Another Option for Increasing Solder Volume in SMT – Solder Preforms

“Solder preforms” are pre-shaped small solder pieces that can be used to locally increase solder paste volume. They help improve solder quality and strength, addressing the limitations on solder paste caused by using a stencil, and are especially useful as electronic components continue to shrink in size.

In today’s electronics, components like resistors, capacitors, and inductors, known as small chip components, have reduced in size from the original 1206 dimensions to 0805, 0603, 0402, 0201, and almost the limit of 01005. As these electronic components get smaller, the required amount of solder also decreases. Consequently, the thickness of the stencil has to reduce, going from the commonly used thickness of 0.18mm to the current 0.08mm. However, not all electronic components can be reduced in size this way. Some connectors responsible for external communication, such as phone jacks, network port connector (RJ45), SIM card readers, smart card readers, etc., still require a certain amount of solder to ensure the strength and quality of their solder joints. Additionally, for through-hole components (THD) used in the Paste-In-Hole (PIH) process, extra solder volume is needed to meet the minimum soldering requirements.

Is there a way to use thinner stencils while still locally increasing solder volume for SMT process?

One method involves using a “step-up & step-down stencil”, a stepped stencil that partially thickens or thins the stencil. However, this type of stencil has limitations on the amount of solder it can add, and it cannot be increased indefinitely. Sometimes, there is still an issue of insufficient solder volume. To address this, “solder preforms” have emerged in the market. These pre-shaped solder pieces can be pressed into various shapes to meet specific needs. Essentially, they are solder blocks formed by extrusion, serving to supplement the insufficient solder paste caused by stencil printing limitations. Moreover, these solder preforms are often packaged in tape and reel format, making them compatible with SMT machines for pick-and-place operations, saving labor and avoiding manual handling errors.

The benefits and advantages of solder preforms are:

  1. In the paste-in-hole process, solder preforms can be added to increase the solder volume, allowing traditional Through-Hole Device (THD) components to achieve more complete soldering and reduce the occurrence of solder shortages and voids in plating holes.

    (↓Insufficient soldering filled in the plating hole)

    (↓ Solder preforms added in the PIN-IN-PASTE process)


  2. Solder preforms can reduce the occurrence of solder skip or solder empty joints due to insufficient co-planarity of component leads and enhance their solderability. Preforms can be placed on printed solder paste to partially increase the solder volume.

  3. Nowadays, solder preforms are mostly standardized and packaged in tape-on-reel , making them compatible with SMT machines for pick-and-place operations, ensuring ease of handling and low quality risks.

  4. Using solder preforms at the bottom of QFN or BTC (Bottom Termination Components) with Epad can effectively reduce the occurrence of solder voids.

Disadvantages of solder preforms include:

  • It involves additional costs, and the tape-on-reel packaging may be more expensive than the solder preforms themselves. If possible, buying bulk material and manually packaging it may save some costs. It is suggested to consider this preform solution only when other alternatives are not feasible.
  • Preforms must be placed where they can contact the component’s solder pad or, more accurately, where they come into contact with the solder paste. This allows the molten solder to achieve the goal of increasing the solder volume. Additionally, solder paste may stick to the preforms before reflow soldering.

Solder preforms have various applications. For instance, they can serve as contact buffers, directly placed between two metal components. Alternatively, they can function as safety switches or fuses. When the temperature reaches the melting point, a short circuit occurs, interrupting the power supply.

Standard Size Specifications for Solder Preforms

The following are specifications for solder preforms from a particular solder paste manufacturer. Similar specifications can be expected from other solder paste manufacturers.

Spec.  Length (mm)  Width (mm)  Thickness (mm)  packing quantity(7”Reel) 
0201 0.51 ± 0.03 0.25 ± 0.03 0.25 ± 0.03 10,000
03015 0.64 ± 0.03 0.34 ± 0.03 0.34 ± 0.03 10,000
0402 1.00 ± 0.05 0.50 ± 0.05 0.50 ± 0.05 5,000
0603 1.60 ± 0.05 0.80 ± 0.05 0.80 ± 0.05 4,000
0805 2.01 ± 0.05 1.30 ± 0.05 0.76 ± 0.05 3,000

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