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Elements that should be Carefully Considered on BGA Assembly Process Capability

BGA (ball grid array) assembly is totally compatible with soldering assembly technology. Pitch of chip-scale BGA can be 0.5mm, 0.65mm or 0.8mm and plastic or ceramic BGA components feature wider pitch like 1.5mm, 1.27mm and 1mm. BGA packages with fine pitch are more easily damaged than ICs (integrated circuits) with pin packages and BGA components allow selective reduction of contact points to meet the specific requirement on I/O pins. As a cutting-edge technology applied in SMT (surface mount technology) assembly, BGA packages have quickly become a significant selection to conform to fine pitch and ultra fine pitch technology, achieving high-density interconnection with a reliable assembly technology provided, which leads to the increasingly more applications of this type of packages.

Application of X-Ray Tomography Inspection Device in BGA Assembly

Most PCB (printed circuit board) manufacturers and electronics manufacturers haven't noticed too much necessity to apply X-ray inspection in their manufacturing process until BGA components are applied in electronics assembly. Traditional inspection methods were regarded to be sufficient such as MVI (manual visual inspection) and electrical test including MDA (manufacturing defect analysis), ICT (in-circuit test) and function test. However, all those inspection methods fail to find out hidden solder joint issues such as cavities, cold soldering and bad tin soldering adhesion. X-ray inspection system is a type of inspection tool that has been verified to be capable of inspecting hidden solder joints and help establishing and controlling manufacturing process, analyzing prototype and confirming process. Different from MDA, ICT and AOI (Automated Optical Inspection), X-ray inspection system is capable of confirming short circuits, open circuits, cavities and BGA solder ball alignment, monitoring process quality and providing instant feedback data for SPC (statistical process control) with high manufacturing efficiency.

X-ray tomography inspection devices can generate tomography images through capturing images of solder joints that are able to implement automatic solder joint analysis and real-time tomography scanning. Moreover, they can carry out accurate comparison analysis on all solder joints of components on both sides of PCB board within couples of seconds or 2 minutes, leading to a conclusion whether solder joints are qualified or not.

BGA Assembly Process and Variation Source

To use X-ray inspection system more effectively, control parameters of BGA assembly process and parameter control limitations have to be clarified. BGA assembly process conforms to the following sequence:

Elements that should be Carefully Considered on BGA Assembly Process Capability | PCBCart

When BGA components eutectic solder balls are assembled in solder paste during assembly process, their positions are usually corrected through self-alignment of liquid soldering tin. Thus, mounting precision seems not so essential as fine pitch lead components and the leading control phase in BGA component assembly technology is solder paste printing and reflow soldering. In addition, variation in terms of solder joint shape and size is also associated with many other elements.

It's almost impossible to eliminate all the variations, so the key point in manufacturing process control is to reduce variation in each manufacturing phase. The influence of different variations on final assembly products should be carefully analyzed and quantitatively processed. With the whole process from BGA components to PCB assembly process considered, leading elements affecting solder joint quality are:
1. Volume of solder balls;
2. BGA component pad size;
3. PCB pad size;
4. Solder paste volume;
5. BGA component deformation during reflow soldering process;
6. PCB deformation at BGA mounting area during reflow soldering process;
7. Mounting placement accuracy;
8. Reflow soldering temperature curve.

No matter what type of inspection device is used, there must be basis when judging whether solder joints are qualified or not. IPC-A-610C regulates the definition of acceptance criteria of BGA solder joints in 12.2.12 item. Excellent BGA solder joints are required to be smooth, round, clear in edge and with no cavities. Diameter, volume, gray scale and contrast should be the same for all solder joints with position aligned and no displacement or twist.

BGA Assembly Process Capability

A type of BGA component is used as an example in the following discussion. This type of BGA components is PBGA (plastic ball grid array) components with 520 pins and a size 2"x2", featuring eutectic solder balls and leveraging no-clean flux. 6 sigma process capability analysis is implemented to testify BGA placement accuracy, solder joint open circuits and short circuits occurring probability. The assumptions prior to calculation are:
a. No variations take place on BGA component pad or PCB pad;
b. BGA components suffer from no deformation (reflow soldering process);
c. The average deviation is figured out in accordance with the average volume of solder joints after reflow soldering;
d. BGA component weight is assumed to be balanced by flotage and surface tension;
e. Pad and eutectic solder balls should feature good solderability;
f. All the distributions are normal distributions.

• BGA Placement

Standard SMT equipment is used to get BGA components mounted. Ordinary mounting equipment is capable of recognizing BGA eutectic solder ball images with placement process capabilities covered as the following:

Elements that should be Carefully Considered on BGA Assembly Process Capability | PCBCart

Based on the above data, the maximum placement deviation is 6.53mil when process capability is 6sigma. Since diameter of pad is 28mil, the placement deviation can be neglected among components' self-alignment deriving from surface tension when solder paste is melted. When it comes to BGA components placement process, it conforms to 6sigma level.

• Solder Joints with Open Circuits

Assembly process tends to see open solder joints due to insufficient eutectic solder ball collapse. As far as PBGA with 520 pins are concerned, eutectic solder balls are balls with diameter of 30mils whose standard deviation is 500mils3 (with volume participated in) and volume is regulated to be 14,130mils3. Diameter of BGA and PCB pad is 28mil with its solder paste thickness being 6mil. Therefore, the average height of BGA solder ball edge is approximately 24mils. As 6sigma capability reflecting solder ball volume variation is concerned,

Elements that should be Carefully Considered on BGA Assembly Process Capability | PCBCart

After reflow soldering, the height of soldering bonding support determined by average volume of solder joints is 19mils. As process capability is set to be 6sigma, solder paste thickness is measured to be 4 to 8mils. Moreover, BGA solder balls will be collapsed into solder paste for 3mils, which leads to the following calculated data:
Minimum thickness of solder paste below solder balls = 3mils
Minimum collapse = 7mils
Minimum incorporated collapse = 10mils
Minimum security deviation generated to stop open circuits from taking place = 2.2mils

When above variations can be controlled into certain ranges, BGA reflow soldering process can achieve 6sigma.

Unfortunately, deformation at BGA components and PCB usually leads to height inconsistency of soldering bonding during BGA reflow soldering assembly. BGA components and PCB pad features differences that lead to process variation. All in all, even though all variations are taken into consideration, open soldering joints will still take place. Thus, X-ray inspection system can be used to carry out defect inspection on open solder joints.

• Solder Joint Bridging (short circuits)

The same method can be used to estimate the influence of short circuits of soldering joints on assembly process capability. Solder joints differ from each other in terms of diameter and measured data indicates that bonding volume of each solder joint is in the range from 12800 to 19250mils3 under 6sigma process capability. As a result, the height of minimum soldering bonding support is 15mils and then maximum soldering bonding diameter can be as much as 38.5mils. When it comes to BGA components with pitch of 50mils, solder joint bridging will hardly take place.

Statistical Process Control Analysis

Effective BGA assembly process control leads fewer variations occurring to solder connections. In practical assembly process, however, the following variations usually make process fluctuate, calling for consistent monitoring on them.
1. Solder paste height and volume;
2. Diameter of side connection of BGA components;
3. Diameter of side connection of PCB pad;
4. Central bonding diameter of connections;
5. Cavity size and occurrence rate;
6. Tin balls.

Solder paste thickness can be monitored by X-ray inspection equipment and process variations can be controlled within a certain level based on solder joint shape and consistency.

Article wrote by PCBCart editor Dora Yang, originally published on SMT007 Magazine 2018 May issue.

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Helpful Resources
A Brief Introduction of BGA Package Types
An Introduction of BGA Packaging Technology
Factors Affecting the Quality of BGA Assembly
PCBCart''s One Stop Production Service Covers PCB Fabrication, Components Sourcing and Turnkey Assembly

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