Applications of SMT on BGA Packages
Surface Mount Technology (SMT) plays an essential role in leading electronic products towards miniaturization and lightweight. The field of the high-pin electronic package has once seen the leading role of QFPs (quad flat packages) that are a type of surface mount integrated circuit (IC) package with “gull wing” leads stretching from each of the four sides. With the quick development of semiconductor integrated technology and micro-fabrication technology, however, IC gate count and I/O end number become increasingly more with electronic products’ functions increasing and volume constantly shrinking. Therefore, the application of QFP can never meet the development requirement of electronics products.
Although QFP technology is also making continual progress and is capable of dealing with components with pitch as low as 0.3mm, the decreasing pitch gets assembly pass rate reduced to some extent. To successfully solve this issue, BGA (ball grid array) technology comes into being and receives a wide range of focus from the industry.
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As a relatively new type of surface mount devices (SMDs), BGA features ball-shaped leads that are distributed in arrays at the bottom of the package. BGA components can have a large pin pitch and a high number of pins. Moreover, BGA components can be assembled on PCBs (printed circuit boards) through the application of SMT.
Structure and Properties of BGA
As a new type of SMDs, BGA evolves from PGA (pin grid array), generally composed by core cavity, base, leads, cover, and ball-shaped pins. The properties of BGA include:
• Higher Number of Pin Count. Within the same package size of SMDs, BGA can have more pins. Generally, a BGA component carries 400+ ball-shaped pins. For example, a BGA with an area of 32mm*32mm can carry as many as 576 pins while a QFP with the same area can only hold 184 pins.
• Smaller Assembly Area. With the count of pins, BGA holds smaller assembly area. For example, as a QFP with 304 pins is compared with a BGA with 313 pins, although the latter carries more pins, it accounts for less area by one third.
• Lower Assembly Height. The assembly height of BGA is lower than the sum of package thickness and solder ball height. For example, the height of QFP with 208 pins or 304 pins is 3.78mm while that of BGA with 225 or 313 pins is only 2.13mm. Furthermore, the assembly height of BGA will be reduced after soldering since its solder balls will be melted during soldering.
• Larger Pin Pitch. Based on the physical standard for BGA released by JEDEC, pin pitch between solder balls in BGA should be: 1.5mm, 1.27mm or 1.0mm. With the same package size and pin count, pin pitch for QFP is 0.5mm while that for BGA is 1.5mm.
• Excellent Thermal Dissipation. The temperature of BGA package circuits is closer to that of environment and the working temperature of chips is lower than that of any other SMD.
• Compatibility with SMT. BGA package is compatible with standard SMT. Besides, because BGA components feature larger pin pitch and excellent coplanarity, pins won’t suffer from bending issue and corresponding assembly technology is simpler than other SMD assembly with leads.
• Better Electrical Performance. Since BGA components carry shorter pins and higher assembly integrity, they share better electrical performance, which is especially true when they are applied in a higher frequency range.
• Lower Manufacturing Cost. Because BGA packages account for smaller assembly area and higher assembly density, the manufacturing cost will be reduced. Especially as BGA package output increases and receives wider applications, it’ll be obvious to get manufacturing cost reduced.
• Higher Reliability and Fewer Quality Defects. As solder balls on BGA packages are implementing soldering, the melting solder balls will be automatically aligned due to surface tension. Even if an error of 50% does occur between solder balls and pad, the excellent soldering effect can be obtained.
In spite of some obvious advantages of BGA packages, some disadvantages will be protruding during SMT assembly, including:
• Solder joints are difficult to be inspected. Solder joints inspection calls for X-ray inspection equipment that leads to higher cost.
• More difficulties have to be overcome on BGA rework. Since BGA components are assembled on circuit board through solder balls that are distributed in the array, rework will be more difficult.
• Partial BGA packages are so sensitive to humidity that dehumidification is called for prior to their applications.
Comparison between QFP and BGA
Compared with traditional SMT assembly, BGA shares simpler assembly technology thanks to its large pin pitch and excellent coplanarity of pins. BGA assembly requirement will be discussed below in this part.
• Moistureproof Principle for BGA
Some BGA components are so sensitive to humidity due to epoxy resin in adhesive inside the BGA core chamber that tends to absorb humidity in daily life that will be later vaporized with large stress generated inside the epoxy resin. Water vapor will cause the generation of bubbles at the bottom base, leading to cracks between the core chamber and base. Therefore, it’s necessary to carry out dehumidification before BGA components application. Due to the constant progress of technology and people’s increasing concern on dehumidification, some BGA packages have reached qualified moisture sensitive level and they can be placed for 48 hours in the environment of 30°C and 60%RH with no cracks generated during soldering. Some BGA components, CBGA (ceramic ball grid array) for example, are no longer sensitive to humidity. As a result, dehumidification procedures should be implemented on BGAs based on their classifications, environmental temperature, and humidity. Moreover, dehumidification has to be carried out according to package instructions and shelf life.
• BGA Solder Ball Coating and Printing
BGA solder balls are usually 25mm*0.0254mm high with diameter 30mm*0.0254mm. Different types of BGA components feature different alloy compositions. Generally speaking, TBGA, CBGA, and CGA depend on solder with high melting point while majority BGAs on solder with low melting point. High-temperature solder balls are applied mainly to stop solder balls from excessively collapsing. BGA solder ball coating and printing refers to the process in which flux or solder paste is coated on solder balls that will then be stuck to PCB, aiming to eliminate oxide on the pad and leading good connections to be generated between solder balls and PCB through melting solder.
• BGA Mounting
Thanks to larger pin pitch, BGA components are more easily mounted on PCB board. Up to now, some advanced mounters can get BGA components mounted. Furthermore, because BGA components can get self-aligned and even an error of 50% still can make it, mounting accuracy won’t be rigorously regulated.
• Reflow Soldering of BGA
In reflow soldering oven, BGA gets heated with solder balls or solder paste melted to form connections. To obtain excellent connections, it’s necessary to optimize temperature curve within the oven and optimization method is equivalent to that of other SMDs. It’s worthwhile to note that solder ball ingredients should be known so as to determine temperature curve of reflow soldering.
• BGA Inspection
BGA inspection covers soldering quality inspection and function inspection. The former refers to soldering quality inspection on solder balls and PCB pad. The arrangement mode of BGA adds difficulty to visual inspection and X-ray inspection is required. Function inspection should be implemented on online devices, which is equivalent to SMD test with other types of packages.
• BGA Rework
Similar to BGA inspection, it’s equivalently difficult to carry out rework on BGAs and professional rework tools and equipment are required. In the process of rework, broken BGAs need to be eliminated first and then modifications have to be done to PCB pad with flux coated on. New BGAs need preprocessing and instant soldering should be carried out.
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