Do you always feel dizzy confronted with some short terms such as SMT, LED, CMRR, CCD or even PCB? It's beneficial that these short terms make our communications easy, convenient and fast. However, sometimes the short terms lead to our quick memory loss about the meaning of these short terms. This series of articles will talk about these short terms. Let's start with BGA today!
What is BGA?
The appearance of BGA (ball grid array) stems from people's expectations for the numerous functions, high performance, small size and light weight of electronic products. In order to reach this goal, the size of integrated circuit (IC) chip must be gradually reduced and complexity be increased so that the packaging I/O density has to be increased. High density and low cost packaging methods are in great need and BGA is one of them.
BGA is basically a form of surface mount technology (SMT) or a type of surface mount packaging for integrated circuits (ICs). Normally, conventional surface mount packages use the sides of the packages for connection so that there's limited area for pins' connection. However, BGA package uses underside for connection and more space can be provided for connection, which makes it possible for the high density of PCBs and high performance of electronic products.
BGA is widely used as a packaging method based on its advantages:
• Highly-effective Use of PCB Space
Using BGA packaging means the fewer components' participation and smaller footprints also help save the space on custom PCBs, which both highly increases the effectiveness of PCB space.
• Increase in Thermal and Electrical Performance
Since the small size of PCB based on BGA packaging, heat can be dissipated more easily. When silicon wafer is mounted on top, most of the heat can be transmitted downwards to the ball grids. When silicon wafer is mounted at the bottom, the back of silicon wafer is connected to the top of packaging, which is considered as one of the best heat dissipation methods. BGA packaging has no pins that can be bent and broken, which makes it become stable enough so that the electrical performance can be ensured on a large scale.
• Increase of Manufacturing Yields Based on the Soldering Improvement
Most BGA packaging pads are relatively big, which makes it easy and convenient to solder on a large area so that PCB fabrication speed increases with manufacturing yields improved. Besides, with larger soldering pads, it's convenient to re-work on it.
• Fewer Damage Leads
BGA leads are composed by solid solder balls that can't be easily damaged in the process of operation.
• Lower Cost
All the advantages displayed above all contribute to the final benefit that is lower cost. Highly-effective use of PCB space provides opportunity to save material. Increase in thermal and electrical performance is capable of ensuring the quality of electronic components and decreasing the chance of defects.
There are mainly three types of BGAs: PBGA (plastic ball grid array), CBGA (ceramic ball grid array), and TBGA (tape ball grid array).
Normally, BT resin/glass laminate is used as the substrate, plastic as the packaging material. Solder balls can be classified into lead and lead-free solder. No other solder is needed to connect the solder balls with packaging body.
CBGA has the longest history among the three types of BGAs. The material of substrate is multi-layer ceramic. Metal cover is soldered onto the substrate by packaging solder in order to protect the chip, leads and pad. High temperature eutectic solder is used as the material of solder balls.
TBGA is a structure with a cavity. There are two kinds of interconnections between chip and substrate: inverted solder bonding and lead bonding.
The features (advantages and disadvantages) comparison between the above three types of BGAs is shown in Table 1.
Step#4: Inspection of BGA
In order to inspect the quality of BGA, X-ray inspection is widely used. X-ray inspection technology is a technology used to inspect the hidden features of target objects or products with X-rays as its source. You can find why is X-ray inspection technology so important from our blog.
Usually, the main four inspection parameters are obtained through X-ray laminography technology:
• Position of Solder Joint Centers
The relative positions of solder joint centers can reflect the positions of electronic components on PCB pad.
• Solder Joint Radius
Solder joint radius measurement can show the number of solder in solder joints on a specific layer. The radius measure on the pad layer indicates any change caused in the process of paste screening craft and pad pollution. The radius measurement on the ball level layer indicates the solder joint coplanarity problems about surpassing components or PCBs.
• The thickness of solder on each loop that is obtained with solder joint as a center
Loop thickness measurement indicates the distribution of solder in solder joints. This parameter is used to judge the humidity and existence of voids.
• Deviation from Circular Form
Deviation from circular form indicates the uniform of solder distribution around solder joints, autoregistration and humidity.
These four parameters inspected are quite significant to determine the integrity of solder joint structure and to understand the performance of each step in the process of the implementation of BGA assembly craft. To know the information provided in the process of BGA assembly and the relationship between these physical inspections is capable of stopping displacement and improving craft so as to eliminate defects. What's more, X-ray laminography inspection can be used to indicate the defects occurring at any step in the process of BGA assembly.