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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.

BGA Component Inspection Methods

It's extremely important to test physical features of BGA component solder joints and determine how to consistently contribute to reliable connections during assembly process during technical process research period. The feedback information provided by all the tests is related with modifications of each technical process or of solder joint parameters.

Physical test is capable of marking situation changes of paste screening and the situation of BGA component connections in reflow soldering process. Moreover, it can showcase the situation of all BGA components on the same circuit board and across boards. For example, during the process of reflow soldering, extreme humidity changes with changes of cooling time, which can be reflected on cavity count and size of BGA solder joints.

As a matter of fact, there are not so many inspection devices for accurate measurement and quality inspection for the whole technical process of BGA component assembly. Automatic laser inspection devices are capable of testing solder paste printing situation prior to component mounting, but they run at a low speed, incapable of reflow soldering quality inspection for BGA components.

With X-ray inspection device applied, solder paste at pad indicates an image of shadow because solder paste is placed above solder joint. When it comes to non-collapsible BGA components, shadow can be also seen due to preposed solder balls that surely make it difficult to determine that. That's because shadow effect aroused by solder paste or preposed solder balls prohibits the work of X-ray inspection devices that can only roughly reflect process defects of BGA packaging. Furthermore, periphery inspection suffers from challenges including insufficient solder paste or open circuits as a result of contaminant.

Cross-section X-ray inspection technology is capable of overcoming the limitations mentioned above. It can inspect hidden defects of solder joints and display connections of BGA solder joints.

Essential Defects of BGA Solder Joint

• Open Circuits

Open circuits always occur to non-collapsible BGA solder joints because of pad contaminant. Because solder paste fails to get pad on PCB (printed circuit board) wetted, it will climb to component surface across solder balls. As is mentioned above, electronic test can determine open circuits but fails to distinguish whether open circuits result from pad contaminant or defects of solder screening. X-ray inspection devices fail to indicate open circuits as well, which derives from shadow effect of preposed solder balls.

Cross-section X-ray inspection technology is capable of capturing slice image between pad and components and thereafter confirming open circuits as a result of contaminant. Because open circuits due to contaminant generates fine pad diameter and relatively large component diameter, difference between component diameter and pad diameter can be used to determine whether open circuits take place as a result of contaminant. As far as open circuits as a result of insufficient solder paste are concerned, only cross-section inspection devices can make it.

• Void

Void happening to collapsible BGA component soldering is generated because flowing steam is held back at solder joints with low eutectic point. Void can be regarded as a leading defect occurring to collapsible BGA components. During the procedure of reflow soldering, flotage as a result of void focuses on the surface of components so majority of solder joint failures take place there as well.

Void issues can be eliminated through preheating and increasing transient preheating time and low preheating temperature during reflow soldering process. Once voids exceed a certain range of size, count or density, reliability will be definitely decreased. However, another school is available believing that voids shouldn't be limited but should be accelerated for their breaking and expansion so that they can be found failed and eliminated as soon as possible.

PCBCart: A Professional SMT Assembler for BGA Components

PCBCart features specialized SMT assembly line containing solder paste printer, chip mounter, on-line and off-line AOI equipment, reflow soldering oven, AXI equipment and BGA rework station. The automatic assembly procedure provided by PCBCart is capable of dealing with BGA components with a pitch as low as 0.4mm. All the services and products provided by us are compatible with regulations of ISO9001:2008 system, which is a solid foundation for customers' expectation achievement. Contact us for more information about our SMT capabilities for BGA components. Or, you may click below button to request FREE and non-obligation PCBA quote!

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Helpful Resources
Four Steps to Know BGA
An Introduction of BGA Packaging Technology
A Brief Introduction of BGA Package Types
Factors Affecting the Quality of BGA Assembly

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