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SMT Soldering Defects: Examples & Causes

Key Takeaways

  • What the SMT prep process entails.

  • Common SMT defects, including tombstoning.

  • How RoHS affects SMT solderability.

Solder defect induced failure

Whether by hand or machine, poor soldering can be the cause of a host of issues on a PCB, including catastrophic failure

Surface mount technology, usually abbreviated as SMT, has revolutionized the capabilities of high-density design. Before SMT, standard components of a circuit like caps, resistors, and inductors came in relatively large packages with a slow and unwieldy method of attaching to the board. SMT components have drastically reduced not only the size of components to allow for greater routing and other copper features on the outer layers, but the amount of non-via plated through holes, which simplifies and expedites the drilling stage of fabrication. Due to its newfound prevalence in design, preventing SMT soldering defects at both the design and manufacturing levels has become an important consideration. 

Preparing a PCB for SMT Assembly

PCB manufacturing is not a simple task; immense care must be taken not only by the operator but also by the designer providing the layout information. A major aspect of assembly is soldering, where the placed components are joined to the board via a solder joint formed between the leg of the component and the surface of the pad. SMT soldering implies two further processes:

  • Solder paste stencil - In order to coat only the areas where a solder bond is intended to be formed, a stencil is used to block out everything but the solderable pads. There are two important aspects at play when applying solder paste: 1) To apply the correct amount of solder paste necessary to form the solder joint and 2) To apply the solder paste at a uniform height to provide an even part placement and joint formation.
  • Solder reflow - Once one side of the components is placed, the board is ready for the staged reflow process. To convert from a tacky paste to a solid metal joint, the boards must pass through a reflow oven paced with heating and cooling cycles. A double-sided PCB will pass through reflow twice - once for each side.

SMT Soldering Defects: Tombstoning and More

One such SMT soldering defect is tombstoning, which will occur due to a difference between surface tension on the pads. Normally, the wetting process creates the idealized heel-toe solder joint, but the process also exerts a pull on the pins, resulting in a centering effect that reinforces part placement. However, if the solder paste on all of the components' pads does not melt concurrently, the net force created by the surface tension pulls the component in the direction of the faster freezing pad. For chip components, this results in a component forming an out-of-the-plane angle from 0 to 90° with the board. There are a few causes that promote tombstoning, and from the layout designer’s perspective, it is most important to ensure the pads possess the same heat flux capacity. Heat flux capacity is influenced by the number of connections to a pad, their width, the presence of vias close to or within the pad, and large, nearby copper features within clearance that can function as heat sinks. Some additional defects are listed below, but it is important to note that what is acceptable in a lower-rated class electronic may or may not be a defect in a higher-rated class:

  • Insufficient/excess solder
  • Nonwetting/dewetting - Solder does not adhere to the terminal or coats the surface in an irregular fashion, forming a poor connection.
  • Disturbed joint - Movement or vibration during joint formation leads to an uneven surface when cooling. 
  • Fracture/cracked joint
  • Pinhead/blowhole - A small (pinhead) or large (blowhole) hole in the solder joint due to escaping gasses or voids in the solder.
  • Bridging - Solder-formed shorts due to excessive application or flow issues during heating/cooling steps.
  • Mounting adhesive on land -  Adhesive applied in the termination area.
  • Solder splashing, balls, fines, or incomplete reflow - All issues related to insufficient heating or rate of heating as well as the presence or absence of solder in a particular location.
  • Open connection - Misalignment takes pins and pads out of coplanarity and prevents solder joint formation entirely. 


The Effects of RoHS and Lead-Free Solder on Defects

A major development within SMT has been the introduction of RoHS. Adopted in the mid-00s to help combat toxic organics and heavy metals inclusion in PCBs that were making their way into the environment, the directive has been a success from a consumer and ecological perspective.

However, lead-based tins have a number of notable properties that endear them to long-lasting PCB design:

  1. Tin-silver-copper solder alloys (collectively, alloys of this mixture are known as SACs) possess a melting point approximately 19% greater than eutectic tin-lead. 
  2. Tin whiskering is a well-observed yet not fully understood phenomenon that occurs in the absence of leaded solder, though other alloys have been used to tamp down on their occurrence. 

While leaded solders and finishes still find use in higher-class electronics, consumer electronics have had to contend with the switch from excellent performing materials. Some defects that are more likely to occur in lead-free solders than in standard solders include the following:

  • Bridging or solder balls due to a solder paste not adhering to the higher heat requirement of lead-free solders.
  • Poor wetting can result from excessive temperature, preheat time, or oxidation on the pins and pads.
  • Voids may or may not constitute a defect - vacancies of no more than 25% volume reduction exhibit no change in reliability. 

Ultimately, these defects and others are best handled by selecting materials with the requisite properties to perform in the relatively heightened temperature environment of lead-free soldering.

Building around the reality of SMT soldering defects is important for any designer working with today’s dense boards; Cadence’s PCB design and analysis software offers a complete package of tools to assist designers in limiting their occurrence. 

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