# Selecting Reflow Oven Length

Folks,

You are putting in a new assembly line to assemble some large boards for which your company just received a three-year contract. The boards are 45cm long and you expect the cycle time from the component placement machines to be 40 seconds per board. Your boss is pressuring you to get another 5-zone oven, as they are cheaper and take up much less space than a 7- or 10-zone oven. But, you are concerned that a 5-zone oven may not have the capacity that is needed to keep up with the component placement machines. Let’s make some calculations and see if your concerns are justified.

Table 1 shows some typical reflow oven metrics:

Let’s assume that you will be using a typical modern SAC solder paste. By studying the reflow profile above, we see that the amount of time needed in the heated zone is about 4.5 min. or 270 sec.

So if we choose the 5-zone oven the belt speed will be:

Belt Speed = BS= Heat Tunnel Length/Time in Heated Tunnel = HTL/Time = 180 cm/270 sec. or 0.66 cm/sec

The component placers will be presenting a 45cm board every 40 sec., so the belt speed needs to be:

BS = Board Length/Cycle Time = BL/CT = 45cm/40 sec = 1.125cm/sec

So clearly a 5-zone oven won’t work. What about a 7-zone oven? Let’s calculate the belt speed for this oven.

BS = HTL/Time = 250cm/270 sec. or 0.926cm/sec

Now we can see that the 7-zone oven won’t do the job either.

How about the 10-zone oven? Let’s see if the belt speed is greater than the 1.125 cm/sec needed.

BS = HTL/Time = 360cm/270 sec. or 1.33cm/sec

Success! Since 1.33cm/sec is greater than 1.1125cm/sec, this 10-zone oven will work. The extra belt speed will permit a small amount of spacing between the boards. Let’s calculate what it will be:

BS = (BL + Spacing)/CT = 1.33cm/sec => BL+ Spacing = BS x CT => Spacing = BS x CT – BL

Spacing = 1.33cm/sec. x 40 sec – 45cm = 53.2cm – 45cm = 8.2cm

To summarize: For our 45cm board that has a cycle time of 40 sec., we need a 10-zone oven with a heated tunnel length of 360cm. There will be an 8.32cm spacing between the boards in the oven.

If you would like an Excel spreadsheet to make these calculations send me an email at rlasky@indium.com.

Cheers,

Dr. Ron

# Soldering 101: II: The Miracle of Soldering

Folks,

Pity Ötzi, The Iceman, circa 3500 BC. It is believed that he was involved in copper smelting as both copper particles and arsenic, a trace element in some copper ores, were found in his hair. Not only was he being slowly poisoned by the arsenic, but to smelt the copper he had to achieve a wood fire temperature of about 1085ºC (1985ºF), as discussed in my last post. The arsenic in the copper did have a benefit, as it gave the copper a little more strength than if it were pure.

Shortly after Otzi’s time, metal workers discovered that adding 10% tin to the copper produced bronze. Bronze is not only markedly harder than copper, but it melts at almost 100ºC lower than pure copper, making metal working much easier. The Bronze Age had begun. This period coincided with what scholars would recognize as the beginning of modern civilizations, such as those in Egypt and Greece.

Since it melts at a lower temperature, bronze also fills molds better. This improved mold filling is evident in Figure 1. This photo shows a copper and bronze hatchet that I had made. The copper hatchet on the left shows evidence of poor mold filling.

Figure 1. Copper, on the left, and bronze hatchets that were made for Dr. Ron’s Dartmouth College course ENGS 3: Materials: The Substance of Civilization. Note that the copper hatchet shows poor mold filling due to copper’s higher melting temperature..

In my opinion, it is almost certain that the Bronze Age is related to the development of soldering. The first evidence of soldering was about 3000º BC where, arguably the first civilization, the Sumerians assembled their swords with high temperature solders. Since the base metal for most copper-to-copper soldering is tin, the early metal workers almost certainly learned that tin could be used to join copper or bronze pieces together at much lower temperatures than smelting.

Until the European Union’s restrictions on lead in solders in 2006, most electronics solders were tin-lead eutectic. Eutectic is a Greek word that roughly translates into “easy melting.” Figure 2 shows the tin-lead phase diagram. Note that the melting point of tin is 232ºC and that of lead is 327ºC, yet at the eutectic concentration of 63% tin/37% lead, the melting temperature drops to 183ºC. This concentration and temperature is known as the eutectic point.

Figure 2. The tin-lead phase diagram. Note the eutectic point at 183ºC.

After the EU’s lead restriction went into effect, most electronics solders are based on a tin-silver-copper alloy that melts in the 217-225ºC range. The most common of these alloys being SAC305 (Sn96.5Ag3.0Cu0.5, where the numbers are weight percentages.)

Although the eutectic point is an interesting and usually beneficial phenomenon due to its lower melting point, the true miracle of soldering is that two pieces of copper that melt at 1085ºC can be bonded together with a tin-based solder at less than 232ºC. The value of this benefit cannot be overstated. Nature has allowed us to mechanically and electrically bond two pieces of copper together at a low enough temperature that we can do this bonding in the presence of electrically insulating polymer materials. Without this feature of solder, we would not have the electronics industry! An added benefit is that the bonding is reworkable, so that if a component fails, it can be replaced without scrapping the entire electronics printed circuit board.

It is natural to ask how this bonding takes place. The tin in the solder forms intermetallics with the copper. Typically Cu6Sn5 forms near the tin and Cu3Sn forms near the copper (Figure 3).

Figure 3. Copper tin intermetallics from Roubaud et al, “Impact of IM Growth on the Mech. Strength of Pb-Free Assemblies,” APEX 2001.

So next time you use your smartphone, laptop, tablet, or other electronics device, don’t forget that without the miracle of soldering it wouldn’t exist.

Cheers,

Dr. Ron

# Soldering 101: The First Copper Smelting

Folks,

Soldering is an ancient technology. It is estimated that soldering was first discovered as long ago as 4000 BC. So soldering was much more ancient to Julius Caesar  (100 BC – 44 BC) than Caesar is to us today. Before considering soldering, let’s discuss early copper smelting, as copper is usually the metal soldered to.

My Cornell colleague Steve Sass wrote a book, Materials: The Substance of Civilization, on which I based my course of the same name on. In his book, Sass points out that the importance of the firing of clay can’t be overstated as it is the first time humankind changed the nature of a material. Once clay is fired it forms ceramic, a material much stronger than dried clay. Artisans first performed this feat about 26,000 years ago in what is now the Czech Republic.

While I agree with Sass’s assessment, it could also be argued that the beginning of modern technology can be traced back to the first smelting of copper. The firing of clay is too simple a process to encourage much further experimentation, which is needed for technology growth. The process of smelting of copper, the first metal liberated from its ore, is quite complex and this complexity led to further experimentation that gave us iron and steel. Continued working with metals likely developed the scientific method, hence led up to all of the breakthroughs to this day.

Consider the novelty of the first smelting of copper. To smelt copper, our ancestors had to grind copper ore, malachite (Figure 1), into a powder, mix it with carbon, and heat it to greater than 1085ºC (1985ºF). By the way, you can estimate the Fahrenheit temperature by multiplying the Celsius temperature by a factor of two and will only be off by <10% from 100º-1700ºC.

Figure 1. Malachite (copper ore) is quite attractive. Perhaps this attractiveness brought it to our ancient ancestors’ attention as a candidate for smelting. (Copyright 2018, Ronald C. Lasky, Indium Corp.)

After I cook on my outdoor grill, I clean the grates by turning the propane to maximum to cook off the grease. Typically the grill’s thermometer will read about 600ºF during this process. The grill gives off so much heat that it is oppressive to approach it to turn it off. Needless to say, noting what 600ºF feels like suggested to me that it is very hard to achieve 1985ºF with a wood or charcoal fire.

Anyway, I recruited some graduate students to try and smelt copper as described above. They purchased many bags of charcoal, used a leaf blower to supply air and worked for two hours on two different attempts and failed both times. The next year some students built a tower with vents and put charcoal on the bottom with the copper ore and carbon in a crucible on top. Their tower was similar to a roman smelting furnace for iron (Figure 2). They were successful and produced a piece if copper about the size of a penny.

Figure 2. A Roman style furnace. Dr. Ron’s students built a similar tower from cinderblocks.

These two attempts demonstrate how amazing our distant ancestors were. How did they think to do it? There were certainly many failed attempts. How did they persevere? One thing is certain, they started the trend of discovery, in about 5000 BC, that led us to today. We owe them much.

Cheers,

Dr. Ron

# Intermetallics and Kirkendall Voids Continue to Grow at Room Temperature

Folks,

In my last post, I discussed intermetallic compounds (IMCs) and what I referred to as the “miracle of soldering.” I also mentioned that research focused on the brittle nature of IMCs suggests that failures in stress tests are more likely due to failures between the interfaces of the IMCs and the solder, the IMCs and the copper, or the IMCs (Cu6Sn5 with Cu3Sn) themselves and are not related to any perceived brittle nature of the IMCs.

Another weakening mechanism in soldering and thermal aging of solder joints is Kirkendall voids. Kirkendall voids form when one metal diffuses more rapidly into another metal than vice versa. A copper-tin interface displays such a mechanism. Copper diffuses into the tin more rapidly than the tin into the copper. This mechanism can result in actual voids in the copper at the metal interface. See the image below. In addition to causing a possible weakness at the interface, the excess copper that diffuses into the tin creates compressive stresses than can result in tin whiskers.

Kirkendall voids

IMCs and Kirkendall voids are formed quite quickly at soldering temperatures. However, even at room temperature IMCs and Kirkendall voids continue to grow, albeit at a much reduced rate. The reason for this continued growth is that on the absolute temperature or Kelvin scale, room temperature is a considerable fraction of the melting temperature of solders. As an example, the melting temperature of SAC is about 219°C, this temperature is equal to 492K (219+273), whereas room temperature is 295°K, so room temperature is 60% of the way to the melting point of SAC solder (295/492 = 0.60). Compare this situation to steel, which melts at about 1480°C. The steel would be red hot at 60% (780°C) of its melting point on the absolute scale. So, since room temperature is 60% of the way to melting, the IMC and Kirkendall forming processes don’t stop at room temperature. Hence, IMCs and Kirkendall voids continue to grow, as do related effects such as tin whiskers.

Stay tuned. Next time we will discuss IMC growth rates and resulting effects in stress testing as we wrap up this series on IMCs.

Cheers,

Dr. Ron

# Wicked Wicking

This PCB assembly challenge involved attaching a solar panel to one side of a pad using solder paste with a pass through an SMT reflow soldering oven.

Solder wicking through the unmasked vias to the back side forms unacceptable “bumps” on top of the vias.

The attachment or bond itself wasn’t the issue; but after the first trial runs, it was clear that solder wicking through the unmasked vias was going to be. Solder would wick through the unmasked vias to the back side and form “bumps” on top of the vias.

These bumps made the surface nonplanar and of course were unacceptable. It wasn’t an issue of using excess solder paste. But the “wicked wicking” had to be stopped, or at least prevented.

Kapton tape is applied to cover the unmasked vias; it will block the molten solder from leaking through.

But how? Clearly, to keep the solder where we wanted it to remain during reflow, we had to find a way to prevent it from wicking up, collecting at the opposite ends of the vias and forming bumps. We had to find a solution that was simple, temporary, and tolerant of reflow soldering temperatures. The answer was Kapton polyimide tape, a familiar product to PCB assemblers for many years, and a material that does not degrade at reflow temperatures.

Kapton tape was applied to cover the unmasked vias in order to block the molten solder from leaking through the vias to the back side during reflow. After reflow and cooling, it was a simple matter to peel off the tape. This temporary masking solution worked; there were no more solder bumps on the back side of the assembly, and the cost of the fix in terms of time and material was very low.

Figure 3. This temporary masking solution worked; there are no more solder bumps on the back side of the assembly.

Roy Akber

www.rushpcb.com

# Solder Defects Causes and Cures Webinar

If you missed the SMTA International preshow webinar supported by CIRCUITS ASSEMBLY you can view it online here.

Printing solder paste or other conductive material requires zero defects printing if a high first-pass yield is to be achieved when using fine-pitch components. Monitoring and control of paste height and volume are becoming the norm in many markets, but what capability can we expect?

Correct printer setup, good stencil design and manufacture plus consistent printing materials are key to successful manufacture but inspection and monitoring the performance makes a process more robust. The same three-dimensional inspections are required in other AOI applications like solder joint analysis. There are common process defects during printing and reflow, Willis says, and the webinar shows causes and cures to help yield improvement.

The webinar is presented by Bob Willis and covers:

• Solder paste inspection standards
• Soldering yield impact with poor printing
• Common solder paste defects
• Impact on reliability based on paste thickness
• Solder joint inspection defects
• Common process defects causes and cures

Results of survey of 98 engineers from last week’s webinar on process defects.

# Down the Drain

Figure 1 shows a closeup photo of a PCB assembly, it seems as though solder has flowed “down the drain” and away from the solder joint where it’s needed.

In fact it has, because the customer has inconveniently located a via right through the center of one of the two topside SMT pads for a surface mount component. When the assembled PCB is run through reflow, the molten solder drains away through the barrel of the via and out the other side of the PCB. There isn’t enough solder remaining post-reflow to create an acceptable solder joint per IPC-A-610. The joint is “starved”; this is unacceptable. What to do?

Figure 1. Insufficient solder, i.e., “starved” solder joint on an SMD pad.

The via is there to stay, by virtue of the customer design. So, no matter how many times solder is added to the joint, every time the PCB is run through the reflow oven the solder is going to drain away because the PCB , including the via, is at reflow temperature.

Obviously, more than one run through the oven makes no sense. The only practical solution is to manually add solder to the individual solder joint, post-reflow, without running the entire PCB through another thermal cycle. It’s a touchup procedure that’s required to create a robust SMT solder joint that meets acceptability criteria. This is a manual PCB assembly soldering process that should be performed by a skilled hand-soldering or rework operator. Solder is added only to the joint, via cored wire solder or solid wire with flux, in order to build up the volume of solder at the solder joint to provide strength, connectivity, and an acceptable meniscus per IPC standards, covering the via drain-hole. The solder won’t flow through the via because only the surface joint area is heated.

Figure 2. The solution: Add solder to the joint manually via a touchup procedure.

It may seem tedious, but a skilled operator can touch up the joint in a few seconds, and if there is only one instance per assembly it won’t appreciably cause production delays.

Roy

rushpcb.com/rushblog

# Using the Newest Gen Arm, Part II

I’m a bit behind in my blog work — well, way behind, actually. I started this series back in January with the intro post.

Here’s where I am right now:

1. I have three different sets of PCBs.
2. One set, I took home to see if it’s possible to solder a micro-BGA at home. (As someone working at a car manufacturer might want to see if they could balance a crankshaft at home, for fun)
3. Two sets, from our partner, Sunstone Circuits, are here in my desk with parts, ready to go through our machines.

After I’ve got all three sets built, I’ll have them x-rayed to see how they look under the hood. Finally, I’ll solder through-hole headers on and fire up the chips to see if the shared escape system works.

And, here’s the shared escape:

The main concern I have is that Reset is on one of the inside pins (B4). I’m not sure if I can get the chip to a state where it will operate properly without unobstructed access to reset.

The routing I’ve chosen is probably the only possible option for reset. Pin A4, right above, is used for the single-wire debug (SWD) clock. I’m assuming that can’t be shared. B5 is Vdd, so that’s out. It might be possible to go down. C4 defaults to one of the crystal pins, and D4 defaults to a disabled state.

In the route I’ve chosen, B3 is an ADC input, so it should start out high-impedance, and therefore not interfere. A3 defaults disabled, so it won’t get in the way.

Next step: solder time!

One other thing – The images above show non-solder mask defined (NSMD) pads. Those are standard for BGAs 0.5mm pitch and higher. This part is 0.4mm pitch. Some manufacturers recommend solder mask defined pads (SMD) for 0.4mm and smaller. I’m actually testing several pad styles: SMD, NSMD and solder mask opening = copper.

Duane Benson

Run it up the flag pole and see who solders

http://blog.screamingcircuits.com

# Putting the ‘United’ in United States

In Ken Burns‘s excellent miniseries The Civil War, Shelby Foote states that the Civil War  made us a country by uniting North and South.  He argues that before the War, its citizens might say the United States are a good place to live, noting the feeling of separation. He points out that before the Civil War, most people had never traveled more than 30 miles from where they were born and therefore had only a theoretical notion of the US as a country. During the War, millions of men and women had walked its fields and hills and cast their eyes on her valleys. They came home with a solid feeling for what this great land is. So, today, everyone, North and South, would say the United States is a good place to live.

If the Civil War united us North and South, gold united us East and West. Shortly after the Louisiana Purchase  in 1803, President Thomas Jefferson commented that it would take 25 generations (a little more than 500 years) to settle the West. Most of us today would balk at this estimate, yet in 1803 it was very reasonable. Consider that by 1803, the US had been settled for 200 years and the vast majority of people lived with a few hundred miles of the Eastern seacoast. Yet by 1850 California had become a state and twenty years later the country was united East and West by the transcontinental railroad .  The sole driving force for these amazing events was gold. Gold was discovered in 1848 at Sutter’s Mill and within months the California gold rush  had begun.  By 1855 more than 300,000 people had come to California from all over the world.  It was the biggest gold find in the world up to that date, but, to put it in perspective, only 750 metric tons (MT) of gold were mined in 10 years of this Gold rush.  All of this gold would only be a cube only 3.4 meters (11.1 feet) on a side. This fascinating story is documented in The West, another documentary produced by the prolific Ken Burns.

Today over 2,000 MT of gold are mined each year, worldwide. Modern mechanized and automated mining techniques enable this tremendous increase. About 75% of all of the gold mined in the world has been mined in the last 100 years or so.

Today, about 50% of gold is used for jewelry, 40% for investments, and 10 % for industrial uses. Gold has one of the best surface electrical conductivities of any metal, making it a top choice for high-performance electrical contacts.  Its resistance to corrosion enables gold solders to be very robust in harsh environments.  Gold’s malleability and ductility also make it ideal for bonding wires  in semiconductor packages.  Gold is so ductile that a 0.5mm diameter ball can be pounded out into 0.5 square meters of gold leaf (see the image).  In electronics assembly, gold is used in Electroless Nickel Gold (ENIG) surface finishes for PWB pads and some corrosion resistant, mechanically-strong solders. Some gold solders have tensile strengths seven times greater than SAC305. With their 280C liquidus temperature, these robust solders can also be used in high temperature applications.

But remember, without the California Gold Rush of 1849, and the Alaskan Gold Rush of 1897, the United States would be a dramatically different country today indeed.

Cheers,

Dr. Ron

# Musings on Metals: Copper

It could be argued that civilization began with the smelting of copper.  Although thousands of years before, humans fired clay to make figurines and containers, smelting required several non-obvious steps.  After all, the firing of clay, at some level, can be accomplished by simply dropping clay into a fire.

To smelt copper, our ancestors had to:

1. Take malachite (see photo) or another copper ore, grind it up or break it into small pieces
2. Mix the ground malachite with carbon
3. Heat the mixture in a vessel to 1,085oC.

Malachite Ore

Achieving this temperature with a wood fire is, to me, astounding.  Think about those days when you are grilling some burgers.  You leave the grill on after the burgers are done, to burn off the grease.  You come back 20 minutes later and the grill is at 500oF.  You can feel the heat.  Even touching the knob to turn the gas off is intimidating, as the heat drives you back.  This temperature, 500oF, is only 260oC!  The ancients reaching 1,085oC with wood and bellows is, indeed, impressive. By the way, a good rule of thumb to convert degrees C to degrees F from 100oC to 1,5000C is that 2XC=F, this fast approximation is accurate to about 10% in this range.

The confluence of the three procedures is not only non-intuitive, but think how many times the smelter of old could only reach 900oC and failed.  I have argued that if copper melted at 1,200oC or so, civilization would have never gotten started.  This temperature is perhaps a little too high to reach with a wood fire.  The smelting of copper encouraged investigations into other metals, eventually resulting in the discovery of the processing of iron, an even less intuitive process than smelting copper.  So, I believe that the success with copper was necessary to the production of steel.

Copper smelting became an industry that encouraged permanent settlements and stimulated trade, which encouraged writing and ciphering.  An effective copper smelter would likely keep secret some of his craft as he wanted a competitive advantage.  He could make more by smelting copper than doing anything else, so he almost certainly was an early specialist.

Considering all of this, I believe that without the discovery of copper smelting, we might still be living in huts or teepees, using stone tools, and living a nomadic existence without commerce, writing, or mathematics.  Examples to support this thesis are the state of native peoples in the Americas in the 1400s.  These native peoples had never learned to smelt metals and hence also lacked the follow-on aspects of civilization mentioned above.

Today, copper is a foundation material for electronics, given its excellent electrical conductivity, second only to silver.  Copper’s ductility likely aids in the formation of PWB traces and plated through-holes in that it resists cracking.

Additionally, copper’s ability to form an electrical and mechanical bond with solder is another trait that makes it a winner as an electrically-conductive assembly material in modern electronics.

Copper has been used for more than 10 millennia, but, as with most metals, 90 to 95% of it has been mined since 1900.  About 15 million metric tons (MT) are used each year, third to aluminum’s  22 million MT and steel’s unequaled 1 billion MT.

In the next installment, we will discuss tin and how it forms an intermetallic with copper during soldering.  Thus making solder paste, solder wire, and solder preforms critical components of electronics assembly.

Cheers,

Dr. Ron