PCB Fabrication

Do You Really Need Soldermask And Silkscreen

by:A-TECH      2020-12-19

Temporary adhesive for soldering double-sided SMT PCBs – A vital problem of assembling double-sided SMT circuit boards is the way to solder parts on the bottom-facet of the board. Running the top-side of the board by way of the reflow oven isn’t a problem as a result of gravity helps maintain the parts in place because the solder melts after which solidifies. When the board is flipped to run the underside aspect, you are counting on the floor tension of the solder to hold the parts into place. That may fit for smaller components, however some type of adhesive might be needed to hold bigger components in place. Spot masks is often applied to the corners of BGAs and other massive elements.

When designing a solder mask outlined pad, make your copper pads as large as you possibly can so a complete copper pad might be exposed in the masks. A good rule of thumb is to have a big enough copper pad that can let you print at least 3mils (zero.076mm) of mask on all sides of your copper pad.

In addition, the adhesive on tape can depart residue behind that may create wetting issues for conformal coating. Masking boots are preformed, which requires extra planning and lead time than many contract manufacturers can afford. Boots also must be cleaned as they're coated with baked-on flux, or discarded.

Because inexperienced turned the default shade early on, masks producers placed more focus on finetuning green for optimal performance. One of an important traits for a solder mask is adhesion to the PCB laminate. Good adhesion permits the power to print very slim mask dams between tight SMD pads.

Green additionally reacts very nicely to UV exposure, which is the process that cures and bonds the mask to the laminate. Green was the best-looking shade possible given the process on the time.

The good contrast between traces, planes and no-copper areas but it's considerably lower than that of the inexperienced PCB board. So it would require magnification to verify whether or not there are defects on the fine traces on the board. Screen printing performed properly on a pink solder mask, and residual flux was eliminated properly, identical to on a green circuit board.

The unique chemistry for solder mask comprised of a base resin that was a brown/yellow and a darker brown hardening agent was added to remedy the mask. Attempts have been made to add completely different mixtures of pigments to the mixture, but reds and blues simply obtained darker, were not very engaging, and made it troublesome to see the circuitry underneath. The finest result was achieved by including yellow and a touch of blue to make inexperienced. In the picture above, the sunshine green line is the solder masks layer.

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