“Solder preforms” are tiny pieces of solid tin alloy that are already shaped before use. They come in two types of flux contain and flux free. These preforms can be put in certain pads to increase the amount of solder paste. This helps when the solder paste volume is constrained due to the limitation of stencil thickness. It makes the solder joints stronger and better, and it also helps to improve voids in the solder.
Electronic components are getting smaller and smaller. The small chip (resister, capacitor, inductor) used to be bigger, like size of 1206, but now they’re tiny like 0805, 0603, 0402, and even 01005. When components are smaller, they need less and accurate solder paste volume. So, the thickness of stencil has also become thinner from regular of 0.18mm to 1.0mm or 0.08mm in order to get accurate solder paste volume. Unfortunately, not all electronic components can be made this small. Some components that connect to outside devices, like phone jacks, network ports (RJ45), smart card reader, still need a certain amount of solder to ensure its strength and quality of solder joint. And some traditional Through Hole Device (THD) do SMT by Paste-In-Hole process also require additional solder volume.
I think most of us know that smaller electronic components need less solder paste. So, we use thinner stencil to control the solder paste volume and avoid issues like solder short or non-wetting. But on a same PCB, there can be both tiny components that need just the less and right amount of solder v.s larger components like I/O connectors that need more solder.
Is there a way to use a thinner stencil that can accurately control the volume of solder paste and provide more where needed? Well, one approach is to use “step-up & step-down stencil” technology, which involves adjusting the thickness of specific areas of the stencil to control the amount of solder used. However, this method has its limitations and may still result in insufficient solder in certain cases. This is why there’s a solution known as “solder preforms.” These are pieces of solder that are already shaped to match various requirements. Essentially, they’re solder bits created by compressing solder. They can compensate for solder shortages caused by stencil limitations. These preforms are often packaged in tapes and reels, allowing placement machines to accurately position them. This saves time and avoids errors that could occur if handled manually.
The Benefits and Advantages of Solder Preforms:
Solder preforms can be used in the paste-in-hole process. This helps traditional through-hole components (THD) get enough solder and reduces the occurrence of insufficient solder and voids in plating holes.
(↓Insufficient soldering filled in the plating hole)
(↓Apply solder performs to the PIN-IN-PASTE process)
It can lower the non-wetting defect rate and reduce the soldering voids caused by inadequate co-planarity of component leads, while also enhancing their solderability. Preforms can be placed on pads to selectively increase solder volume. It’s important to note that preforms should be placed in areas that come into contact with solder paste, which is necessary for achieving the desired increase in solder volume. Additionally, solder paste can hold the preforms in place before reflow soldering.
The solder preforms are often standardized and available in tape-on-reel packaging, making them compatible with SMT machines for easy placement and minimizing quality risks.
It can be customized the shape and size if quantity and cost met profit.
Using solder preforms under the Epad of QFN or BTC (Bottom Termination Components) can effectively reduce the solder voids.
The Disadvantages of Solder Preforms:
This incurs an additional cost, and the tape-on-reel packaging might be more expensive than the preforms themselves. If possible, purchasing bulk preforms and handling the packaging process in-house may lead to some cost savings. You may choice alternative cheap solution to increase the solder paste volume before select this solder preforms solution.
The solder preforms must put on the solder paste just like the small chip components to prevent it from moving or dropping during SMT conveyor transfer.
Solder preforms have different uses too. For instance, they can act like a cushion between two metal parts, or they can work as safety switches. When the temperature gets really high, they melt and cut off the power.
Here are the standard sizes for solder preforms.
These specifications are from a specific solder paste company, but other companies probably have similar ones.
|Specification||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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- Selectively increase solder paste volume solutions
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