How To Get Perfect Results PCB Soldering

Perfect PCB soldering made easy! It is also very rare. What can explain this discrepancy? If perfect soldering is easy, why is there so much touch-up and rework? The answer, of course, is that if you know it is easy. And everything the soldering “experts” have told us is wrong. The “industry standard” soldering rulebook guarantees failures and high costs.

Surprisingly, even though PCB soldering is a fundamental electronics assembly process, few people know how to solder reliably. They often become adept at hiding flaws, but this is a completely different and unacceptable ability. Visually acceptable connections are not necessarily reliable connections.

The time and money the electronics assembly industry spends on training and certification is largely a waste of resources. No one learned perfect soldering by attending “industry standard” training.

This and upcoming articles will explain why PCB soldering reliability is so disappointing, how we got here, and the necessary corrective actions.

Why we don’t have (usually) perfect PCB soldering.

Here’s the problem: Training focuses on the desired appearance of the solder rather than getting the connection. And an “acceptable” appearance can mask pending failures. How the connection is made determines not only whether the soldered connection itself is reliable, but whether there is catastrophic damage to the component being soldered.

At the temperature of the soldering iron, the solder will adhere to oxides and contaminants to produce a visually acceptable connection. However, the connection lacks an intermetallic bond and high temperatures reduce the bond within the components. Altered bonds change electrical values ​​and shorten component life. In just a few seconds of misuse, a soldering iron can reduce the life expectancy of components by decades.

But because the connection looks acceptable and component loss cannot be seen, the truly desperate state of modern soldering goes largely unrecognized.

How To Get Perfect Results PCB Soldering

A Brief History of PCB Soldering Procedures

Electronics do not always consist of solid-state components. Until decades before the advent of devices like transistors and microprocessors, vacuum tubes represented the state of the art. Electrical connections were made by soldering wires to the socket lugs into which the tubes were inserted. Some of the wires and lugs were quite large and absorbed a lot of heat. Meanwhile, soldering irons were not very efficient at converting electricity to heat.

Therefore, the thermal challenge in PCB soldering was how to prevent the solder from freezing before it completed its flow. So techniques were developed to maximize the amount of heat applied. (It was not necessary to protect the tubes from heat. The tubes were not inserted into the sockets until after soldering. They were never exposed to the soldering heat.)

The advent of solid-state components meant that, for the first time, solder was applied directly to the component rather than to wires and sockets. In other words, the components were subjected to soldering heat. And it had profound reliability consequences because the heat reduced the electrical properties of the components.

To prevent heat damage during soldering, metal clamps were attached to the leads along the body of the component. The heat from the soldering iron flows to the component but is absorbed by the clamps before reaching the body of the component. The clamps were called “heat sinks” and provided complete protection against heat loss.

Since the beginning of solid-state electronics, every work instruction has called for the use of a heatsink. (See, for example, J-STD-001G, Sec. 4.6.) But no one uses a heatsink! How can they? The leads (if there are even leads) are too short. There is no room to attach a heatsink. But all training programs still ask students to warm up like it’s 1960!

No reflow soldering.

It gets worse. During the years when soldering procedures were being written, nearly all component leads had tin or tin/lead plating. These surfaces were melted during “soldering” and the molten solder flowed together with the molten surface metal. The oxides, being lighter than the pure metal, floated on top of the liquid metals where they contacted the flux (even lighter than the metal) and were dissolved.

It is quite easy to make a connection by mixing molten metals but this is not soldering. (The term “reflowing” was often, and appropriately, used.) Soldering is the process of forming intermetallic bonds with metal surfaces that do not melt. (They do not “reflow”.) And it requires additional processing steps that do not involve mixing the molten metals. (Sadly, “reflow” continues to be widely used even though it is no longer correct.)

The distinction between soldering and reflow (simply joining the molten metals) gained greater relevance when Europe banned lead in electronics. The transition to a lead-free world is focused on new alloys. However, apart from a few points, new soldiers do not present great problems.

Lead-free solder is less forgiving of defective processes than conventional tin/lead alloys but performs reasonably well with properly controlled processes. (Since most companies had a faulty PCB soldering process, difficulties were encountered in switching to new compounds that were incorrectly attributed to the solder instead of the process.)

The biggest challenge is related to the new lead plating. The tin/lead plating disappeared of course. But, because of tin whiskers, fewer and fewer component leads (especially multi-lead surface mount parts) come with tin surfaces. These new surfaces do not melt at soldering temperatures. In other words, they must be soldered. But our industry also often sticks to limited measures that only work for reflow. And most common training and certification only guarantee defects and failures.

Soldering is simple science – if we let it be

The fact is that soldering is a science – mostly chemistry but also a fair amount of metallurgy and physics. The people who wrote the book of rules did not approach it this way. He worked based on observation, not realizing that the main foundations of science were not clear. If they found results that seemed correct, they created this entity. If we want a product that works and functionality that makes profit possible, things need to change.

Interestingly, reliability varies inversely with the amount of handling. The most reliable products are manufactured most efficiently. Our industry has the worst of both worlds – excessive costs and too many failures.

Use of flow

I just said perfect soldering is easy. But simple doesn’t mean slapping molten metal on parts and hoping everything goes well. Successful soldering requires knowledge and discipline. And it all starts with solderability.

Solderability is a relatively recent concern in electronics soldering. Until recently, most component leads were plated with tin or tin/lead. Soldering is the process that creates intermetallic bonds with metal surfaces that do not melt when the joining material (solder) is used. However, tin and tin/lead PCBs melt at soldering temperatures, and the solder simply fuses with the molten plating. This is not soldering; This is “reflow” and much easier than actual soldering.

Reflow is easy.

In reflow, there is no need to remove the oxide before soldering. The oxides, being lighter than the pure metal, float on the combination of liquid plating metal and liquid solder. The flux, being lighter than the liquid metal, also floats on the molten metal, where it can easily contact the oxides and break them. In reflow, the flow only makes the final connection shiny and cosmetically pleasing.

Most of the beliefs about soldering arose during this era of reflow. One such belief, with disastrous consequences today, states that liquid flux should not be used during hand soldering. The belief is that the flux in the wire solder is sufficient to do the job. While this may be true for reflow, relying specifically on flux in solder leads to incomplete wetting during soldering.

What are the main PCB soldering defects?

Banning lead in electronics changed our business tremendously by eliminating tin/lead component levels. Meanwhile, tin plating has become increasingly uncommon due to concerns about tin rusting with pure tin. The risk of whisker shorts is very real for multi-level surface mount parts such as I.C.s.

The new component levels are not tons or tons/lead; They are metals with high melting temperatures that do not reflow during soldering. In other words, these are metals that are soldered, not reflowed. And surfaces should be thoroughly deoxidized before soldering.

This will not happen if the flux is present in the wire solder; The solder cannot be released until the solder melts. Molten solder forms a barrier between the flux and the surface metal, preventing complete deoxidation and causing incomplete wetting.

What are the main PCB soldering defects?

Fluid flow is essential.

The only way to be sure before the solder melts is to use a liquid flux first. And more than just a trace amount of flux is needed. Flux acid (the part that removes oxides) is neutralized during the deoxidation chemical reaction. Trace amounts of the flux will be neutralized before the fraction is completely deoxidized.

In soldering, flux is more than our friend – it’s important. Yet, every few days, industry “experts” write strongly worded instructions that using liquid flux is a sin. There’s even a widely used “Seven Sins of Hand Soldering” video that states that “the best way to reduce the use of excess flux is to use only the flux in the solder wire.” (The video is sold by the trade association that publishes standards like J-STD-001 and A-610. They really should know better.)

Complications

Unfortunately, there’s more to the flux business than randomly picking off-the-shelf flux. We’ll look at the science of flux selection next time.

Heat control

The basic principles of soldering – the approach that, in many cases, is still used today – began about 70 years ago. The most advanced electronic components then consisted of vacuum tubes. Soldering was used to connect the wires to the lugs on the sockets into which the tubes were inserted after soldering. All soldering was done by hand.

Wires and lugs could not be damaged by overheating and sensitive components – the tubes – entered the picture only after soldering was complete. The heat was a different problem, however: some of the wires and lugs were quite large and the iron’s ability to convert electricity to heat was modest.

Taken together – large pieces of metal and scrap iron – keeping the material hot enough for the solder to melt and flow well presented significant challenges. To prevent the solder from freezing, the training emphasized making the parts very, very hot before soldering. (The term “cold solder” originated at this time and is appropriate. As I’ll discuss next time, “cold solder” is almost non-existent in modern electronics but is often used, albeit mistakenly, as a diagnostic for wet problems. appears.)

Heat sink

The advent of solid-state components (mainly leaded resistors and capacitors in the early days) meant that the active elements of the circuit were exposed to the heat of the soldering iron. An epidemic of component failures occurred until the heat-sensitive nature of these new components was recognized.

The solution was to use a metal clamp (“heat sink”) to protect the component. The clamp was clamped to the lead near the body of the component. The heat from the iron flowed to the body but was absorbed (“sunk”) by the clamp. Component failure immediately fell.

(Reliability also benefited from machine soldering, which at the time was entirely by a wave.

Read More: How to Drill Super-Straight Holes

At what temperature should I solder the PCB?

Heat damage to components is invisible and, as the saying goes, “out of sight, out of mind.” Static damage also occurs within components and is no more visible than heat damage. Yet no respectable electronics manufacturing facility would consider operating without strict ESD prevention measures in place. Why the difference? This is probably the result of market forces.

Anti-static requires the use of both hand tools and disposable materials, the combination of which adds up to a huge amount of money at the global industry level. Large revenues support large advertising budgets which, in turn, lead to the universal recognition that static represents a significant threat to credibility. The same applies to humidity.

Preventing heat damage does not involve purchasing any materials. Since there is no big dollar market, there is no advertising budget. Therefore, there is limited recognition.

Yes, I exaggerated the lack of concern about overheating components. Some companies are concerned enough about the heat that they spend big dollars on soldering irons that maintain a constant temperature.

Some companies even go so far as to monitor the temperature of the iron and, if possible, reset it once the deviation from the set point starts to cause concern. And they are all wasting money. Constant temperature irons will cause just as much damage as less precise tools. Heating is not controlled by iron temperature. It is about how iron and solder are used together.

At what temperature should I solder the PCB?

Institutional apathy.

During the 1980s, I conducted several soldering workshops for engineers at the Naval Weapons Center Soldering Standards site in China Lake, CA.

“How do you prevent heat loss?” I asked the director (a legendary figure in industry standards who is the final authority on all DoD soldering requirements). “Quick solder,” he said. “And how fast is coffee?” I answered. Immediately, he announced “three seconds.” I was stunned by the lack of science behind the comment. “Sometimes three seconds can be fine,” I agreed. “But isn’t it sometimes too long and other times not long enough?”

In the next class, I demonstrated a technique that ensures that the temperature of the components will stay close to the melting temperature of the solder. “I agree that what you’re showing works,” the director told me. “But do you expect me to tell the Admiralty that we are doing wrong?” I never came back. And, 30 years later, standards keepers continue to promote the wrong practices.

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