Designing a stencil for a conventional rigid printed circuit board (PCB) is challenging enough in this day and age, but designing a stencil for a much smaller IoT rigid-flex or flex circuit takes on a considerably new meaning.
On its surface, a stencil looks a bit like kitchen tinfoil, but is made of extremely thin stainless steel and not overly flimsy like tinfoil. Based on a specific stencil design, an automated laser cuts small, specified openings on that stencil for each surface mount (SMT) component and device supporting a particular IoT product.
The stencil’s job is to serve as a guide for transferring and printing the dispensed solder paste onto the PCB. In the case of smaller IoT products, solder paste is dispensed on rigid-flex or flex circuits to solder SMT joints to a bare PCB. The pick-and-place system in the assembly process then places components and devices on to the tiny boards.
At this point, fixtures come into the picture. Fixtures are small, flat metal carriers used to move the circuits along in the assembly processes. They’re key to making sure the paste is properly dispensed during the paste-printing process. Fixture sizes vary, but for IoT circuits, they’re about the size of a 3-by-5-inch notecard as IoT products are typically smaller in size. The main purpose of a fixture is to ensure surface mount pad stability and preciseness during the printing process.
So, that’s easily said. However, actually designing the stencil is another matter. Why is it so important? The answer is that we’re now dealing with smaller device packages such as the tiny 0201 and 01005 package types that are difficult to print due to their minute sizes.
This means the stencil must be so precisely designed that the exact amounts of solder paste are accurately dispensed. The goal is to ensure sturdiness and stability by making sure those micro-packages are solidly soldered on and connected to the circuit board. Here, the burden falls on the electronics manufacturing services provider to assure that certain steps are taken to properly design an IoT rigid-flex or flex circuit stencil. It calls for three major criteria:
- Make sure that the printing surface is completely flat. That’s vital for even solder paste distribution. If you’re dealing with a combination IoT rigid-flex circuit, the rigid board is generally 0.062 mils thick and the flex board is five to 10 mils thick. Therefore, dispensing solder paste on the pads and ball-grid array (BGA) package balls becomes challenging.
A step stencil could be an answer to resolving this difference in board-thickness printing. A step stencil is multilevel; its purpose is to either reduce or increase the thickness at certain points on the stencil. Hence, this technique dispenses the solder paste on the different thicknesses while printing is performed on the combination rigid-flex circuit.
- The stencil must also provide optimum stability and control solder paste spillage to avoid electronic connection flaws. Keep in mind that an IoT flex circuit will incur a multitude of bends and twists during the product’s routine use. If the stencil surface is not flat and stable during the assembly process, there’s a high probability of solder paste spillage occurring between pads or joints. The end result is the creation of shorts or bridges, or latent flaws that occur once the IoT product is being used in the marketplace.
- Good stencil design uses techniques like window paning and overprinting or underprinting depending on package differences. Window paning refers to altering the paste dispensing area or openings of a stencil when typically dealing with center ground or thermal pads of a device leadless package like a quad flat lead or dual flat no-lead.
To cap this discussion off, designing the right stencil is extremely important and takes good experience in process engineering to design it. Some of today’s more advanced rigid-flex and flex circuits have a number of analog components on them. The experienced process engineer must have expert knowledge about those specific components. For example, a heavily populated analog component board may require overprinting. That key aspect of the design must be factored in and calls for an extra amount of solder paste. But if the IoT rigid-flex or flex circuit is mostly populated with digital devices and micro BGA balls are closer to each other, the stencil designer might consider underprinting with less solder paste to avoid shorting two adjacent balls.
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