Sunday, October 1, 2023

How To Select Your Own Benchtop SMD Assembly

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Another interesting one is the PNP machine by ProtoVoltaics, which can speed up the system by using compute unified device architecture (CUDA) accelerated computer vision from an NVIDIA graphics card to speed up the processing of video tasks like rotational vision correction.

The PCBGRIP electronics assembly system being offered is not a complete PNP machine, but holds PCBs, components, stencils and other items so that you can improve the quality and speed of your soldering.


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Of stencils and printers
The paste you use along with a stencil is like toothpaste, but it is grey and has to be kept refrigerated in order to last at least six months. If it dries even after being kept in the fridge, you can add alcohol and rejuvenate it. You do not need a lot of it, though. To apply the paste, you need to have a stencil, a piece of material with holes.

Vaishnav adds, “Solder paste comes in different compositions, leaded and without lead to comply with the restriction of hazardous substances (ROHS) directive. There are a few good Chinese brands that are there in the market.”

How do you build stencils? You can laser-cut these stencils pretty easily. Materials can be high-durability or high-temperature plastic. You can also use vellum, which is a translucent paper that you can get at a stationary store. It can be used for about 50 boards and can be easily laser-cut.

How do you select a laser cutter? Laser cutters have a limitation of the smallest hole these can cut. So this may restrict you from using the latest lead-less parts that have a pin pitch of 0.5mm or 0.4mm. If you need to cut a hole of 0.25mm×0.25mm the typical desktop cutters face the limit of their capability here. For steel stencils you need to use ultraviolet lasers that are very expensive. You only find these at big contract manufacturers or you can get a custom stencil by sending your gerber to China from where you will get a steel stencil for about US$ 80. Only do this for super-fine pitch parts with, say, 0.5mm pin pitch.

You may also want to go for steel if you want to do volume as vellum will not stand up to high volumes. You can get a steel stencil framed with an aluminium frame that is designed for mounting into the stencil printer.

You might be able to order a stencil at US$ 80 in China, but to get that to India you will end up paying another US$ 200. Venu suggests you try the many local companies in India that make quality stencils.

When you are dealing with 0.25mm pitch holes, getting the stencil aligned to the board can be tricky business. Stencil printers can be used here. While manual ones are not really useful, automatic ones are very expensive.

Reflow ovens
Reflows are generally categorised in two ways.

Batch reflow. This happens in batches of PCBs.

Continuous reflow. With the help of a conveyor, it moves through the oven, all controlled by PID/PLC control with multi-zones of temperature for better reflow soldering. There are a few Chinese vendors that have very good desktop models of this type.

There are a number of reflow ovens in the market from reputed Chinese suppliers. There are some small reflow ovens that can handle up to 10mm2 and larger ones that can go up to 330mm by 220mm, which is almost as big as an A4-size paper, and that is a good size to work with for office production.

When it comes to reflow ovens, get the biggest one that you can afford. When you get bigger ones, you require good ventilation as these generate fumes and smell.

In the case of smaller desktop ones, you do not need to worry about fumes and smell under normal conditions. But if you are using these throughout the day, then you do need to worry about the fumes.

You are either done or you need to rework

4C4_majorNo matter how good a machine is, you always have to inspect what the machine did for you and how the reflow went. Coggeshall says, “The biggest challenge that I have is the paste application and whether the paste is getting on in a uniform manner, or whether you have solder bridges due to too much or too little paste.”

Other cases are where there are big inductors, which could take a long time to heat up due to their thermal mass and the profile of the board. This could cause you to not get complete reflow. So when the boards come out, you need to check these out in a rework station.


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