PCB Assembly

PCB Assembly

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PCB Assembly

General Procedure

This article aims to present a general circuit board assembly procedure that you can apply in most of the DIY (Do It Yourself) projects appearing on this site.

Tools needed:

A. Soldering Iron

The quality of electronics PCB assembly work depends to a large extent on the quality of the soldering job. Hence it makes sense to put out some extra peso to get the best soldering equipment your money can buy.

Temperature controlled low secondary voltage soldering iron (Soldering Stations) are best for electronics assembly. Unfortunately, they are quite expensive, hence are never popular to typical hobbyists and occasional circuit builders. Cheap clones of popular brands are, incidentally, available on the local market. These are a poor copy of the original, but still, are much better compared to the ordinary types.

Good quality general purpose soldering irons does a neat job when used with extra care.

Soldering stations may be pricey, but makes lot economic sense, especially when you work often on expensive and sensitive circuits.

Soldering at just the right temperature is extremely important. Insufficient heating control in poor solder quality flavored with bad connections. Too much heat, on the other hand, causes similar soldering quality problems and more: component damage, PCB de-lamination (copper trace peeling off), oxidation, etc.

The pcb shown on the left shows what a simple soldering iron can do when left into the hands of an untrained amateur.

A lot of articles had been published covering the subject of good soldering practices. A particularly good one is available as free download from the following site:

http://www.cooperhandtools.com/brands/electronic_applications/55578.pdf

It makes a good reference for the humble beginner and experienced veterans who may think they knew better.

B. Soldering Lead

Just as with the soldering iron, you should also be picky when it comes to the selection of the soldering lead. Don’t look for the cheapest, look for good quality. Soldering lead with 60/40 or 63/37 tin-lead alloy ratio melts at lower temperature, and are always recommended for electronics assembly.

Use rosin core solder lead, never use solder paste that is corrosive (acid based) .

C. Desoldering Tool

A need to de-solder arises as almost as soon as you solder something. Without proper de-soldering tools and knowledge of technique, things can go from bad to worse.

Desoldering tools and equipment. Clockwise from top left: a) Desoldering braid. b) Desoldering Pump c) Desoldering Iron with vacuum pump d) Desoldering Station.

D. Hand Tools

This includes diagonal pliers, long nose pliers, screwdriver set. Get a small electric drill. It quickly lets you drill missing component mounting holes on your PCB, sparing you from the annoying inconvenience that goes with it.

E. Ground Strap

The more expensive your component is, the more likely it will be damaged. There is more than a dozen way to kill a sensitive components,. The most serious threat may be the one we neither feel nor see, the dreaded ESD (electrostatic discharge). ESD damage occurs when a static charge that builds up in a moving object ( example, you ) suddenly finds an irresistible discharge path (and go for it) through one of your sensitive components.

Fortunately, there is one simple yet effective way to guard against this- use ground strap! A good ground strap is not exactly cheap, but is a sensibly worthwhile investment.

Recommended Assembly Procedure

Check the PCB carefully, especially if you made it yourself. Remember, this is the start of both the assembly process and your troubles. A trouble free PCB makes everything else easy while the opposite is true. You can usually find PCB defects with greater ease if you illuminate the opposite side of the PCB during inspection. The picture on the right shows one setup. The PCB is put on top of a illuminating box during visual inspection. Look for possible short circuit on densely spaced traces.

Microscopic breaks are hard to detect, it will take extra effort to successfully detect one. Microscopic breaks are very common with PCBs fabricated using pre-sensitized (photosensitive) boards. And don’t blame the PCB. This problem is usually caused by sloppy handling during the exposure and curing process.

Check your components. Do you have everything you need? Do you have the right components?

Take time to familiarize yourself with the components, especially the strange looking ones. Avoid touching the component leads with your bare fingers: Aside from exposing your sensitive components to ESD threat, your body fluids may corrode these and degrade the solder-ability of the components.

Solder all components working first with PCB jumper connections, if there is any. Next, mount and solder all resistors and capacitors. Some capacitors have polarity and must not be inserted the wrong way. Then, mount and solder all non sensitive components, such as connectors, IC socket, switches. It is prudent to remind you keep the temperature of your soldering iron in check.

Polarized capacitors have their (+) leg made longer for easy polarity identification.

Mount and solder all sensitive components last. This includes ICs, transistors, and similar fragile components. All semiconductor components, needing a correct polarity to work, have to be mounted the way they are intended to. MOS/CMOS devices are dreadfully allergic to static electricity, hence, should only be removed from their protective wrappings at the time of mounting. MOS/CMOS devices are most vulnerable to ESD damage when they are not connected in a circuit and out of their protective shield.

A static shielding bag, antistatic IC tube,

and conductive foam protects your ICs from

harmful ESD. Do not remove them from this package

till circuit board installation time.

After assembly, some checking and test procedure is usually required before the finished circuit can be put to use. Read the instructions very carefully and follow each to the letter. Should you decide to do it your own way, good luck, I trust you know what you are doing.

Every project may require additional assembly procedure very specific to it, you should find this information in their corresponding write ups. The procedure presented above may apply in part or in its entirety to a project. Enjoy building!

11/21/2006

Last Update

Written By:

Henry L. Chua

comments?

hlc@e-gizmo.com

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