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Get the right solder paste for your IoT PCBs

The right solder paste must be used to ensure components are solidly connected to IoT rigid-flex and flex circuit boards. Otherwise, without getting a good handle on solder paste, both large and small IoT product companies may be throwing away thousands of dollars, as the wrong solder paste can create flaws such as shorts, opens and latent defects can occur, which could be very costly and time-consuming.

The industry uses solder paste by types. There are types 1 through 7, according to the Institute for Interconnecting and Packaging Electronic Circuits standard J-STD 005A dated Dec. 2011. Types 1-5 are the most popular, while types 6 and 7 are not as prevalent in PCB manufacturing. Powder metal serves as the paste’s basis, and it’s made up of millions of metal particles or so-called “solder spheres.”

That powder metal is combined with flux to form the solder paste, thus creating the necessary adhesion. Then stencil printing applies the paste on the IoT rigid-flex or flex circuit board to temporarily hold the components on the board.

The board is run through pick and place for component placement, and the assembly subsequently goes through the reflow oven so that the IoT components are soldered on to the board.

The key part of this process is to understand the size of the solder spheres comprising the solder paste. At one end of the spectrum, types 1 and 2 have significantly larger solder spheres compared to types 4 and 5. Type 5 has the relatively smallest solder spheres.

Also important to keep in mind is the larger the solder sphere size in the paste-flux combination, the greater the demand for a thicker stencil design. To meet proper dispensing requirements, large solder spheres work better and more effectively when thicker stencils are used. The reason is better solder paste can be more easily dispensed using the slightly larger stencil openings. For example, a 5-mil stencil might dispense better paste compared to a 4-mil stencil while using type 3 or 4 paste.

Types 1 through 3 work for larger conventional PCBs, but not for the smaller IoT ones. The IoT PCB stencil design requires a considerably smaller stencil design.

Therefore, it’s prudent to focus on types 4, 4½ or maybe 5 for IoT rigid-flex and flex circuit boards. Aside from its association with stencil design, solder sphere size again takes center stage because we have to keep in mind that extremely small components populate an IoT rigid-flex and flex circuit boards. These are micro ball grid array (BGA), quad flat no-lead (QFN) and dual-flat no-lead (DFN) component packages. The pitch between package leads ranges from 0.25 mm or less, although in today’s technology people are beginning to use 0.15 mm pitch or in extreme cases, 0.1 mm.

These micro packages have literally thousands of leads, bumps or tiny balls used to make connections to the circuit board. The tiny spacing between each lead, bump or ball is called “pitch,” and it is less than the diameter of a human hair strand.

In effect, the smaller, finer pitch of these micro packages calls for smaller solder sphere size associated with a given solder paste. The smaller metal particle or sphere provides greater viscosity for fine pitch devices so that the solder paste performs considerably more accurate stencil printing within those tiny crevices. These smaller solder spheres can be dispensed more accurately because they are designed to be used on fine pitch geometries with finer pitches in the range of 0.25 mm or lower.

With IoT technology still in its infancy, inexperience can creep into promising IoT innovations and sabotage what can potentially be highly rewarding market opportunities. Here’s where a case of using the wrong solder paste — as mundane as it may sound – can derail those highly promising IoT projects.

PCB manufacturing and assembly have been so traditional over the years that using the same type solder paste for larger PCBs can be the norm at some contract manufacturers and electronics manufacturing services providers. That’s not where you want to go with your new IoT project. In cases like that, the manufacturer may inadvertently rely on types 1, 2, or 3. So, here are some of the consequences. The larger metal particles or solder spheres in those types of solder paste means the stencil may need to be adjusted accordingly. And under stencil printing may need to be performed, which should be avoided because the size of devices and their pads are smaller to begin with. With underprinting, there may not be enough paste to create a solid solder joint. Also, if too thick a paste is used, overprinting is the result, leading to shorts between BGA or QFN/DFN bumps. Other problematic areas can occur, requiring costly rework and longer assembly and manufacturing time. All this is avoided by using the right type of solder paste.

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