Underfill Techniques Automated Dispensing and Jetting
BY ALAN LEWIS
ALAN LEWIS, director of dispensing technology
Devices that use underfill have proliferated both in package type and volume. The need for underfilling a wide variety of packages for reliability is well established. The equipment used to dispense underfill materials has matured, but the technological improvements for needle dispensing have only been incremental in speed and capability. The recent introduction of jetting technology to underfill dispensing is a different approach that can increase speed several times over and can successfully underfill packages not possible with needle dispensing.
Before exploring the use of jetting in underfill applications, a review of the underfill process is in order.
Underfill is used in a wide variety of packages and board-level assemblies. Flip chips, direct chip attach on boards, stacked die packages, and various ball grid array (BGA) assemblies are some of the many permutations that are being used in production today. The “part” to be underfilled can be either a die or the BGA substrate. While these packages often have their own material sets and reliability drivers, the underfill process used for their production has many similarities.
The automated underfill process includes:
• Pre-treatment
• Parts handling
• Heating
• Part location with vision and height sensing
• Dispensing the underfill material with one or several timed dispensing passes
• Material flow out (material flowing by capillary action under the part)
• Dispensing of a seal or fillet pass (optional)
Pre-treatment
For good, reliable underfill, make certain the substrate is moisture-free. The parts must be dried sufficiently
before underfilling. This is easy for ceramic substrates, but it can be a challenge for organic and polymer substrates. If there is moisture trapped in the substrate, there is potential for voiding and delamination during cure.
Plasma cleaning is an established process for liquid encapsulation of wire-bonded parts.Work is in progress to show how the use of plasma cleaning improves reliability of the underfill process; published results are pending.
Part Handling
Parts are conveyed into a dispenser generally one of two ways: 1) Packages are usually conveyed on a carrier, such as an Auer boat or a Joint Electronic Device Engineering Council (JEDEC) tray. 2) Board-level assemblies usually do not use carriers;
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