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Understanding Electronic Assembly from a
Manufacturing Engineer Prospective
The Tyranny of Numbers in Hand Soldering
The Tyranny of Numbers in Hand Soldering
Let’s be honest — your odds of assembling this soldering kit and ending up with a 100% functional LED American Flag on the first try (with zero troubleshooting, rework, or repair) are extremely low.
It doesn’t matter how much experience you have with electronics or soldering. The reason is what we Electronics Manufacturing Engineers call “the tyranny of numbers.”
Every component you place, and every solder joint you make, is another opportunity for something to go wrong. The more parts, the more chances for mistakes. It’s simple math — and it dominates every assembly process, from kitchen-table kits to billion-dollar factories.
Understanding “The Tyranny of Numbers”
A small soldering kit with just a few components in most cases gives you a much higher chance of success—fewer parts mean fewer opportunities for defects.
A large soldering kit — no matter how well designed or precise the assembly instructions — dramatically increases the odds of assembly errors. My LED Flag, with its 266 components, is no exception.
From experience, I estimate that 85–95% of builders, regardless of knowledge, skill, or experience will need to troubleshoot, rework, or repair something before their flag works perfectly.
Why Humans Struggle to Build Modern Electronics
After decades as an Electronics Manufacturing Engineer, I’ve learned one unshakable truth:
Humans are not built to assemble modern electronics
Machines never blink, tremble, or lose focus. We do. And that’s why defects happen.
Modern surface-mount components are far too tiny and delicate for consistent human soldering, especially with a traditional iron. Some, like BGAs, even have their contacts hidden underneath the components body— making it completely unsolderable with a hand soldering iron. That’s why every major manufacturer today uses robotic pick-and-place systems and reflow ovens. Robots provide the precision and repeatability that make modern electronics cheap and reliable.
The True Reality Regarding the Use a Soldering Iron for Electronic Assembly
Reality: Considering a career as a professional soldering technician today?
That’s like saying, “I’ll specialize in typewriters!” in the 1980s.
While the traditional soldering iron still has its place — the use of a soldering iron in modern electronic assembly is extremely limited for rare cases where certain components cannot be included in standard machine assembly. This may be due to their size, sensitivity to the heat of the soldering process, or incompatibility with typical manufacturing processes such as circuit board cleaning. Soldering irons can also be used for rework and repairs, but only on a very limited basis, since hot air rework tools are usually the more effective method.
Additionally, outside of circuit assembly — where soldering irons once held some relevance — soldering for external connections, such as wiring to connectors and components, has been almost entirely replaced by solderless contacts and terminals. Soldering is now typically reserved for a few specialized applications that demand exceptionally high reliability of electrical connections.
Just as the traditional hand axe was rendered obsolete by the introduction of the handsaw and, later, by modern machining, the traditional soldering iron is likewise disappearing from electronic assembly manufacturing.
Think of it like this: imagine a modern furniture factory deciding to ditch CNC machines and power saws and instead build every chair by hand with axes and handsaws.
The result?
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Rough, inconsistent furniture
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Sky-high labor costs
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One chair per worker per week
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Zero scalability
That’s exactly where the soldering iron stands in 2025 electronics manufacturing — a proud relic of the past. Perfect for minor repairs, prototyping, teaching the mechanics of soldering, obsolete for production.
Why My LED American Flag Design-
Does Not Lend Itself Well to Automated Manufacturing
The Two Big Manufacturing “Sins”
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Mixed technology: through-hole LEDs plus surface-mount resistors and a diode.
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Components on both sides of the PCB.
In mass production, the golden rule is using either 100% through-hole or 100% surface-mount components, place them on one side of the board, and solder everything in a single pass through the appropriate soldering machine.
Circuit boards that incorporate mix technology components, that use both through-hole and surface mount components adds complexity and makes machine assembly far more difficult, and costly.
In addition, even if only one type of component package is used, mounting components on both sides of the circuit board, for example, a board with surface-mount components on both sides, similar to mixed-technology assemblies, adds process steps, increases manufacturing complexity, and raises production costs. This approach is typically justified only when it enables a smaller enclosure, eliminates the need for a secondary circuit board, or provides performance benefits in high-frequency applications.
My LED flag breaks both rules. Through-hole LEDs on one side, surface-mount parts on the other. That means more process steps, higher complexity, and lower yield — all of which drive up cost.
If someone handed me my own design and asked for a million units, I’d just smile and think,
“Ah, so you’ve never actually seen how this thing goes together.”
Automating it would make even the robots unionize in protest.
My LED American Flag soldering kit is specifically designed to be 100% hand-assembled and soldered using a soldering iron. The design intentionally prioritizes manual craftsmanship over manufacturability and was never intended for automated mass production.
Although the LED American Flag could be populated and soldered using automated processes, the design is far from optimized for that purpose.
If the design had used exclusively through-hole or exclusively surface-mount components, automated population and soldering would have been considerably easier to implement. However, multiple design factors influenced the final configuration.
In addition to the circuit board layout, the enclosure design for the circuit board was a key consideration. Due to cost considerations and simplicity my objective was to develop a single-piece, open-face enclosure that did not require a transparent cover over the top side of the PCB. Incorporating such a cover would have increased the cost and complexity of the enclosure while diminishing the visual appeal of the exposed LEDs.
The use of surface‑mount LEDs was incompatible with an open‑face enclosure design because their electrical contacts are exposed on the top side of the circuit board. Although surface‑mount LEDs are well‑suited for automated assembly, they would have significantly complicated manual placement and soldering compared to through‑hole LEDs, which are far better suited for hand assembly. Moreover, the exposed contacts of surface‑mount LEDs would have required a protective cover to prevent accidental contact or damage.
To avoid these issues, through‑hole LEDs were chosen for the top side of the board, simplifying hand assembly and preserving the open‑face aesthetic. Surface‑mount resistors and diodes were instead placed on the bottom side of the circuit board, as using through‑hole versions would have left their leads exposed on the top, making an open‑face design impractical.
Below is an Example of:
The Added Process Steps & Assembly Complexity of My LED Flag Design
would require using automated assembly equipment for large-scale production. Which is why my design would be a poor candidate for mass manufacturing, but is an ideal design for hand assembly as a soldering kit.
This is due to the fact that my design includes both through-hole and SMT components, it requires additional, more complex and added manufacturing steps, and specialized equipment— for applying adhesive to the SMT components, curing, and soldering.
Since my LED flag is a simple, low-end technology device that generally cannot justify the added cost in real-world manufacturing, therefor- it is extremely unlikely that you will ever see a mass produced, fully assembled, LED American Flag being marketed with the same configuration of components as my LED American Flag soldering kit.
The Process of Assembling My LED Flag- Using Fully Automated Assembly Equipment:
(This method has existed since the 1980s — and it’s notorious for problems)
The Process of Assembling My LED Flag- Using Fully Automated Assembly Equipment:
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Solder paste application: Solder paste is screen-printed onto the bottom side of the board at the pads where surface-mount resistors and diodes will be placed.
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Surface-mount adhesive (SMD adhesive) application: A small dot of SMD adhesive (also known as surface-mount adhesive or SMA, typically a one-part epoxy, red or orange in color) is dispensed directly beneath the center of each component body to temporarily secure it to the PCB during handling and wave soldering. This adhesive meets IPC-SM-817 standards and is applied via automated syringe, stencil, or pin-transfer.
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Surface-mount component placement: The surface-mount resistors and diodes are precisely placed onto the solder paste and SMD adhesive dots using a pick-and-place machine.
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Adhesive curing: The SMD adhesive is heat-cured (typically 100–150°C for 1–5 minutes in a dedicated oven or inline process) to fully bond the components to the board before further handling or wave soldering.
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Board flipping and through-hole component insertion: The board is flipped, and through-hole LEDs are inserted on the top side using an insertion machine or manual process.
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Final soldering: The fully populated board passes through a wave soldering process, which simultaneously solders the surface-mount components (held in place by cured SMD adhesive and solder joints) on the bottom side and the through-hole LEDs on the top side in a single operation.
Surface Mount Technology Component
Although, invented decades earlier:
The evolution of SMT was a collective effort across several organizations during the late 1960s and early 1970s. It was in the early 1980's that surface mount components first started to be manufactured in scale, utilized in designs, and first started to appear in electronics assembly floors.
I know for certain that the method of soldering mixed-technology circuit boards using wave-flow soldering first began in the early 1980s, as I working for the premier soldering equipment manufacturer at the time, and installed one of their very first mass soldering machines, featuring a specialized chip-wave/turbulent-wave designed specifically for this very purpose. They would later add ultrasonics to the wave to further improve the soldering results.
Common Process Difficulties in Mixed-Technology
(SMT + THT) Wave Soldering
Below are real-world manufacturing challenges in the mixed-technology wave soldering process, focusing on SMD adhesive and wave soldering interactions. These can lead to defects, yield loss, or rework.
1. Adhesive-Related Issues
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Adhesive stringing or tailing Dispensed dots form strings/tails due to high viscosity, wrong needle, or poor parameters. → Contaminates pads, causes misalignment or solder bridging.
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Insufficient adhesive volume Too little adhesive → weak bond. → Components shift or fall off during flip or wave.
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Excessive adhesive volume Too much adhesive oozes onto pads or under component. → Solder non-wetting, insufficient joints, tombstoning.
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Adhesive on solder pads Misaligned dispensing contaminates pads. → Poor solderability, voids, open joints.
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Incomplete adhesive curing Under-cured (low temp/time). → Components dislodge in wave, outgassing causes solder splatter.
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Over-cured or brittle adhesive Excessive heat/time. → Cracks under stress; adhesive chars and contaminates flux/solder.
2. Component Placement & Handling
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Component shifting before cure Pick-and-place pressure pushes part into wet adhesive. → Misalignment, rotated parts, solder defects.
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Board warpage after cure Thermal mismatch (adhesive vs. PCB). → Poor wave contact, skipped joints, bridging.
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Adhesive curing oven bottlenecks Dedicated curing step adds time/space. → Throughput reduction, WIP buildup.
3. Wave Soldering Interactions
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Adhesive outgassing in wave Moisture/uncured monomers vaporize at 250°C+. → Solder balls, voids, flux contamination, nozzle clogging.
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Adhesive residue on wave Charred adhesive floats in solder pot. → Contaminates solder bath, needs frequent dross removal.
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Solder wave turbulence High wave height/pump speed. → SMT parts swept away if bond is weak.
4. Material & Compatibility Issues
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Adhesive-flux incompatibility No-clean flux reacts with adhesive. → Poor wetting, residue, reliability issues.
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Adhesive not wave-resistant Adhesive softens in wave. → Component detachment, defective boards.
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Color misidentification red adhesive mistaken for flux in AOI. → False defects.
5. Inspection & Quality Control Challenges
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AOI false calls Red/orange dots flagged as contaminants. → Unnecessary rework, yield loss.
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Adhesive voiding under component Air trapped in large dots. → Weak bond, popcorning risk in wave.
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Post-wave adhesive residue Charred adhesive around component. → Cosmetic defects, cleaning needed (if not no clean).
Bottom Line: The SMD adhesive step is critical but fragile in wave soldering. Biggest risks: under/over-dispensing, incomplete curing, and adhesive-wave interaction.
What 30 Years in the Industry Taught Me
Back in the mid-1980s, I worked as a Field Engineer for the largest soldering equipment manufacturer in the United States. I traveled the country installing high-volume assembly systems, provided operational and maintenance training, and solving process issues.
Over a hundred plant visits — from small startups to global giants — I saw one pattern again and again:
Persistent yield problems often had nothing to do with the soldering process itself. They came from poor circuit board or fixturing designs. That lesson stuck with me until today:
A great product must also be manufacturable.
You can design the most brilliant product in the world, but if it’s difficult or inconsistent to build, it’s a poor design. Manufacturability determines true quality.
What Makes This Kit Special
What really sets this kit apart isn’t its difficulty — it’s how easy it is to fix when something doesn’t work.
Each LED acts as a built-in diagnostic tool. In a radio kit, a silent speaker could mean 25 different things. But in this flag, the LEDs instantly show which section has an issue.
I designed the 266 parts in small groups of 6–8 LEDs. So, when something fails, you don’t have to hunt across the entire board — you only check a tiny section. The LEDs literally light the way to the problem. Follow the included troubleshooting guide, and your chances of finishing with a fully functional LED flag are extremely high.
This Isn’t Just a Soldering Kit. It’s How You Learn to Think Like an Engineer.
You’ll learn how to solder, yes. But more importantly, you’ll understand how engineers think about manufacturability, yield, and design trade-offs. It’s hands-on learning project, with a dose of real-world engineering insight — and a bright, patriotic payoff when your flag lights up.
Putting Real Numbers to the Cost of
Hand Assembly vs. Automation
We’ve talked about how design choices affect manufacturability — but what about cost? It’s one thing to understand that automation is faster and more consistent, but another to see just how dramatic the difference really is. Let’s look at what it would actually take to build one million LED American Flags by hand versus by machine.
So, let’s put some real-world numbers to it. Imagine scaling this LED American Flag kit — the same one you’re soldering by hand — to 1 million units per year. What would it actually take to build that many by hand… and how would it compare to a modern, fully automated production line?
The Cost Reality: Why Hand Assembly Doesn’t Scale
There are many ways to assemble circuit boards — from 100% manual hand soldering to hybrid or semi-automated processes, to fully robotic production lines.
But to clearly show how costs explode when humans replace machines, let’s compare two extremes:
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All Hand Assembly — built entirely by people using soldering irons and hand tools.
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Fully Automated Surface-Mount (SMT) Assembly — built entirely by robotic placement and reflow ovens.
Both methods will get you a working board, but only one is practical when you start talking about serious production volume.
Scenario A: All Hand Assembly
Imagine producing 1 million LED American Flag boards per year, each with 266 components — 37 chip resistors, one chip diode, and the rest 5 mm through-hole resistors.
Assembly time:
• Through-hole parts: about 30 seconds each × 228 parts = 1.9 hours
• Chip parts: about 1 minute each × 38 parts = 0.6 hours
• Total assembly time per board: approximately 2.5 hours
Labor cost:
At a typical manufacturing rate of $35 per hour (fully loaded, including benefits, payroll taxes, and overhead), labor alone costs about $87–$105 per board, including inspection and rework.
Scaling that up to 1 million boards per year means:
• Around 1,200 to 1,500 full-time assemblers working year-round
• Roughly $87–$105 million in annual labor cost
• Significant extra overhead for rework, quality control, and management
Even before adding workspace, tools, and yield losses, this approach is economically impossible. Hand assembly is fine for small runs or learning, but at a million units per year it’s completely unfeasible.
Scenario B: Fully Automated SMT Assembly
Now, imagine the same board redesigned for 100% surface-mount components on one side of the PCB — ideal for robotic pick-and-place and reflow soldering.
Modern SMT lines can place thousands of components per hour with exceptional accuracy. Typical placement cost ranges from $0.02 to $0.05 per component. At 266 components per board, that’s about $8 for placement.
Add reflow, inspection, and handling, and the total assembly cost is about $10 per board.
Scaling that to 1 million boards per year:
• One or two SMT lines can handle the entire volume
• Around 30 to 50 total staff including operators, maintenance, and QA
• Total annual assembly cost of roughly $10–$13 million
Hand Assembly vs. SMT Automation — Quick Comparison
• Components per board: both have 266
• Time per board: hand assembly ~2.5 hours; automated SMT ~1–2 minutes
• Labor cost per board: hand assembly $87–$105; automated SMT about $10
• Annual assembly cost (1 million units): hand assembly ~$87–$105 million; SMT ~$10–$13 million
• Workers required: hand assembly 1,200–1,500; SMT 30–50
• Yield and consistency: hand assembly low yield, high rework; SMT high yield, low rework
• Scalability: hand assembly extremely poor; SMT excellent
• Feasibility: hand assembly unrealistic beyond small runs; SMT ideal for mass production
The Takeaway
There are countless ways to build a circuit board, but at high volume, labor cost and yield dictate survival. Hand soldering is perfect for learning, prototyping, and small-run craftsmanship. Automation is the only viable path for large-scale manufacturing.
Modern electronics manufacturing runs on precision, repeatability, and throughput — the very things humans struggle with and robots excel at. That’s why the traditional soldering iron, while still an amazing educational tool, has become a symbol of the past in production environments — a bridge between how electronics were built and how they’re built today.