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Skinny PCBs: Challenges with BGA packages

Throughout electrical design course of, sure design decisions should be made. One instance is USB C kind connector-based design with a straddle-mount connector. In such situation, the general PCB thickness is constrained whereas utilizing a straddle-mount connector whose thickness governs the general thickness. For historic causes, the usual PCB thickness is 0.063” (1.57 mm).

Earlier than the arrival of PCBs, transistor-based electronics have been usually assembled utilizing a technique referred to as breadboarding, which concerned utilizing wooden as a substrate. Nevertheless, wooden was fragile, resulting in delicate assemblies. To deal with this, bakelite sheets, generally used on workbench surfaces, turned the usual substrate for digital assemblies, with a thickness of 1/16 inch, marking the start of PCBs at this thickness.

Determine 1 A PCB cross part is proven with a straddle-mount kind connector. Supply: Wurth Elektronik

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Take the instance of Wurth Elektronik’s USB 3.1 plug, a straddle-mount connector with half quantity 632712000011. The half datasheet recommends a PCB thickness of 0.8 mm/0.031” for an optimum use. This board thickness is widespread amongst varied board fabrication homes. The 0.031” board is comparatively simple to manufacture as many fab homes do a 6-layer PCB with 1 Oz copper on every layer.

Nevertheless, designing and dealing with skinny PCBs presents a number of challenges. One of many main issues is their mechanical fragility. Skinny PCBs are extra versatile and liable to bending or warping, making them troublesome to deal with throughout meeting and extra inclined to break throughout dealing with. The dealing with consists of choose and place meeting course of, holes drilling, in-circuit testing (ICT) in addition to useful probes through the useful testing.

The second stage of dealing with is by the top consumer, for instance dropping the machine containing the PCB meeting (PCBA). Moreover, skinny PCBs usually requires specialised manufacturing processes and supplies, resulting in elevated manufacturing prices. Element placement turns into extra vital as nicely, as traces could should be positioned nearer collectively, growing the chance of quick circuits and sign interference.

Moreover, skinny PCBs face challenges in warmth dissipation resulting from their decreased thermal mass. Addressing these challenges calls for cautious consideration through the design, manufacturing, and meeting phases to make sure the reliability and efficiency of the ultimate product.

These points are particularly vital when a designer mounts a ball grid array (BGA) element on a 0.031” thickness board. Most of main fabrication homes advocate a minimal thickness of 0.062” when BGAs are mounted on the board.

Methods to check sturdiness

The mechanical sturdiness of PCB assemblies is mostly assessed utilizing a drop check. Drop check necessities for a PCBA sometimes embody specifying the drop peak, drop floor, variety of drops, orientation through the drop, acceptance standards, and testing requirements. The drop peak is the space from which the PCBA can be dropped, sometimes starting from 30 to 48 inches, relying on the appliance and trade requirements.

The drop floor, equivalent to concrete or wooden, can also be outlined. Producers decide the variety of drops the PCBA should stand up to, often between 3 to six drops. The orientation of the PCBA through the drop, whether or not face down, face up, or on an edge or nook, can also be specified. Acceptance standards, equivalent to performance after the drop and any seen injury, are clearly outlined.

Testing requirements like IPC-TM-650 or particular buyer necessities information the testing course of. For a medical machine, the drop check necessities are ruled by part of IEC 60601-1 Third Version 2005-12. By establishing these necessities, producers be sure that their PCBAs and merchandise are sturdy sufficient to resist real-world use and keep performance even after being subjected to drops and impacts.

The soldering joint won’t be captured throughout a drop check till a useful failure is noticed. The BGA can fail resulting from poor assembly-related points just like the thermal stresses throughout soldering or poor soldering joint high quality. A skinny board weakens resulting from extreme mechanical shock and vibration meeting.

These defects could be captured throughout a drop check because the BGA half could not stand up to the stresses encountered throughout a drop check, as proven within the figures under. The BGA failures could be inspected utilizing X-ray, optical inspection, or electrical testing. An in depth evaluation could also be carried out utilizing cross part evaluation utilizing scanning electron microscopy (SEM).

Determine 2 The BGA solder joint exhibits a line crack. Supply: Keyence

Determine 3 The above picture shows a cross part of a wholesome BGA. Supply: Keyence

Determine 4 Here’s a view of a number of the BGA failure modes. Supply: Semlabs

Methods to repair BGA failure on skinny PCBs

Pad cratering is the fracturing of laminate underneath Cu pads of floor mount parts, which regularly happens throughout mechanical occasions. The preliminary crack can propagate, inflicting electrically open circuits by affecting adjoining Cu conducting strains. It’s extra widespread in lead-free assemblies resulting from totally different laminate supplies. Mitigation entails lowering stress on the laminate or utilizing stronger, extra pad cratering-resistant supplies.

The problem could be mounted by mechanically stretching the PCB or altering the laminate materials. It may be performed with any of the next steps.

  • Thinner boards are extra liable to warping and will require further fixturing (stiffeners or work board holders) to course of on the manufacturing line if the necessities under will not be met. A PCB stiffener will not be an integral a part of the circuit board; quite, it’s an exterior construction that gives mechanical assist to the board.

Determine 5 An aluminum bar is proven as a mechanical PCB stiffener. Supply: Compufab

  • Nook adhesive/epoxy on the BGA corners or use BGA underfill. For instance, an adhesive that can be utilized for this objective is Zymet UA-3307-B Edgebond, Korapox 558 or Eccobond 286. The epoxy alongside the BGA corners or as an underfill strengthens the PCB, thereby stopping PCB flexion and therefore the failure.
  • Strict limitations on board flexure throughout circuit board meeting operations. For example, supporting the PCB throughout dealing with operation like through gap drilling, choose and place, ICT, or useful testing with flying probes.
  • Matching the advisable soldering profile of the BGA. The problem could be made worse if the BGA manufacture’s advisable soldering profile will not be adopted, leading to chilly solder joints. There ought to be sufficient thermocouples on the PCB panel to observe the PCB temperature.
  • Be certain that the BGA pad dimension is as per manufactures advice.

Managing skinny PCB challenges

A skinny PCB (0.031”) can weaken the PCB meeting, thereby making it inclined to mechanical and thermal forces. And the challenges are distinctive when mounting a BGA to the skinny PCB.

Nevertheless, the design challenges and dangers could be managed by fastidiously controlling the PCB dealing with processes after which strengthening the skinny PCB with design options mentioned on this article.

Editor’s Word: The views expressed within the article are creator’s private opinion.

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

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