Some Engineer-Friendly Methods to Achieve Optimal Solder Joints in BGA SMT Assembly Process

Constant progress of scientific technology has led modern society to be closely associated with electronic technology. Rigorous requirement has been laid to miniaturization and light weight of electronics products like cell phones, portable computers, storage, hardware drivers, CD-ROM drive, high-resolution TVs etc. To obtain such targets, study has to be implemented in terms of manufacturing technology and components. SMT (surface mount technology) conforms to such trend, laying solid foundation for miniaturization of electronic products.

1990s has seen SMT stepping into a mature phase. However, higher requirement has been laid to electronics assembly technology with electronic products quickly developing towards portability, miniaturization, networking and multimedia among which BGA (ball grid array) package is a type of high-density assembly technology stepping into a pragmatic phase. Solder joint quality plays such a key role in determining reliability and performance of SMT assembly that BGA solder joint quality should be focused. Thus, this article will provide some effective measures to guarantee the quality of solder joint of BGA components based on which final reliability of SMT assembly can be fulfilled.

A Brief Introduction of BGA Packaging Technology

BGA packaging technology started as early as 1960s and was first applied by IBM Company. Nevertheless, BGA packaging technology didn't enter a pragmatic phase until the beginning of 1990s.

As early as 1980s, people laid higher requirement for electronics miniaturization and I/O pin number. In spite of miniaturization characteristic held by SMT, more rigorous requirement has been set on high I/O pin count and fine pitch components and lead coplanarity. Due to limitations in terms of manufacturing accuracy, manufacturability, cost and assembly technology, however, limit pitch of QFP (quad flat package) components is 0.3mm, restricting the development of high-density assembly. Moreover, because fine-pitch QFP components call for strict demands on assembly technology, which makes their applications confronted with limitations, component manufacturers switch to R&D on BGA components that are more advantageous than QFP components.

Limitations of fine-pitch components lie in their leads that are easy to be bent and to be broken and suffer from fragility, laying high requirement for lead coplanarity and mounting accuracy. BGA packaging technology takes advantage of a new design thinking mode, that is, circular or column-shaped solder balls are hidden below package so that lead spacing is large with leads being short. As a result, BGA packaging technology is capable of defeating the issue deriving from coplanarity and warpage that usually take place on fine-pitch components.

Therefore, BGA components perform better in reliability and SMT assembly than ordinary SMDs (surface mount devices). The only problem of BGA components lies in their difficulty in terms of solder joint test, difficulty to guarantee quality and reliability.

BGA Component Solder Joint Issues

Up to now, reliable electronic assemblers, PCBCart for example, BGA component soldering defects are exposed through electronic tests. Other methods to control technical process quality of assembly and determine defects during BGA component assembly include sample test on paste screening, AXI and result analysis on electronic testing.

Meeting quality assessment requirement is a challenging technology because it's difficult to pick up test points under packages. When it comes to BGA component defect inspection and identification, electronic test is usually incapable, which to some extent adds cost for defect elimination and rework.

During the process of BGA component defect inspection, electronic testing is only able to judge whether current is on or off once BGA components are connected. If non-physical solder joint test is implemented as assistance, it's beneficial for technical process of assembly and SPC (statistical process control) improvement.

BGA component assembly is a type of basic physical connection technical process. To be able to confirm and control the quality of technical process, physical elements have to be known and tested affecting their long-term reliability such as solder paste volume, alignment of leads and pad and wettability. Otherwise, it's worrying to make modifications just based on the result generated by electronic testing.