In the packaging of flip chips, the C4 between substrates and IC chips is pretty vulnerable to mechanical stress. That is due to the extensive CTE (coefficient of Thermal Expansion) mismatch that lies between the substrates and the silicon chips. Thus in case of a high thermal load, this CTE mismatch brings about significant stress to the C4 leading to reliability concerns. Manufacturers hence use a single underfill layer to efficiently fill this gap that exits between substrates and ICs. In doing so, they encapsulate various solder joints, which helps minimize reliability concerns. Over the years, this practice has been of much help to manufacturers. It is actually one of the most basic packaging procedures in the packaging of electronic gadgets. But what does this process entail entirely?
What exactly is an underfill?
An underfill is a thermoset epoxy that manufacturers apply onto flip chips to effectively reduce thermal stress. The thermal stress occurs due to the CTE mismatch between organic substrates and integrated circuit, of course.
Today’s underfills come in many different variations. Manufacturers utilize this variation widely to boost the board level (BL) reliability of various Ball Grid Array (BGA) components. They also utilize them to boost the performance of their PCBs.
BGA underfill substances achieve this feats due to their low mechanical and thermal load. These two features help them function well in harsh conditions providing optimal outputs each time.
However, before utilizing an underfill, manufacturers have to consider:
- The underfill’s intended use
- The underfill’s properties
In doing so, they get to assess how much BL reliability the underfill will offer. A miscalculation in this stage could cause catastrophic results.
Underfilling BGA Process, what does it entail?
BGA, as seen earlier, stands for Ball Grid Array. It is a special surface-mount packaging type that manufacturers use for:
- The assembly of circuit boards
- To permanently mount various microprocessor
- To permanently mount other devices onto a circuit board
Ball Grid Array underfilling also lets you add interconnection pins. That is because, when working with a BGA component, you can utilize its entire bottom part instead of just using its perimeter. It hence provides a net result of interconnection pins that exceed those of flat and dual-inline packages. Interconnection pins are pretty fragile and also susceptible to a lot of damage from impact and moisture. Therefore, manufacturers also utilize underfilling BGA to shield the assembly of their circuit board by boosting:
- Their thermal properties and,
- Their Mechanical properties
How the process occurs
The BGA process helps create stable component attachments onto a PCB. It also helps create a circuit board that has impeccable thermal and moisture performance. However, this process is not easy to undertaking. Many people have to utilize special gadgets to achieve this feat.
Mostly, manufacturers use epoxies as their coating substance. However, other substances such as silicone and acrylic are also not so uncommon. With this substances in check, manufacturers then move on to underfilling BGA stage.
The underfilling BGA process is as follows:
- Manufacturers apply the underfilling onto a particular corner or in a specific line along the BGA’s or Micro CSP’s edge
- After applying the BGA, they then heat the micro CSP or the BGA using temperatures within the range of 125 – 1650C
- They then utilize capillary action to effectively absorb the applied underfill substance under the Micro CSP and BGA
- To cure the underfill, manufacturers maintain a steady temperature for about one hour or so. The time range shifts depending on the underfill component in use.
When the underfilling BGA process is over, you get a PCB that is super-efficient and durable.
How does the underfilling BGA process shield the PCB assembly process?
The underfilling BGA process provides an impeccable mechanical bond between PCBs and BGAs. It does so by shielding various solder joints from any physical stress.
The underfill substance also aids in efficient heat transfer between the PCB and various BGA components.
BGA reworking solution
Most underfill substances that manufacturers use in the underfilling BGA process are re-workable. What this means is that, in case of a defect, you, or the manufacturer, can replace or repair the parts. In addition, the repairing and replacing processes are easy to undertake, making it better than getting another device.
The ultimate goal of underfilling BGA is to reduce operating stress while at the same time reducing:
- Processing time and,
- Manufacturing cost
Special underfilling BGA process
A unique underfilling BGA process exists, which utilizes made-to-order components. These components are frozen to minimize the chances of contamination.
However, to utilize this component, you have to have a specific requirement in mind. That is because manufacturers apply this BGA underfills to precision and according to your needs.
Identifying various underfill BGA defects
To identify underfill defects in BGA, you need to utilize extreme magnification and cross-section testing. These two methods are almost always accurate.
Factors to consider in your test include:
- Low-quality profile development
- Heat damage
- Improper print circuit board assembly preparing
- Operator error
- Inappropriate equipment
Common defects that you are likely to encounter include:
- Open circuitry
- Excessive voids found in the solder balls
- Eccentricity of the solder balls
- Coplanarity between PCB and BGA assembly
These are common defects that you are likely to encounter after carrying out your testing. However, after finding these defects, do not panic, as you can easily carry out BGA reballing or reworking.
Underfilling BGA applications
You can apply underfilling BGA in the following areas:
- Inland grid array (LGA) devices – BGA application in LGA helps account for chip-scale consideration. It also helps account for the packaging density.
- In Chip Scale Packages (CSP) – BGA application in CSP helps prevent any damages that may occur due to:
- Vibrations
- Shock
- Weight
- In chip-on-chip gadgets – you can apply the underfilling BGA on various chip gadgets.
Utilizing underfilling BGA helps reduce mechanical and thermal stress. It also helps boost the reliability of a Print Circuit Board via the underfilling process.
BGA packaging shields PCBs against cracking while at the same time reducing solder-joint stress. The net result of all this is a PCB that functions impeccably.
Conclusion
The modern market has a wide range of PCB variations to offer. Therefore, most people are now looking for reliability and efficiency, not just variation. The underfilling BGA process offers these two qualities at a low price. Using this simple yet efficient process, you can produce a PCB that can withstand great thermal and mechanical stress. Your PCBs will hence become unique in the modern market, and this is all it takes to take your PCB game to the next level.
