What is BGA Assembly?

BGA assembly is the process of mounting Ball Grid Array (BGA) components onto a PCB and connecting them electrically to its copper trace circuit. In this process, the BGA package, which has solder balls underneath in a grid pattern, is placed on the PCB which has solder paste applied in the form of small beads in the same grid pattern as the BGA component. When heat is applied, the balls of solder paste melt, and it forms electrical connections with the solder balls of the component. This method of soldering is called reflow soldering. Reflow soldering is the most common method of BGA assembly and is ideal for complex and space-constrained PCBs due to its precision and cost-efficiency.

BGA Assembly Process

The BGA assembly process involves several key steps to ensure the accurate placement and soldering of BGA components:

Stencil Creation and Inspection

A solder stencil is a thin sheet with a circuit pattern cut into it that's used to apply solder paste to a PCB. It is created using digital design files of the PCB and has a layout that includes pads for each solder ball on the BGA. The solder stencil is fed into a solder printer, which is a machine that aligns the stencil on top of the PCB and applies solder paste over the holes. When the stencil is separated from the board, the solder paste remains. Following this, the PCB with applied solder paste is subjected to an examination using a solder paste inspection machine to verify the precise application of the paste onto the solder joints.

BGA Placement

Once the solder paste is applied, the BGA components are positioned on the PCB using an automated pick-and-place machine. The solder balls on the BGA must be aligned with the corresponding pads on the PCB. Pick-and-Place machines perform this process with exceptional speed and accuracy.

Reflow Soldering

The PCB, with the BGA components in place, is heated in a reflow oven. This process gradually melts the solder paste forming solid solder joints between the BGA and the PCB and securing them firmly to the board. Improper reflow profiles can lead to defects like voids, incomplete reflow, or thermal damage to the components. Hence, reflow soldering ovens operate with precise control over their temperature to perform the soldering process efficiently.

Cleaning and Inspection

Once the solder has cooled, the board may undergo a cleaning process to remove flux residues except for assemblies using no-clean solder paste. The assembled PCB undergoes rigorous inspection and quality control checks to ensure proper solder joints and the verify overall quality of the assembly. X-ray inspection machines are used to verify the integrity of the solder joints, especially since BGAs have hidden connections that are not visible to the naked eye. Automated Optical Inspection (AOI) systems may also be used to check for surface-level defects.

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Key Considerations for a Successful BGA Assembly

• Accuracy in printing solder paste is very crucial to avoid cases of short circuits and slippery alignment.
• The pick-and-place machine should apply only the required pressure during placement since excess pressure can damage components.
• Adequate heat should be applied so that all the balls in the grid melt to realize a strong bond for every BGA solder joint.
• The surface tension of the molten balls locks the BGA package in place until the solder cools and solidifies. A temperature-controlled soldering process is essential for establishing quality solder joints. This practice also prevents solder balls from short-circuiting each other.
• The configuration of the solder alloy and soldering temperature is chosen precisely so that the solder doesn’t completely melt but stays semi-liquid. It allows each ball to stay separate from its neighboring balls.

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