BALL GRID ARRAYS by KRISHNA TEJA KARIDI

INTRODUCTION: The evolution of ball grid arrays (BGA). The necessity of ball grid array packages. The assembly process of BGA. The popcorn phenomenon. The under fill technology evolution.

EVOLUTION OF BGA: The circuits involved in the early stages are slow and the overall packaging is easy. As the circuit complexity increased along with the speed, the packaging method changed. The quad flat packages with less I/O connections are not much helpful. Area array packaging came into existence. BGA descended PGA.

NECESSITY OF BGA: The pins or leads are very fragile and the chance of breakage is more. They also exhibit some inductance which deteriorates the signal quality and also includes the time delay. The BGA can offer more number of I/O connections compared to quad flat package in the same area. The short interconnects does not add any inductance.

TYPES OF BGA’S: There are number of types of BGA’S and the popular ones are plastic ball grid arrays(PBGA), Tape array BGA, Metal BGA are some of the popular arrays. The PBGA has several advantages smaller footprint, high I/O interconnects, superior thermal and electrical characteristics.

ASSEMBLY PROCESS: The assembly process is same for all the types of BGA except for the substrate used. Types of substrates used: ceramic and organic substrates. Encapsulation process. Dam dispensing. Ball placement. Reflow process.

POPCORN PHENOMENON: Moisture absorption ability of PBGA due to organic substrates like glass epoxy, polyimide. Study of this phenomenon using three substrates like substrate without thermal vias, with closed thermal vias and open thermal vias. This phenomenon is studied assuming perfect adhesion between chip and substrate and considering some delamination at die adhesive layer.

POPCORN PHENOMENON: In perfect adhesion, the moisture can expand during reflow, so there will not be much effect.

POPCORN PHENOMENON: Consider the delamination, and the moisture is converted to vapour which results in the cracking of the package.

POPCORN PHENOMENON: The cracks occurs in two types: Type I and Type II.

POPCORN PHENOMENON: To prevent this open thermal vias are used so The popcorn phenomenon occurs when the moisture is penetrated through the substrate and molding interface and the vapour is much more than the flexible strength of the package. To prevent this open thermal vias are used so that the vapour goes out of them.

UNDER FILL TECHNOLOGY: It is important for the reliability of the package i.e. to bear the thermal fatigue in the package. It is important to extend the solder joint life of flip chip technology. Earlier as the number of components in an IC are less there is no issue of thermal fatigue. Due to organic substrates introduction, thermal fatigue played a major role in the reliability.

UNDER FILL TECHNOLOGY: A material used for underfill should have the following properties: 1. CTE match with the solder joint 2.lower absorption rate 3. good adhesion 4. cure time 5. good flowability

UNDER FILL TECHNOLOGY: The conventional method of epoxy underfilling is: 1. Flux dispense 2. Reflow 3. Flux Cleaning 4. Underfill dispense 5. Underfill cure Mechanical removal of chip.

UNDER FILL TECHNOLOGY: Mold release layer (parylene) underfill which is expensive. So reworkable underfills are needed like chemically reworkable underfills and thermally reworkable underfills. Epoxy based underfills that are soluble in organic acids. It is time consuming. Thermally reworkable underfills are quick. So the epoxy is thermally degraded and heated to remove the underfills and replace the chip but this method left some residue.

UNDER FILL TECHNOLOGY: Additives are added to epoxy create explosions thus removing the chip and also underfill is removed. Thermoset resins are also used which is a solid but gradually turns to liquid. Thermoplastics are also used but they don’t have good adhesion. Polymerizable thermoplastics are created and coupling agents are used for increase in adhesion.

UNDER FILL TECHNOLOGY: no-flow underfills are invented with the following properties: 1.fluxing for solder interconnects 2.latency to form solder joints 3.fully cured before reflow. Wafer-level-applied underfills

REFERANCES: http://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=550897 http://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=367494 http://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=904159