PCB Blog - Failure analysis of welding connection between FPGA and PCB board

FPGAs are used in most electronic PCB board systems, including many commercial and defense fields, and most FPGAs use BGA packages. As soon as BGA appeared, it became the choice for high-density, high-performance, multi-function and high I/O pin packaging of VLSI chips such as CPU and north-south bridge. Its features are:
1. Although the number of I/O pins increases, the pin spacing is much larger than that of QFP, thus improving the assembly yield;
2. Although its power consumption increases, BGA can be welded by the controlled collapse chip method, referred to as C4 welding, which can improve its electrothermal performance:
3. The thickness is reduced by more than 1/2 compared with QFP, and the weight is reduced by more than 3/4;
4. The parasitic parameters are reduced, the signal transmission delay is small, and the frequency of use is greatly improved;
5. Coplanar welding can be used for assembly, with high reliability;
6. The BGA package is still the same as QFP and PGA, which occupies too much substrate area;

Solder joint failure failures frequently occur in FGPAs, in all types of commercial and defense products. When the FPGA is packaged in a BGA package, the FPGA is susceptible to solder connection failures. The cause of weld failure cannot be isolated, early detection is very difficult, and intermittent failures can escalate over time until the equipment provides unreliable performance or becomes inoperable. However, as often happens, this problem can be solved, and it is Ridgetop-Group SJ-BIST. Generally speaking, welding failure means that the welded joint breaks under certain conditions due to various factors (such as: stress, temperature, material, welding quality and actual working conditions, etc.). Once the joint fails, the components that are closely connected to each other will be partially separated, torn and expanded, resulting in damage to the welded structure, causing equipment downtime and affecting normal production.

What factors can cause a welded connection to fail?
Common failure reasons:
1) Stress-related failures-for devices in operation
Usually, the defects of the material itself (such as chemical composition inhomogeneity, local micro-cracks), hot and cold cracks, incomplete penetration, slag inclusions, pores and undercuts in the weld due to various reasons, etc., during the welding process. The high residual stress in the near seam area (including the structural stress of the phase transformation of the weld and the heat affected zone), as well as the softening of the structure at high temperature during the welding process and the embrittlement after cooling, are the root causes of joint failure, and also contribute to the failure of the joint. The brittle fracture or expansion of the joint provides the conditions. The current method for predicting the failure of welded connections is the statistical degradation model. However, since statistics are only meaningful when large numbers of samples exist, statistical-based models are at best a stopgap solution. SJ-BIST from Raytor Group can provide a direct, real-time means of measuring and predicting failure of welded connections.

2) Failures related to manufacturing production
Because solder joint failures also occur during the manufacturing process. Ridegtop-Group SJ-BIST can detect unmounted FPGAs. These manufacturing-related failures have their own set of detection challenges. Visual inspection is the method currently employed to determine failure in a manufacturing environment. The main disadvantage is the inability to test and inspect solder joints. Visual inspection is limited to the solder joints on the outer row of the FPGA, while board size and other surface mount components limit further visibility. As the BGA package array density increases, the solder ball deviation becomes more stringent. In fine pitch BGA packages, there are thousands of solder balls with 1.0mm pitch and 0.60mm ball diameter. Under these conditions, insufficient pad tuning and soldering become the main causes of pad disconnection and partial disconnection failures. Even a 100% x-ray inspection is not guaranteed to find a solder joint break when the solder does not wet the pad. Another defect involving solder balls and sticking capillary penetration into plated through holes is not easy to identify, even with X-ray imaging. As an embedded soft core, Ridgetop-Group SJ-BIST is really suitable for PCB board-FPGA monitoring in a manufacturing environment.
Definition of BGA package connection failure (for thermal cycling):

The industry's definition of BGA package connection failure is:
1) Peak resistance greater than 300 ohms for 200 ns or more.
2) 10 or more failure events occur 10% of the time after 1 failure event.

Types of Weld Failures:
1) Solder ball crack
Real-time failure detection of soldered connections between a working FPGA and a PCB board. Over time, welds can develop cracks due to damage from accumulated stress. Cracks are common at the edge where the device is soldered to the PCB. Cracks can cause solder balls to separate from parts of the BGA package or PCB. A typical crack location is between the BGA package and the solder balls, and another typical crack is between the PCB board and the solder balls. Continued damage to a cracked solder ball can lead to another type of failure - solder ball fracture.
2) Solder ball breakage
Real-time failure detection of soldered connections between a working FPGA and a PCB board. Once there is a crack, subsequent stress can cause the solder ball to break. The fracture causes the solder ball and the PCB board to separate completely, resulting in an open circuit state for a long time, contamination and oxidation of the fracture surface. The end result is from a degraded connection to a short intermittent open circuit all the way to a longer open circuit.
3) Missing solder balls
Subsequent mechanical stresses that lead to cracks, and ultimately fractures, may also lead to dislocation of the fractured solder balls. Not only does a missing solder ball fail that pin's connection, but a misplaced solder ball could get stuck in another location causing an unimaginable short in another circuit.

Electrical Signs of Solder Ball Failure: Periodic opening and closing of solder ball fractures can lead to intermittent electrical signal failures. Vibration, movement, temperature changes, or other stresses can cause broken solder balls to open and close, resulting in intermittent failures of electrical signals. The flexible materials used by the PCB board factory also make this intermittent signal possible, such as breaking open and close caused by vibration stress, and the unpredictable opening and closing of solder ball circuits cause intermittent signals. This intermittent Faults are difficult to diagnose. In addition, the I/O buffer circuit around the FPGA makes it nearly impossible to measure the resistance value of the solder network. A device that fails in a working FPGA may pass without any fault finding (NTF) on the test bed because the solder joints are temporarily connected. Many users find that the FPGA is not working properly, and it is for this reason that the FPGA will work normally by pressing P by hand.

Real-time failure detection of soldered connection between FPGA and PCB board at work: SJ-BIST detects soldering status in real time. Currently, techniques such as visual inspection, optical, X-ray and reliability testing used in manufacturing are difficult to work because the reflection is electrical. Signal failure faults are largely invisible when the device is not powered. Through early detection of impending failures, SJ-BIST supports condition-based equipment maintenance and reduces intermittent failures. Its superior sensitivity and sensitivity allow SJ-BIST to detect and fail high resistances down to 100 ohms within two clock cycles without false alarms. As a scalable solution, it can be attached to a user's existing test hub without adding additional resources.

FPGAs are used in most electronic systems, including many commercial and defense fields, and most FPGAs use BGA packages. As soon as BGA appeared, it became the choice for high-density, high-performance, multi-function and high I/O pin packaging of VLSI chips such as CPU and north-south bridge. Its features are:
1. Although the number of I/O pins increases, the pin spacing is much larger than that of QFP, thus improving the assembly yield;
2. Although its power consumption increases, BGA can be welded by the controlled collapse chip method, referred to as C4 welding, which can improve its electrothermal performance:
3. The thickness is reduced by more than 1/2 compared with QFP, and the weight is reduced by more than 3/4;
4. The parasitic parameters are reduced, the signal transmission delay is small, and the frequency of use is greatly improved;
5. Coplanar welding can be used for assembly, with high reliability;
6. The BGA package is still the same as QFP and PGA, which occupies too much substrate area;

Solder joint failure failures frequently occur in FGPAs, in all types of commercial and defense products. When the FPGA is packaged in a BGA package, the FPGA is susceptible to solder connection failures. The cause of weld failure cannot be isolated, early detection is very difficult, and intermittent failures can escalate over time until the equipment provides unreliable performance or becomes inoperable. However, as often happens, this problem can be solved, and it is Ridgetop-Group SJ-BIST. Generally speaking, welding failure means that the welded joint breaks under certain conditions due to various factors (such as: stress, temperature, material, welding quality and actual working conditions, etc.). Once the joint fails, the components that are closely connected to each other will be partially separated, torn and expanded, resulting in damage to the welded structure, causing equipment downtime and affecting normal production.

What factors can cause a welded connection to fail?
Common failure reasons:
1) Stress-related failures - for devices in operation
Usually, the defects of the material itself (such as chemical composition inhomogeneity, local micro-cracks), hot and cold cracks, incomplete penetration, slag inclusions, pores and undercuts in the weld due to various reasons, etc., during the welding process. The high residual stress in the near seam area (including the structural stress of the phase transformation of the weld and the heat affected zone), as well as the softening of the structure at high temperature during the welding process and the embrittlement after cooling, are the root causes of joint failure, and also contribute to the failure of the joint. The brittle fracture or expansion of the joint provides the conditions. The current method for predicting the failure of welded connections is the statistical degradation model. However, since statistics are only meaningful when large numbers of samples exist, statistical-based models are at best a stopgap solution. SJ-BIST from Raytor Group can provide a direct, real-time means of measuring and predicting failure of welded connections.

2) Failures related to manufacturing production
Because solder joint failures also occur during the manufacturing process. Ridegtop-Group SJ-BIST can detect unmounted FPGAs. These manufacturing-related failures have their own set of detection challenges. Visual inspection is the method currently employed to determine failure in a manufacturing environment. The main disadvantage is the inability to test and inspect solder joints. Visual inspection is limited to the solder joints on the outer row of the FPGA, while board size and other surface mount components limit further visibility. As the BGA package array density increases, the solder ball deviation becomes more stringent. In fine pitch BGA packages, there are thousands of solder balls with 1.0mm pitch and 0.60mm ball diameter. Under these conditions, insufficient pad tuning and soldering become the main causes of pad disconnection and partial disconnection failures. Even a 100% x-ray inspection is not guaranteed to find a solder joint break when the solder does not wet the pad. Another defect involving solder balls and sticking capillary penetration into plated through holes is not easy to identify, even with X-ray imaging. As an embedded soft core, Ridgetop-Group SJ-BIST is really suitable for PCB board-FPGA monitoring in a manufacturing environment.
Definition of BGA package connection failure (for thermal cycling):

The industry's definition of BGA package connection failure is:
1) Peak resistance greater than 300 ohms for 200 ns or more.
2) 10 or more failure events occur 10% of the time after 1 failure event.

Types of Weld Failures:
1) Solder ball crack
Real-time failure detection of soldered connections between a working FPGA and a PCB board. Over time, welds can develop cracks due to damage from accumulated stress. Cracks are common at the edge where the device is soldered to the PCB. Cracks can cause solder balls to separate from parts of the BGA package or PCB. A typical crack location is between the BGA package and the solder balls, and another typical crack is between the PCB board and the solder balls. Continued damage to a cracked solder ball can lead to another type of failure - solder ball fracture.
2) Solder ball breakage
Real-time failure detection of soldered connections between a working FPGA and a PCB board. Once there is a crack, subsequent stress can cause the solder ball to break. The fracture causes the solder ball and the PCB board to separate completely, resulting in an open circuit state for a long time, contamination and oxidation of the fracture surface. The end result is from a degraded connection to a short intermittent open circuit all the way to a longer open circuit.
3) Missing solder balls
Subsequent mechanical stresses that lead to cracks, and ultimately fractures, may also lead to dislocation of the fractured solder balls. Not only does a missing solder ball fail that pin's connection, but a misplaced solder ball could get stuck in another location causing an unimaginable short in another circuit.

Electrical Signs of Solder Ball Failure: Periodic opening and closing of solder ball fractures can lead to intermittent electrical signal failures. Vibration, movement, temperature changes, or other stresses can cause broken solder balls to open and close, resulting in intermittent failures of electrical signals. The flexible materials used by the PCB board factory also make this intermittent signal possible, such as breaking open and close caused by vibration stress, and the unpredictable opening and closing of solder ball circuits cause intermittent signals. This intermittent Faults are difficult to diagnose. In addition, the I/O buffer circuit around the FPGA makes it nearly impossible to measure the resistance value of the solder network. A device that fails in a working FPGA may pass without any fault finding (NTF) on the test bed because the solder joints are temporarily connected. Many users find that the FPGA is not working properly, and it is for this reason that the FPGA will work normally by pressing P by hand.

Real-time failure detection of soldered connection between FPGA and PCB board at work: SJ-BIST detects soldering status in real time. Currently, techniques such as visual inspection, optical, X-ray and reliability testing used in manufacturing are difficult to work because the reflection is electrical. Signal failure faults are largely invisible when the device is not powered. Through early detection of impending failures, SJ-BIST supports condition-based equipment maintenance and reduces intermittent failures. Its superior sensitivity and sensitivity allow SJ-BIST to detect and fail high resistances down to 100 ohms within two clock cycles without false alarms. As a scalable solution, it can be attached to a user's existing test hub without adding additional resources on PCB board.