Flip chip underfill repairability
COB technology (Chip-on-Board referred COB), also known as direct chip mount technology (Direct Chip Attach referred DCA), is the use of an adhesive or automatic with solder, wire bonding, flip chip and other methods, the bare IC chip is directly mounted on a circuit board technology. Flip-chip is a COB one (the other two species of wire bonding and tape automated bonding carrier) will face the substrate, the chip active region, rendering the chip through the solder bumps arranged in an array to implement the chip and the substrate at the end of the interconnect.
It provides a lot of advantages; eliminates requirements for wire bonding connections; increased input / output (I / O) density of connections; and the space used by the printed circuit board is very small. Compared with wire bonding together, it implements more I / O counts, speeds up the operation.
Flip chip underfill
In order to meet the reliability requirements of general use flip chip underfill technology. This is because at the time of reflow, the solder balls under a lot of stress at the moment mechanical force and thermal cycling stress may cause damage to their cracking phenomenon. And the material used to fill the bottom by means of a strong adhesive type epoxy resin adhesive which taut, reducing and redistributing the stresses. The thermal expansion coefficient of the chips, circuit board and the solder joints between (coefficient of thermal expansion referred to as CTE) mismatch is reduced to a minimum.
For flip chip, the most cases, when the die is connected and not be able to fill the bottom of the later testing. If you find a defective chip, manufacturers must discard the entire board. This is because there is no guarantee in the PCB without damaging the substrate, and removed the adhesive residue.
As an alternative way for some components manufacturers rely on expensive and time-consuming off-line testing and treatment to ensure that they use the die to be reliable. In the offline test, flip chip and circuit board by reflow soldering process, and then remove the offline tests. If the die is proven reliable, the board is re-placed to fill the bottom line; if there are problems, they replace the other one chip, and then press process re-test until qualified.
For these reasons, the flip chip technology is a relatively time consuming process method. Board-level flip chip is often considered a bottleneck in the manufacturing process, only the production of high reliability and high prices when using electronic components.
However, the industry has recently introduced a repairable underfill technology. It allows testing and quickly replace a defective chip. The new repair will allow the underfill material in a flip-chip after the test, to confirm the replacement operation when the chip is defective, the entire board will thus be discarded may be reduced to a minimum. This is especially important for complex and expensive multi-chip module package (multichip packages referred MCMs) it.
Repairable underfill
US Loctite Corporation developed and manufactured out of a repair material is a liquid epoxy-based underfill material, and when it is at ambient temperature of 150 ℃ to 165 ℃ can be cured in a time of 5-15 seconds . This fixes the thermoset underfill material is suitable for polyimide passivation (polyimid passivated) flip chip, CSP and BGA assembly. It was developed from a proprietary resin article out. When heated to the design temperature (at least 210 ℃, a heating process at a temperature generally of between 220 ℃ to 230 ℃) when the melting phenomenon may occur. Residual adhesive by rubbing down the handle, you can go to get rid of the PCB substrate.
This fixes mainly relies on a proprietary material of a single molecule of epoxy resin the object, it is added in the end portion of the filling material formulation, the components after heating to meet the repair work. US Loctite Corporation and Cornell University to jointly develop and optimize the synthesis of this new single-molecule body. This innovative epoxy material in the US national and international chemical management organizations (TSCA and EINECS) were registered. This can be mass-produced using conventional methods and materials developed by the repair process is the key to the smooth conduct of.
Compared with the conventional underfill
In most cases, the new repair materials and conventional underfill underfill material having the same capillary flow capacity. The maximum difference between them is that the bottom of the repair filler material in the molten state, subjected to a solder reflow processing temperatures.
Repairable and traditional rapid curing, fast flowing underfill material is 0-50% filler composition. They can be reliably used in the temperature range of -55 ℃ to 125 ℃, which fills a gap of 25 microns quickly. For conventional consumer product applications, they can well meet the requirements. These materials can be cured within 5 minutes or within a shorter period of time. In the implementation of the coating process, you can use the same equipment and processes, slowly underfill Process. This same device may be used in one hour a number of processing components.
The key development that fixes the underfill material to find a method. It can quickly remove the components, easy to clean up and prepare for the next step in the on position for the replacement of components. When the second component is positioned well then, it can maintain good reliability.
Repair process is actually divided into three parts. Components to be repaired by heating for 1 minutes or less time at a temperature of 220 ℃ to 230 ℃. Then die by removing or tweezers to destroy the last of the adhesive by vacuum adsorption device utilizes a slight twist. When most after the adoption of the relevant equipment to remove some of the residue at the bottom of the filler material will remain on the PCB substrate, especially in the fillet area.
You can then use a simple process of order does not involve solvents and acids, and to carefully removed the remaining residue. A high-speed brushing device, commonly referred to as "cleaner", the rest can be removed after the defective components have been removed any residue. Due to the excessive pressure is applied to the circuit board solder mask or threatening to gold pads, work pressure "cleaner" to be imposed must be carefully controlled. Began to develop during the treatment process, cleanliness can be visually examined by FT-IR analyzer to determine. During the cleaning process, even if some gold to be cleared away, before resettlement device does not further require that location for processing.
In order to remove the underfill residue, people continue to search for a non-physical treatment. This may require the development of a novel single-molecule form, it is now below the body of a single molecule of epoxy resin fixes below 240 ℃, can effectively begin to decompose.
Reliability Test Results
For the repair operation for the last one is the ability to work with a new chip to replace the original test chip. As a new assembly operation, the repair assembly should have the same with the previous reliability. In order to re-confirm the replacement of flip chip assembly on the reliability of the entire lifetime of the case, it is necessary to measure various parameters. Meet certain target value determined, in general, can be predictive of a good reliability. For the underfill material, the test parameters for the test of thermal expansion coefficient is generally (CTE), the transition temperature (Tg), moisture resistance (increased airway), and modulus. Performance characteristics of the thermal cycle properties and adhesion properties. Other parameters to be measured, and the minimum capacity of the ion radiation, and a stable dielectric constant.
Fixes the reliability and performance of the underfill material has been shown to be able to fill the bottom of the board-level comparable to conventional. For CSP devices and BGA devices, in order to be able to prove the case of the original component reliability and reconditioned components, the use of drop test (drop testing). The results were tested using the drop test showed that: The component repair process has better reliability than the original components. During this embodiment produced the result is due to clearance in the repair phase, the surface of the circuit board need to be friction, thus enhancing the bonding effect. Because of the roughened surface of the circuit board, it is possible to produce a strong bonding effect of the bonding substrate.
Also need the device to thermal cycling tests, in order to confirm whether the repair work diminished thermal cycling characteristics replacement device. For flip-chip assembly, the results of the thermal cycle tests showed that: after experiencing the heating and cooling cycle test over 500 times, the device is similar to the case of the original component and the repair position.
Conclusion
The new repair underfill material under study in development, including one of silicon nitride passivation compatible (silicone nitride passivated) flip-chip material, and the ability to withstand multiple cycles without breakage phenomenon under reflux temperature material.
Currently repairable underfill material can satisfy the polyimide passivation die, but not be able to meet the requirements of one of silicon nitride passivation die.
Study of new bottom filler fixes based single molecule having the same patent, it can be repaired in the usual operation of the reflux temperature. These materials will provide a higher than normal temperature reflow soldering temperature repair, allowing the underfill material to withstand multiple reflow temperature cycle impact, as many are now irreparable underfill material did. At the same time, improve the operating speed underfill repair work is also carried out in.
