Thin Printed Circuit Boards – The Challenges and Risks of BGA packages

Do you know how PCB thickness affects the reliability and lifetime of electronics? Maybe more than you think. In this article, you will learn the most common challenges and risks associated with the design and manufacture of thin PCBs, especially when using BGA encapsulated components. You will also learn why mechanical fragility, heat dissipation or incorrect solder profile can be a major problem and how to prevent these complications at the design stage. In the last chapter, you will also learn tips on methods for testing and repairing damaged BGA packages.

Several factors affect the thickness of a printed circuit board. For example, one factor may be the use of a USB Type-C connector with a straddle-mount connector. The thickness of this type of connector will significantly limit the overall thickness of the PCB itself. Currently, the standard PCB thickness is 1.57 mm (0.063").

Figure  1  - Straddle-mount connector

In the days of transistor-based electronics, components were mounted on wooden boards using the breadboarding method. However, these electronics were very susceptible to damage as the wood is very fragile. After the invention of the conventional printed circuit board, the substrate of bakelite boards started to be used. These bakelite boards were already used for workbenches at that time, and their thickness was 1.5875 mm (1/16 inch). This thickness was retained for the circuit boards.

A specific example of a connector that affects the thickness of a circuit board might be the USB 3.1 from Würth Elektronik, numbered 632712000011. The manufacturer states that the recommended optimum PCB thickness for this connector is 0.8mm/0.031". A board thickness of 0.8mm is quite common among circuit board manufacturers and relatively easy to manufacture.

However, in the case of thin printed circuit boards, there are several challenges and risks that must be taken into account that may occur during design or manufacturing.

The challenges and risks are as follows:

• Mechanical fragility - Thin PCBs are generally more flexible, prone to bending or twisting. As a result, mechanical fragility increases the risk of damage to the boards during the embedding process and makes them more difficult to handle during the manufacturing process as a whole.
  
• Customer handling - Thin PCBs are also susceptible to damage during customer handling. In fact, the installation of printed circuit boards into end products may involve several steps that can damage the board. Therefore, when designing thin printed circuit boards, the subsequent installation process in the customer's end product must also be taken into account.
  
• Manufacturing processes and materials - Specialized manufacturing processes and materials are a very common factor entering into the design of thin-film printed circuit boards. As a rule, this increases the manufacturing cost of such a board. Related to this is the placement of components on the board, which can be affected by the placement of individual connections closer together, which can ultimately result in board short circuits and signal interference.
  
• Heat dissipation - The lower thermal mass of thin boards can result in heat dissipation issues. Therefore, thermal management must be given extra attention.

Successfully addressing these challenges and risks lies in addressing them in the design, manufacturing and actual assembly phases of the board. This will ensure the long-term reliability and performance of the PCB.

It is safe to say that most major manufacturing companies recommend a minimum board thickness of 1.5748 mm (0.062") when using BGA components, but if you need a board thickness of 0.7874 mm (0.031"), you need to take the challenges and risks listed above especially responsibly.

Durability testing

The so-called “drop test” is used to test mechanical durability. This method is quite common in mechanical resistance testing of printed circuit boards. In order to make the drop test as informative as possible, several requirements must be met.

The requirements for the crash test include:

• Fall height - The distance from which the board was dropped during the test. The usual drop height is between 76 cm and 122 cm. The drop height is based on the application of the board and industry standards.

• Drop Surface - The surface of the drop surface can significantly affect the resulting damage to the slab. Typical surfaces include concrete, asphalt, plastic, or wood.
  
  
• Number of drops - The customer determines how many drops the slab should take without damage. Typically, this is a number of 3 to 6 falls.
  
  
• Orientation of the fall - An equally important factor is the orientation of the slab during a fall. This is because the result of the test may vary depending on whether the board was turned face up, face down, on an edge or corner.

Testing of standard printed circuit boards is governed by the IPC-TM-650 standard and customer specific requirements. If the customer requires a board for medical equipment, the test must be performed according to the requirements of IEC 60601-1 2005-12. By meeting the individual requirements, the manufacturer will demonstrate the durability and quality of the manufactured board when the board is used in practice. At the same time, it can be assumed that such a board will retain its functionality even after an impact or a fall.

In the case of soldered joints, it is not mandatory to record their condition after the test if there is no visible or functional damage. In the case of BGA bushings, damage may occur due to assembly problems. These problems include thermal stresses during soldering, poor quality of solder joint. In the case of thin circuit boards, excessive mechanical shock and vibration during assembly are also involved.

Figure  2 - BGA solder joint crack

The crash test usually catches these defects because BGA components may not withstand the stresses that occur in a crash test. If a failure does occur, the BGA components can be inspected using X-ray, optical inspection or electrical testing. For even more detailed analysis, cross-section analysis technology can be used using a scanning electron microscope (SEM).

Figure  3 - Correct BGA solder joint

Repairing BGA connector failures on thin printed circuit boards

Cracking of the laminate under the copper (Cu) pads of surface mount components (SMT). This phenomenon usually occurs under mechanical stress. The phenomenon creates an initial crack at the junction that begins to propagate and cause electrically open circuits on the board. “Cracking” is more common on boards that have lead-free solder bonded components for which a different laminate is used. To mitigate the risk of this phenomenon, it is possible to reduce the stress in the board laminate or use a stronger material.

Mechanical stretching of the PCB or changing the laminate material can be addressed by applying one of the following steps:

• Additional reinforcement – Since thin PCBs are more prone to deformation, the manufacturer can use additional reinforcements. These typically consist of various stiffeners, board holders, and similar structures that provide mechanical support for the PCBs. These reinforcements are not a permanent part of the PCB but serve as temporary external fixtures used only during handling and processing on the production line.
  
  
• Adhesive or epoxy on the corners of BGA connectors – Adhesives and epoxies strengthen the PCB and prevent bending, reducing the risk of failures and damage.

• Preventing PCB bending during assembly – Bending of PCBs can be avoided by properly supporting the board during handling operations. These operations include, for example, drilling of through-holes, functional testing, ICT testing, or pick and place assembly.
  
  
• Following the manufacturer’s recommended soldering profile for BGA connectors – Failure to follow the specified soldering profile for BGA components can result in cold solder joints. It is therefore necessary to attach enough thermocouples to the board to monitor the temperature at various points. This helps prevent the formation of cold solder joints.
  
  
• The pad size of the BGA connector must match the manufacturer’s recommendation.

Conclusion

In this article, we discussed the challenges and risks involved in the design and manufacturing of thin printed circuit boards, especially when mounting BGA packages. We covered why it is important to address mechanical durability, thermal management, and proper manufacturing processes, and how to prevent these issues.

Key takeaways from the article:

• Thin printed circuit boards are more prone to mechanical damage and deformation.
• Heat dissipation is more demanding in thin boards and requires careful planning.
• Soldering BGA packages onto thin boards increases the risk of cold solder joints and “cracking.”
• Drop tests and X-ray inspections are crucial for detecting defects in boards.
• Risks can be minimized through proper design, additional reinforcement, and strict adherence to soldering profiles.