An analysis of two extensively used packaging technologies QFP and BGA

Traditionally, through hole technology (THT) was vastly used in the PCB design since it replaced its predecessor techniques such as point-to-point construction. That was until surface mount technology ( SMT) gained acceptance in the late 1980s (although it was developed in the 1960s). With the technological evolution in the semiconductors field, IC packages have had to adapt accordingly in terms of the size and pin count. Standard dual-in-line packages (DIP), which fulfilled the basic IC requirements such as protection and heat dissipation, enjoyed extensive usage in the electronics industry for many years. Back in 1986, Quad Flat Package (QFP), a package design developed in Japan, was introduced to world by Electronic industries Association of Japan in an international conference on semiconductor packages called ‘International Microelectronics Conference’. It was groundbreaking in the sense that it provided an excellent format for high pin count SMT integrated circuits. Then came the era Chip Size Packages ( CSPs) during the 1990s. The development of CSPs paved way for even more compact information devices such as the mobile phones. Moving forward, Flip chip assembly process provided further advantages in terms of cost, component performance/reliability and design flexibility. Finally, the use wafer-level packaging (WLP) in sensors and power supervision circuits of modern era smart phones is another great example of steep technological advancement in IC packages.

 

Quad Flat Package (QFP):

 

QFP emerged as the new generation high density surface mount solution to the cost pressure on the component manufacturers and users. QFP typically have 32 to 208 pins placed in a pitch of 0.4 to 1 mm. Its typical structure is flat rectangular body (usually square) and it has leads spreading from all along the perimeter.

 

Ball Grid Array (BGA):

 

Plastic BGA was jointly developed by Motorola and Citizen in 1989 which was followed by the development of ceramic BGA by Motorola and IBM. BGA’s were introduced because other packaging approaches like QFP had reached their limit in terms of the number of I/Os. Whenever I/O count exceeds 250, BGAs are beneficial over QFPs in terms of footprint requirement.  BGA package typically have the lead count between 50 and 500 (theoretically 1000 are possible) and the ‘pitch’ (center-to-center distance of the balls) ranges between 1.0 and 1.5mm. Having larger pitch area than QFPs certainly results in ease in mounting of BGA components. There are several types of BGA packages with subtle differences. BGA packaging technique is less attractive for small volume production and prototyping applications. However, BGA components are sturdy and can still be used even after receiving in impactful force (like falling on the floor) whereas it is not possible for plastic QFP.

 

BGA package types

Figure 1 BGA package types

BGA and QFP Comparison

 

Both packages fulfill the ultimate requirement of modern IC technology i.e. large number if I/Os. However, as discussed earlier, the increasing number of I/Os pushed the peripheral QFP technology to the limits of lead pitch. Graph below clearly depicts that replacement of BGAs with QFPs means higher pin count and smaller package size.

 

Package size comparison for peripheral and BGA

Figure 2 Size comparison of square QFP and BGA packages  (Rörgren, 1995)

 

If we take a specific example of 50 mil BGA and 25 mil PQFP, BGA results in smaller footprint whenever number of pins exceeds 70.

 

Size comparison of square QFP and BGA packages

Figure 3 Package size comparison for peripheral and BGA (Bogatin, 1997)

 

It is also crucial to discuss some drawbacks linked with BGA packaging technique. BGA packages suffer from assembly defects such as misalignments, missing balls, bridges and partially reflowed solder joints. Moreover, excessive collisions during shipping process also results in defects for BGA packages. However, these assembly defects are not the biggest downside of BGA packages. In fact, while comparing with QFP, the major drawback is the cost of BGA packaging. Naturally, the cost difference between the two reduces in the case of bulk production. For volume production, high efficiency of BGA assembly compensates for the downside of high cost. As described earlier, whenever I/O pins count exceeds 250, selection of BGA over QFP is a suitable choice. According to some estimates, almost 50% of the designs require more than 240 pins. Which means the trend towards higher pin count has resulted in the selection of BGA as a suitable choice of packaging technique.

 

Another crucial design difference between these two packaging techniques is the location of connections. BGA components have connections under the package as opposed to QFP having pins at the periphery.  So inherently it becomes difficult to detect any defects as solder joints are not directly visually accessible. That’s why, in case of any defects, BGA inspection requires in-depth examination through imaging techniques such as X-rays. Consequently, reduction in any assembly defects and high pass rate is of extreme importance for BGA assembly.

 

Conclusion:

 

BGA will continue to the packaging technology where lead number is higher than 250. Between 200 and 300 lead count, BGA and QFP are still competing with certain advantages and disadvantages linked with both techniques.

 

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