Archive for November, 2015

Quality Control for PCB Assembly

Written by Admin on . Posted in PCB Assembly and component

pcb assembly

A printed circuit board (PCB) is the foundation of any electronic product and if its quality is not up to par, the entire machine will not be operational. But how can you know if your PCB has been assembled correctly? How can you tell if your product is made with the highest of quality? Here are some ideas to let you know.

What does the PCB do?

A PCB provides an automated method of wiring, allowing components within an electronic device to communicate with each other. The boards are both large and small and the specific wiring can vary depending on the board’s purpose.

Has Your PCB Been Constructed With The Standard Requirements?

You will need to verify that your PCB has been made following the standard PCB manufacturing requirements. This means the physical form should match what the design was meant to look like. The dimensions and position of the interconnecting points should match up with the components. If there is any question to how well it fits, it may not have been built correctly.

Additionally, the PCB is meant to provide insulation between the interconnecting points and the points that are not being used. Verify that there is an adequate insulation. The interconnection of the components however is key. If the points do not match up, the PCB will not do its job and the entire electronic unit will not be able to function.

How Was It Put Together?

Components/parts are soldered to PCB There are three types of soldering that is used in a PCB: wave, reflow, and by hand. Wave solder is mainly used for through-hole components, but not for surface-mount technology (SMT). SMTs will have a reflow solder.

Looking for a quality reflow solder will show that the surface mount components have been adequately soldered to the PCB with solder paste. In hand soldering, a solder iron is used. There is the highest discrepancy with hand soldering as it is not standardized by a computer. The mounts with any method of soldering should be secure in their place.

Check Who Built It

There should be a fab house logo verifying the manufacturer. This indicates the types of materials used by the PCB manufacturer. This logo and type for the manufacturer are given by the Under Writer Labs after testing the samples that had been provided by the PCB manufacturer. The handling and the materials used during fabrication can seriously affect the quality and the reliability of the PCB. The company access and use of raw materials is the biggest factor in the material quality. Companies will need to have evaluated whether their materials are suitable for the manufacturing requirements of a PCB.

Some PCBs will have a conformal coating, or thick paint-like coating to seal and protect the components on the board. This will keep out any foreign bodies, such as dust or moisture, from compromising the functionality of the board. The coating should verify that the PCB has been tested already to verify that it is functional.

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Misfits

Written by Admin on . Posted in PCB Assembly and component

If the shoe doesn’t fit, can you still wear it? You might have to if they are the only shoes available. In this case, the SMD packages for this PCB assembly application are actually wider than the PCB footprint itself. There are any number of reasons for this, from changes in component design to substitution issues, but we won’t get into that here. But the problem is that the leads actually overhang the SMT pads and extend onto the solder mask area (Figure 1).

 

These packages are too large for the corresponding footprints, with leads extending and overlapping the solder mask.

These packages are too large for the corresponding footprints, with leads extending and overlapping the solder mask.

 

This, of course, is unacceptable. But attempting to shorten or “snip” the leads won’t work either; the shear force could easily be too much for the package’s integrity.

The solution was to bend the pins in slightly so that they could fit onto the SMT pad without extending or overhanging off of the pads (Figure 2).

 

Misfit-PCB-Assembly-2

Bending the leads back slightly to fit within the confines of the pads is the only acceptable solution.

 

Certainly some stress and tension is applied in mechanically bending the leads, but not enough that we need to worry about it. And even though the lead is contacting the pad at a changed angle, there’s enough solder to create a robust solder joint. Remember that in the early days of SMT, some through-hole DIPs were snipped off and soldered to SMT pads creating butt joints, and these proved to be robust and reliable.

Misfit-PCB-Assembly-3

The bent leads solder to the pads just fine, forming robust solder joints, and meeting acceptability criteria.

 

An added advantage of not shortening the leads is that retaining lead length provides added spring-like flexibility for the lead to flex with thermal cycling, minimizing the possibility of solder joint failure due to thermally-induced stress. It isn’t much trouble, a good solder joint is created, and the part passes standard acceptability criteria because, in part, the leads are contained within the solderable pad area.

Read: Solder Mask Design Changes Help Prevent BGA Pad Lifting

By Roy

 

A Sticky Situation

Written by Admin on . Posted in PCB Assembly and component

The surface tension of liquidus solder exerts a considerable pulling force on a component during reflow. This is why, once upon a time, small components could be relied on to self-align on SMT pads during reflow. They still can, of course, providing that all things are equal, such as pad dimensions all around. But if they’re not, you can expect problems.

Sticky situation_PCB_1

Figure 1. Two SMDs have misaligned during reflow due to uneven pad sizes and disproportionate solder liquidus surface tension.

 

In this case, two components had shifted away from their center location on the PCB footprint during reflow (Figure 1). This was due to the fact that a large SMT pad on one side of the components, see photo, was exerting a stronger pulling force on the component than the ordinary-size pads on the opposite side. More surface area means more pulling force, and consequently component misalignment. It doesn’t matter that the placement machine put the part in the right location beforehand.

Sticky situation_PCB_2

Figure 2. Temporary solder mask glue applied to two corners of each component prior to reflow keeps them in place.

 

Mechanical fixturing simply wasn’t a practical solution. Instead, two dots of temporary solder mask glue were applied to the corners on one side of each component, prior to reflow, to hold it in place. The glue acts as a temporary adhesive and prevents the parts from moving during reflow because it is stronger than the pull of the liquidus solder. After reflow, the glue is easily removed, and the SMDs are perfectly centered. Problem solved!

Sticky situation_PCB_3

Figure 3. Post-reflow, the adhesive is easily removed, and the SMDs are perfectly positioned as they should.

By Roy

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