Thin PCBs: Challenges with BGA packages

During electrical design process, certain design choices need to be made. One example is USB C type connector-based design with a straddle-mount connector. In such scenario, the overall PCB thickness is constrained while using a straddle-mount connector whose thickness governs the overall thickness. For historical reasons, the standard PCB thickness is 0.063” (1.57 mm).

Before the advent of PCBs, transistor-based electronics were often assembled using a method called breadboarding, which involved using wood as a substrate. However, wood was fragile, leading to delicate assemblies. To address this, bakelite sheets, commonly used on workbench surfaces, became the standard substrate for electronic assemblies, with a thickness of 1/16 inch, marking the beginning of PCBs at this thickness.

Figure 1 A PCB cross section is shown with a straddle-mount type connector. Source: Wurth Elektronik

Take the example of Wurth Elektronik’s USB 3.1 plug, a straddle-mount connector with part number 632712000011. The part datasheet recommends a PCB thickness of 0.8 mm/0.031” for an optimal use. This board thickness is common among various board fabrication houses. The 0.031” board is relatively easy to fabricate as many fab houses do a 6-layer PCB with 1 Oz copper on each layer.

However, designing and working with thin PCBs presents several challenges. One of the primary concerns is their mechanical fragility. Thin PCBs are more flexible and prone to bending or warping, making them difficult to handle during assembly and more susceptible to damage during handling. The handling includes pick and place assembly process, holes drilling, in-circuit testing (ICT) as well as functional probes during the functional testing.

The second level of handling is by the end user, for example dropping the device containing the PCB assembly (PCBA). Additionally, thin PCBs often requires specialized manufacturing processes and materials, leading to increased production costs. Component placement becomes more critical as well, as traces may need to be positioned closer together, increasing the risk of short circuits and signal interference.

Furthermore, thin PCBs face challenges in heat dissipation due to their reduced thermal mass. Addressing these challenges demands careful consideration during the design, manufacturing, and assembly stages to ensure the reliability and performance of the final product.

These issues are especially critical when a designer mounts a ball grid array (BGA) component on a 0.031” thickness board. Most of major fabrication houses recommend a minimum thickness of 0.062” when BGAs are mounted on the board.

How to test durability

The mechanical durability of PCB assemblies is generally assessed using a drop test. Drop test requirements for a PCBA typically include specifying the drop height, drop surface, number of drops, orientation during the drop, acceptance criteria, and testing standards. The drop height is the distance from which the PCBA will be dropped, typically ranging from 30 to 48 inches, depending on the application and industry standards.

The drop surface, such as concrete or wood, is also defined. Manufacturers determine the number of drops the PCBA must withstand, usually between 3 to 6 drops. The orientation of the PCBA during the drop, whether face down, face up, or on an edge or corner, is also specified. Acceptance criteria, such as functionality after the drop and any visible damage, are clearly defined.

Testing standards like IPC-TM-650 or specific customer requirements guide the testing process. For a medical device, the drop test requirements are governed by section 15.3.4.1 of IEC 60601-1 Third Edition 2005-12. By establishing these requirements, manufacturers ensure that their PCBAs and products are robust enough to withstand real-world use and maintain functionality even after being subjected to drops and impacts.

The soldering joint might not be captured during a drop test until a functional failure is observed. The BGA can fail due to poor assembly-related issues like the thermal stresses during soldering or poor soldering joint quality. A thin board weakens due to excessive mechanical shock and vibration assembly.

These defects can be captured during a drop test as the BGA part may not withstand the stresses encountered during a drop test, as shown in the figures below. The BGA failures can be inspected using X-ray, optical inspection, or electrical testing. A detailed analysis may be performed using cross section analysis using scanning electron microscopy (SEM).

Figure 2 The BGA solder joint shows a line crack. Source: Keyence

Figure 3 The above image displays a cross section of a healthy BGA. Source: Keyence

Figure 4 Here is a view of some of the BGA failure modes. Source: Semlabs

How to fix BGA failure on thin PCBs

Pad cratering is the fracturing of laminate under Cu pads of surface mount components, which often occurs during mechanical events. The initial crack can propagate, causing electrically open circuits by affecting adjacent Cu conducting lines. It’s more common in lead-free assemblies due to different laminate materials. Mitigation involves reducing stress on the laminate or using stronger, more pad cratering-resistant materials.

The issue can be fixed by mechanically stretching the PCB or changing the laminate material. It can be done with any of the following steps.

• Thinner boards are more prone to warping and may require additional fixturing (stiffeners or work board holders) to process on the manufacturing line if the requirements below are not met. A PCB stiffener is not an integral part of the circuit board; rather, it’s an external structure that offers mechanical support to the board.

Figure 5 An aluminum bar is shown as a mechanical PCB stiffener. Source: Compufab

• Corner adhesive/epoxy on the BGA corners or use BGA underfill. For example, an adhesive that can be used for this purpose is Zymet UA-3307-B Edgebond, Korapox 558 or Eccobond 286. The epoxy along the BGA corners or as an underfill strengthens the PCB, thereby preventing PCB flexion and hence the failure.
• Strict limitations on board flexure during circuit board assembly operations. For instance, supporting the PCB during handling operation like via hole drilling, pick and place, ICT, or functional testing with flying probes.
• Matching the recommended soldering profile of the BGA. The issue can be made worse if the BGA manufacture’s recommended soldering profile is not followed, resulting in cold solder joints. There should be enough thermocouples on the PCB panel to monitor the PCB temperature.
• Ensure that the BGA pad size is as per manufactures recommendation.

Managing thin PCB challenges

A thin PCB (0.031”) can weaken the PCB assembly, thereby making it susceptible to mechanical and thermal forces. And the challenges are unique when mounting a BGA to the thin PCB.

However, the design challenges and risks can be managed by carefully controlling the PCB handling processes and then strengthening the thin PCB with design solutions discussed in this article.

Editor’s Note: The views expressed in the article are author’s personal opinion.

Jagbir Singh is a staff electrical engineer for robotics at Smith & Nephew.

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