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Application of Bottom Filling Technology in Printed Circuit Board Assembly

Classification of Bottom Filling Technology

Bottom filling can be classified into fluidity bottom filling based on capillary flow theory and nonfluidity bottom filling. Up to now, bottom filling technology suitable for chips of BGA, CSP etc. mainly includes: capillary bottom filling technology, SMT hot melt adhesive sheet technology, ACA (Anisotropically Conductive Adhesives) and ACF (Anisotropically Conductive Films) technology, ESC (Epoxy Encapsulated Solder Connection) technology and more. For capillary bottom filling technology and SMT hot melt adhesive sheet technology, soldering flux and filler are independent to each other while for ACA and ACF technology and ESC technology, soldering flux and filler are combined as one.

Capillary Bottom Filling Technology

The theory of capillary fluidity goes like this. Liquid with excellent fluidity such as liquid epoxy resin is dripped around BGA and CSP chip and capillary action makes liquid resin sucked in the space between chip bottom and PCB. Then resin, soldered chip and PCB are fixed together through the method of heating or ultraviolet curing so as to protect soldering points, reduce the harm caused by stress and increase the reliability of soldering points.

Capillary bottom filling technology is applied in the fields of PCB chip bottom filling and flip chip packaging. The application of bottom filling technology can distribute the stress suffered by solder ball point at the bottom of chip so as to increase the reliability of the whole PCB. The process of capillary bottom filling should be implemented as the following. First, surface mount chips such as BGA and CSP are mounted on the PCB with solder paste printed on it. Then reflow soldering is carried out so that alloy connection is formed. After chip soldering, distribution technology is applied to fill bottom filling material into one or two edges at the bottom of chip. Filling material flows at the bottom of chip and fill the space between chip and PCB. Although capillary bottom filling is capable of greatly increasing reliability, devices filling bottom filling material, sufficient factory space for device assembly and workers who are able to finish delicate operation are required in order to complete this process. What's more, capillary bottom filling technology can't be implemented until PCB assembly is completed and it features some other disadvantages such as difficult operation, large time and energy consumption and difficulty in terms of filling amount control. Therefore, capillary bottom filling technology is only applied on some key chips or chips whose coefficient of thermal expansion is extremely different from that of PCB substrate so that capillary bottom filling technology isn't massively applied in PCB assembly.

SMT Hot Melt Adhesive Sheet Technology

In accordance with regulations of RoHS and WEEE, SMT hot melt adhesive sheet technology features advantages of non-toxic, halogen free, without heavy metal residue, excellent insulativity, boundary dimension compatible with standard and precise size, convenient for optical identification mounting. SMT hot melt adhesive sheet can be mounted between PCB and BGA or CSP and can be soldered with normal lead or lead-free soldering craft. In the process of melting, adhesive sheet can't be influenced with solder and its attributes of no solvent evaporation and no necessity for cleaning all contribute to its status of ideal PCB filling material. The process flow diagram of SMT hot melt adhesive sheet technology is shown in Figure 1 below.

Figure 1

Based on Figure 1, the application of SMT hot melt adhesive sheet technology is actually adding a step of hot melt adhesive sheet mounting before IC chip mounting, which means that BGA and CSP chip with the need of bottom filling are mounted with hot melt adhesive sheet prior to IC chip mounting. Finally chip soldering and bottom filling are completed in reflow soldering with the step of refilling omitted. It is quite suitable for the bottom filling of PCBs with small batch production.

ACA and ACF Technology

ACA and ACF technology reduces procedures and cost by simultaneously completing soldering and bottom filling. Both ACA and ACF are conductive adhesives that are generally composed by matrix resin and conductive filling material, classified into ICA (isotropic conductive adhesive) and ACA (anisotropic conductive adhesive). ACA is a type of filling conductive adhesive, capable of finishing bottom filling with electric connection completed. Based on the differences of forms, ACA is classified into gelatinous form and thin film form. Generally, ACA of thin film form is also called anisotropic conductive film (ACF). ACA is conductive along the direction of Z axis while it is not conductive along the direction of X and Y axis. An insulation layer is laid over conductive particle layer and particles aren't conductive to each other. Only when particles suffer from stress between chip bump and PCB substrate pad and insulating layer is crushed as a result of stress, conductivity along Z axis can be ensured.

ESC Technology

ESC technology, short for epoxy encapsulated solder connection technology, is a new type of technology with the paste material of "paste particles plus resin" instead of ACF. The process flow of ESC technology starts with solder paste resin adhesive dripping on PCB pad. Then the chip bump is aligned with pad of PCB and mounted on it. Finally, soldering and resin solidification are completed through heating and compression.

Rework of Bottom Filling

Since present technology fails to ensure the good condition of supplied chips so that some defected chips can't be found until PCB test, rework and replacement are in great need. If the bottom filling material of PCB chip has excellent thermal stability and insolubility, more rework difficulties will come into being and even sometimes the whole PCB will be abandoned. If weak chemical bonds are led into epoxy resin of bottom filling material, resin will be decomposed through heating or adding chemical reagent after solidification, which will make rework of bottom filling much easier.

The application of bottom filling technology in PCB can increase the strength of solder joints from some chips such as BGA and CSP and of improving the falling-down resistance, anti-heat cycle performance and reliability of PCB. Therefore, it will be massively applied in PCB assembly in the future.

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