engineering

Selective soldering

Selective soldering is a engineering topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Selective soldering rather than just read about it. In short: Selective soldering is the process of selectively soldering components to printed circuit boards and molded modules that could be damaged by the heat of a reflow oven or wave soldering in a traditional surface-mount technology (SMT) or through-hole technology assembly processes. This usually follows an SMT oven reflow process; parts to be selectively soldered are usually surrounded by parts that have been previously…

Key takeaways

Selective soldering belongs to engineering; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Selective soldering to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Selective soldering from memory before moving on to harder problems.

Reference excerpt

Selective soldering is the process of selectively soldering components to printed circuit boards and molded modules that could be damaged by the heat of a reflow oven or wave soldering in a traditional surface-mount technology (SMT) or through-hole technology assembly processes. This usually follows an SMT oven reflow process; parts to be selectively soldered are usually surrounded by parts that have been previously soldered in a surface-mount reflow process, and the selective-solder process must be sufficiently precise to avoid damaging them.

Processes Assembly processes used in selective soldering include:

Selective aperture tooling over wave solder: These tools mask off areas previously soldered in the SMT reflow soldering process, exposing only those areas to be selectively soldered in the tool's aperture or window. The tool and printed circuit board (PCB) assembly are then passed over wave soldering equipment to complete the process. Each tool is specific to a PCB assembly. Mass selective dip solder fountain: A variant of selective-aperture soldering in which specialized tooling (with apertures to allow solder to be pumped through it) represent the areas to be soldered. The PCB is then presented over the selective-solder fountain; all selective soldering of the PCB is soldered simultaneously as the board is lowered into the solder fountain. Each tool is specific to a PCB assembly. Miniature wave selective solder fountain(s): This typically uses a round miniature pumped solder wave, similar to the end of a pencil or crayon, to sequentially solder the PCB. The process is slower than the two previous methods, but more accurate. The PCB may be fixed, and the wave solder pot moved underneath the PCB; alternately, the PCB may be articulated over a fixed wave or solder bath to undergo the selective-soldering process. Unlike the first two examples, this process is toolless. Laser Selective Soldering System: A new system, able to import CAD-based board layouts and use that data to position a laser to directly solder any point on the board. Its advantages are the elimination of thermal stress, its non-contact quality, consistent high-quality solder joints and flexibility. Soldering time averages one second per joint; stencils and solder masks may be eliminated from the circuit board to reduce manufacturing costs. Less-common selective soldering processes include:

Hot-iron solder with wire-solder feed Induction solder with paste-solder, solder-laden pads or preforms and hot gas (including hydrogen), with a number of methods of presenting the solder Other selective soldering applications are non-electronic, such as lead-frame attachment to ceramic substrates, coil-lead attachment, SMT attachment (such as LEDs to PCBs) and fire sprinklers (where the fuse is low-temperature solder alloys). Regardless of the selective soldering equipment used, there are two types of selective flux applicators: spray and dropjet fluxers. The spray fluxer applies atomized flux to a specific area, while the dropjet fluxer is more precise; the choice depends on the circumstances surrounding the soldering application.

Miniature wave selective solder fountain The miniature wave selective solder fountain type is widely used, yielding good results if the PCB design and manufacturing process are optimized. Key requirements for selective fountain type soldering are:

Process

Nozzle diameter selection according to solder-joint geometry, nearby component clearance, component lead height and wettable or non-wettable nozzle Solder temperature: Set value or actual value on plated through-hole part. Contact time Preheating Flux type: No-clean, organic-based; method of fluxing (spray or dropjet) Soldering: Drag, dip or angle method Solder temperatures are typically lower for tin-lead alloys and 250–270 °C for common lead-free SAC alloys in selective soldering pots, with optimum around 265 °C ± 5 °C for many low-silver variants to ensure proper hole filling and minimize dross.

Design

Temperature requirement (for soldered part) and component selection Nearby SMD through-hole component clearance Ratio of component pin diameter to plated through-hole Component lead length Thermal decoupling Solder masking (green masking) distance from component pad

Drop-Jet The Drop-Jet is an Electromechanical device which is capable of depositing a droplet of flux on demand onto a surface such as a Printed Circuit Board and or component pin.

Thermal profiling The thermal profile of the selective process is critical as with other common automated soldering techniques. Topside temperature measurements within the pre-heat stage must be verified as with conventional flow solder machine, additionally flux activation must be verified as sufficient. As number of miniature profiling dataloggers are now available to make the process more simple such as the Solderstar Pro units.

Selective solder optimization A number of fixtures are available to allow daily checking of the selective solder process, these instruments allow the verification of machine parameters to be performed on a periodic basis. Parameters such as contact time, X/Y speeds, nozzle wave height and profile temperature can all be measured.

Use of nitrogen atmosphere Selective soldering is normally undertaken in a nitrogen atmosphere. This prevents oxidation of the fountain surface and results in better wetting. Less flux is needed with less left-over residue. The use of nitrogen results in clean, shiny joints without the need for PCB cleaning or brushing.

References

Illustrations

Worked examples

Example 1 — a first encounter with Selective soldering

Start with the simplest possible case. Write down what Selective soldering claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In engineering, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Selective soldering before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Selective soldering ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Selective soldering

In research: Selective soldering appears in engineering research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Selective soldering in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Selective soldering is common in secondary-school and first-year university syllabi. It links to neighbouring topics Printed circuit board manufacturing, Soldering, so understanding it makes those chapters shorter.
In everyday life: Look for Selective soldering outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Selective soldering in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Selective soldering means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Selective soldering out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Selective soldering in simple terms?

Why does Selective soldering matter?

Because it connects several engineering ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Selective soldering?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Selective soldering.