Skinny PCBs: Challenges with BGA packages

Throughout electrical design course of, sure design selections must 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 appearance of PCBs, transistor-based electronics have been typically assembled utilizing a way known 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, grew to become 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

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 straightforward 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 major considerations is their mechanical fragility. Skinny PCBs are extra versatile and susceptible to bending or warping, making them tough to deal with throughout meeting and extra inclined to wreck 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 in the course of the useful testing.

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

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

These points are particularly essential 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.

Easy methods to take a look at sturdiness

The mechanical sturdiness of PCB assemblies is mostly assessed utilizing a drop take a look at. Drop take a look at necessities for a PCBA usually embody specifying the drop top, drop floor, variety of drops, orientation in the course of the drop, acceptance standards, and testing requirements. The drop top is the space from which the PCBA might be dropped, usually starting from 30 to 48 inches, relying on the appliance and trade requirements.

The drop floor, resembling 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 in the course of the drop, whether or not face down, face up, or on an edge or nook, can also be specified. Acceptance standards, resembling performance after the drop and any seen harm, are clearly outlined.

Testing requirements like IPC-TM-650 or particular buyer necessities information the testing course of. For a medical gadget, the drop take a look at necessities are ruled by part 15.3.4.1 of IEC 60601-1 Third Version 2005-12. By establishing these necessities, producers make 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 may not be captured throughout a drop take a look at till a useful failure is noticed. The BGA can fail as a consequence of poor assembly-related points just like the thermal stresses throughout soldering or poor soldering joint high quality. A skinny board weakens as a consequence of extreme mechanical shock and vibration meeting.

These defects may be captured throughout a drop take a look at because the BGA half might not stand up to the stresses encountered throughout a drop take a look at, as proven within the figures beneath. The BGA failures may 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 reveals 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 few of the BGA failure modes. Supply: Semlabs

Easy methods to repair BGA failure on skinny PCBs

Pad cratering is the fracturing of laminate beneath Cu pads of floor mount elements, which regularly happens throughout mechanical occasions. The preliminary crack can propagate, inflicting electrically open circuits by affecting adjoining Cu conducting traces. It’s extra widespread in lead-free assemblies as a consequence of completely different laminate supplies. Mitigation includes decreasing stress on the laminate or utilizing stronger, extra pad cratering-resistant supplies.

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

• Thinner boards are extra susceptible to warping and should require extra fixturing (stiffeners or work board holders) to course of on the manufacturing line if the necessities beneath aren’t met. A PCB stiffener shouldn’t be an integral a part of the circuit board; relatively, it’s an exterior construction that provides mechanical help 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 function 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. As an example, supporting the PCB throughout dealing with operation like through gap drilling, choose and place, ICT, or useful testing with flying probes.
• Matching the really helpful soldering profile of the BGA. The difficulty may be made worse if the BGA manufacture’s really helpful soldering profile shouldn’t be adopted, leading to chilly solder joints. There ought to be sufficient thermocouples on the PCB panel to watch the PCB temperature.
• Be sure that the BGA pad measurement is as per manufactures suggestion.

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 may be managed by rigorously controlling the PCB dealing with processes after which strengthening the skinny PCB with design options mentioned on this article.

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

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

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