs, meaning there’s an extremely high reliability requirement. In cases like this, there’s a possibility harsh environmental forces can incur problems to a device’s solder bonds, resulting in poor connections. However, once underfill is applied, those package and board connections are strengthened considerably by upwards of 10 times. Hence, the underfill process dramatically enhances thermal cycling performance and shock resistance of those chip packaging and board connections.