Readers of this blog will remember that I have been a skeptic of self-driving cars emerging in the near term. I am even less sanguine today. A recent article supports my perspective. Humans just do so many things effortlessly that sensors and computers cannot duplicate.
As an example, suppose there are five people at a street corner. These individuals non-verbally communicate intent that other humans easily pick-up on. If they are talking to each other and not facing the road, a human rightly concludes they are not planning on crossing. If they are facing the road and looking at the traffic, a human expects they plan to cross. This intuition is well beyond any AI’s ability to interpret and will be for decades to come.
Figure 1. A human recognizes that these students aren’t planning on crossing the street.
Autonomous vehicles are typically over designed to not cause accidents. Therefore, in some cases, if a pedestrian sticks their hand out into a road to wave at a self-driving car, it will stop. Whereas a human would recognize that the person is just goofing-off or being friendly.
All of this new information makes Elon Musk’s claim that Tesla will have a car on the road in 2022 without a steering wheel hard to accept.
To be fair, self-driving cars in controlled conditions, such as low traffic, well-marked routes, in good weather, will become more common in the decade ahead. However, an autonomous vehicle that can pick me up from my poorly marked 200 foot driveway, off an unmarked country road in Vermont, and then drive me to terminal C at Boston’s Logan airport is many decades away.
So, if you know someone who wants to be a truck driver, I feel that that will continue to be a fruitful career for a long time. In addition, those of us who manufacture electronics can take comfort in the fact that autonomous vehicles will need much more electronics than originally thought.
To the SMT process engineer, the second most important thixotropic material in their lives is solder paste. If solder paste was not thixotropic, it would be difficult to print and would likely slump after printing the paste. What is a thixotropic material? It is a material that has a low viscosity when it is shear stressed and a high viscosity when it is not shear stressed. So, when the solder paste is forced through the stencil aperture by a squeegee, its viscosity plummets and allows it to fill the aperture. See Figure 1.
Figure 1. The viscosity of solder paste dramatically decreases as it is forced through the stencil aperatures.
When the stencil is removed, the resulting solder paste deposit experiences no shear stress so the deposit maintains the shape of a “brick.” See Figure 2. So thixotropy is a very helpful property of solder pastes.
Figure 2. After printing, the solder paste viscosity is high, enabling the depost to maintain the brick shape. Figure courtesy of Ron Lasky, Jim Hall, and Phil Zarrow.
If solder paste was dilatant, it would be a disaster. These materials are the opposite of thixotropic materials. They have a low viscosity when not shear stressed and a high viscosity when shear stressed. So they could not be forced through the stencil aperture and, if they could, they would flow all over the board. Cornstarch and water is an example of a dilatant material.
Oh, yes, what is the most important thixotropic material to the SMT process engineer? Their blood. When getting up from lying down, our heart automatically makes a strong “pump” to rush the flow of blood to our head. Since blood is thixotropic, it shear thins and makes it easier for our heart to get the needed blood up to our head. If blood was not thixotropic, we might faint every time we rise from reclining!
Let’s assume your
company has decided that transfer efficiency (TE) is the key metric in determining
solder paste quality. Transfer efficiency is the ratio of the volume of the
solder paste deposit divided by the volume of the stencil aperture. While you
agree that TE is an important metric, you are a little troubled with the recent
results in a solder paste evaluation. Two out of 10 pastes are fighting for the
top spot and it looks like TE will be the deciding metric. Paste A had a TE of
99.5% and Paste B had a TE of 99%. So management wants to go with paste A. You
are troubled because paste A has a poor response-to-pause. If it is left on the
stencil for 15 minutes or more the first print must be discarded. This weakness
may result in 30 minutes or so of lost production time in a 3-shift operation.
However, the TE test results showed that the TE of paste A was statistically significantly better than paste B. You think about this situation and something doesn’t make sense — 5% and 99% are quite close.
You dust off your statistics textbook and review hypothesis testing. Then it hits you, with very large sample sizes, means that are closer and closer together can be statistically significantly different.
The data show that paste A has a mean of 99.5% and a standard deviation of 10%, whereas paste B has a mean of 99% and also a standard deviation of 10%. The sample sizes were 10,000 samples each. These large sample sizes are important in the analysis. The standard error of the mean (SEM) is used to compare means in a hypothesis test. SEM is defined as the standard deviation (s) divided by the square root of the sample size (n):
So as the sample
size increases, the SEM becomes smaller or in statistics lingo “tighter.” With
very large sample sizes, this tightness enables the ability to distinguish
statistically between means that are closer and closer together. This situation
was not a concern with sample sizes of less than 100, however with the modern
solder paste volume scanning systems of today, sample sizes greater than 1000
are common.
Figure 1 shows the
expected sampling distribution of the mean for samples with a TE of 99.5% and
99.0% and a sample size of 100, both have a standard deviation of 10%. Note
that to your eye you do not see much difference. However, with the means and
standard deviations the same and sample sizes of 10,000 the sampling
distributions of the mean are clearly different in Figure 2.
The reality
though, is that there is no difference in the results in Figure 1 and 2. The
tiny difference in the means (0.5%) may be statistically significant with a
sample size of 10,000, but is it practically significant? Would this small
difference really matter in a production environment? Almost certainly not.
Figure 1. Sampling distribution of the mean for a sample size of 100. Figure 2 Sampling distribution of the mean for a sample size of 10,000.
So, with large
sample sizes, we need to ask ourselves if the difference is practical. For TE,
I think we can be confident that a difference of 0.5% is not practically
significant. But, what if the difference was 2% or 5%? Clearly,
experiments should be performed to determine at what level a difference is
significant.
With the case
discussed above, I would much prefer the paste that has a 99.0% TE and a good
response-to-pause.
Dr.
Ron, when if comes to SMT printing of solder paste, why do some people use the
five-ball rule for rectangular apertures and the eight-ball rule for circular
apertures?
Michel:
The
“Five Ball Rule” is another metric that SMT assembly industry leaders believe,
but it is difficult to find its origin. It states that when selecting a solder
paste, five of the largest solder balls should be able to fit across the width
of the smallest rectangular stencil aperture. See Figure 1a for a 0.2mm wide
rectangular aperture.
Typically,
the largest solder ball diameter is assumed at the 90th
percentile. See Figure 2. So, in this example, a type 4 solder paste would fit
the five ball rule as the largest solder ball is 0.038mm. Five times 0.038 is 0.190mm,
just a little less than the aperture width of 0.2mm. It should be remembered
that this is a “rule.” not a “law.” So let’s say you had
4.5 balls across the aperture with instead of 5, it would most likely be
OK.
Figure 1.A
comparison of the Five and Eight Ball Rules
Figure 2.Solder
Powder Sizes
A
generation ago, the advent of circular apertures to support BGA and CSP
packages necessitated a new “rule.” Figure 1b shows why the five-ball rule is
inadequate for circular apertures. Although five type 3 solder balls fit along
the 0.275 diameter, off the diameter, there is not enough room for many
solder balls. Hence, an insufficient amount of solder paste would be
printed.
For
the same aperture, if a type 4 paste is used, 7 or 8 solder balls span the
diameter and the amount of paste printed would be much closer to the volume of
the aperture.
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:
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:
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 [email protected].
Patty had just finished an
all day workshop on “Common Defects in SMT Assembly and How to Minimize
Them.” The workshop seemed to go really well, and many of the 35 or so
attendees thanked her for a great learning experience.
After most of the people
filed out of the room, two approached her as she was disconnecting and packing
her laptop.
“Dr. Coleman, that was a great workshop. But, I do have one question. You used a term all day that I wasn’t familiar with, ‘SAC’,” a 35-year-old process engineer commented to her.
While saying this, he
presented his business card that referred to him as a “Senior Process
Engineer.”
Patty was trying to
recover from this shock, when the second similar looking fellow asked,
“And what are ‘OSP’ and ‘eutectic’.”
After explaining these
three terms and exchanging a few pleasantries, the two senior process engineers
walked out of the room and bade Patty farewell. As the room became empty, Patty
settled into a chair.
“How can this be?” she
thought. She was stunned that people with enough experience to be called
“senior process engineers” would not know these terms.
Today 6 AM …
Patty was jogging back to her house in Woodstock, VT, when she spied a beautiful red fox. Neighbors had reported seeing the fox numerous times. People believed that the fox was nesting. In addition, a black bear had been sighted by everyone in her family over the past few weeks. Add all of this to the family of deer and the rafter of turkeys in her neighborhood and it was quite an experience for Patty, Rob, and their sons.
The fox, however, created
a new problem. Patty and Rob had bought their twin sons a Yorkshire puppy,
Ellie, about a year ago. At 6 pounds she could be dinner for the fox, so,
unfortunately, they could no longer let Ellie out by herself.
Figure 1. Ellie the Yorkie after a big day. Sadly she has to be
watched when she goes outside of Patty’s house, due to the local predators.
By 7:30AM Patty was
in her office. She was giving a workshop in two weeks at a local chapter
meeting in Boston and decided to create a pre-test to give to the attendees so
that she could assess their current knowledge. Patty planned on having the
students grade each other’s exams and on working the exam in as a leaning
experience at the start of the workshop. By assessing the results of the
pre-test, she wanted to make sure she didn’t use acronyms they don’t
understand, and to also explain topics that the students might not be familiar
with. As she was working on the questions for the pre-test, Pete walked in.
“Hey, Professor C, how
goes it?” Pete asked.
“I’m preparing a pre-test
for the workshop I’m giving in a few weeks,” Patty replied nonchalantly.
“I remember you talking
about doing it a month or so ago. Seems like a good idea to me,” Pete
responded.
“I’m ,glad you approve,”
Patty said wryly. “I just finished it. Do you want to take a look at it?”
she continued.
Patty printed out a few
copies and handed one to Pete. They both looked at it for a few minutes, in
silence.
Finally, Pete commented
sheepishly, “Aaa, Patty your joking, right?”
“Why do you say that?”
Patty asked, a little annoyed.
“It’s just too easy.
Everyone will get 100% and you won’t get any information,” Pete opined.
Patty then reminded Pete
of her experience 6 months ago.
“OK. Maybe you have a
point. But, I still think it’s too easy,” Pete concluded.
“I’ll tell you what. How
about a bet? If the average pre-test grade is above 70%, Rob and I will take
you and your new crush, Mary, out to Simon Pearce. If
it is 70% or less, you treat us,” Patty teased.
“It’s a bet,” replied Pete
quickly.
The Pretest:
What does the letter “S” in SAC stand for?
How much silver is in SAC305?
What is the approximate melting point for SAC305 solder (+/- 4oC)?
Solder paste is approximately how much (by weight) metal (+/- 5%)?
What is not a current common defect in SMT?
Head-in-pillow
Pad cratering
BGA Ball Matting
Graping
Which is a closest to typical stencil thickness?
5 microns
20 mils
5 mils
20 microns
Which is closest to a typical lead spacing for a plastic quad flat
pack (PQFP)?
0.1mm
0.1mil
0.4mm
0.4mils
Which has finer solder particles, a Type 3 or 4 solder paste?
What does OSP stand for?
Place an arrow at the eutectic point of the tin-lead phase diagram
below.
Epilogue (two days after the workshop)
Patty arrived at Ivy U and
couldn’t wait to see Pete. She went to his office but he wasn’t there. Finally,
she found him in the machine shop helping four students with a project that
required some additive manufacturing.
“Hey, Pete! When are you
and Mary going to treat us to our dinner?” Patty teased.
“Don’t tell me the average
was less than 70%,” Pete grumbled.
“Forty-three point zero
eight to be exact,” Patty punctuated.
Figure 2. The Pretest
Scores
“Yikes!” Pete exclaimed,
rubbing the back of his neck. “I guess you were right.”
“It really helped me to
take things slowly and explain all the terms. I think I helped the students
much more than usual,” Patty explained.
“Rob and I both agreed, we
are ordering the most expensive meal that Simon Pearce has,” Patty joked.
At that Pete let out
a deep groan.
Dr. Ron note: All of the events in this post are true. How would you do on the pretest?
Recent
articles have added to the confusion regarding when fully autonomous vehicles
will become common. One
suggests that they around the corner with this quote:
“Alphabet plans to launch a self-driving service later this year,
while GM Cruise has targeted the introduction of a similar service in 2019.
Ford has that it expects to put self-driving vehicles into commercial service
by 2021.”
So
it sounds like autonomous vehicles will be here this year or next. But wait,
here is a counter
article. This article points out the many issues to be resolved before
fully self-driving cars are launched. Consider this one quote from the article:
“There’s still a lot to be worked out. There are scenarios where
the car will have to break the law in order to proceed. One common scenario is,
you’re driving down a two-lane highway—one lane each way—and there’s a Fed Ex
truck in front of you, parked on the curb. You can’t go around it without
crossing the double-yellow line. Are you going to allow the car to break the
law? Now, you’re getting into a whole different set of rules, regulations, and
even morality decisions.”
These
two perspectives were brought home to me recently when I was on a review board
for a student projects course, Technology
Assessment, taught by friend and colleague, Eric Bish.
One of the projects was to assess the viability of bringing fully autonomous
vehicles to market by 2021. Reviewing this project helped to clarify the
dichotomy between the two perspectives discussed above.
It
ends up that the efforts of Alphabet, Ford, and GM are to be launched in very
controlled environments. They will only be used in well mapped out routes, with
good lane markers, no construction, on days with good weather etc. Note also
that the first quote refers to a self driving service, not private autos.
Having
an autonomous vehicle that can completely replace a human is still (many?)
decades away. There are just too many issues such as the FedEx scenario
envisioned above that need to be resolved. I believe that over time, more and
more such issues will be discovered and push the date of such vehicles even
farther in the future.
Even
if, on the whole, early autonomous vehicles are safer, accidents like the one
in Phoenix
earlier this year, will put a spotlight on autonomous vehicles that will
further delay their full advent.
What
does all of this portend for the electronics industry? I think these issues
will require more electronics and sensors than many believe, so in a sense it
is good news for the electronics industry.
For the next few weeks I plan to repost some of the first Patty and the Professor episodes. As I visited several facilities, some of them in other industries, I found that uptime is as vital a topic as ever. Although these facilities were tracking a few metrics, uptime was not one of them. I estimated they were little better than ACME in the following vignette. Let’s all be committed to measuring and improving our processes uptimes. Now on to Patty and the Professor.
Two weeks passed quickly and The Professor returned to ACME. Patty met him at the door. “Professor, it’s great to see you,” Patty said with enthusiasm. “We collected the uptime data in real time on a laptop, no one has seen that results yet. We wanted it to be a surprise,” said Patty. The Professor suggested that he go out on the shop floor to observe the manufacturing activities until shortly after lunch. He pointed out that his observations may help to understand the uptime results.
The morning seemed to drag for Patty, she was very anxious to see the resets of the uptime data. She bet Pete a dinner for two that the uptime would not be more than 50%. If she wins, Pete and his wife will treat her and her boyfriend Jason to dinner at the restaurant of her choice.
Around 1:30 p.m. The Professor suggested that he was ready for the meeting. Patty had written a simple Excel macro to perform the calculations for the uptime. She only had to push a button and he whole room would see the result in a moment, as Patty connected her laptop to a projector. There was tension in the air, friendly wagers had been made, but the entire process team realized that their reputation was on the line.
When the number emerged on the screen, John, the manager’s face became ashen. Pete’s visage was redder than two weeks ago. John thought, “I should be fired. How could I manage this team for five years and not know that our uptime was only 9.7%.” Patty was thinking about her choice of restaurants.
“How can we be so bad?” John asked The Professor. The Professor responded, “The good news is that there are tremendous opportunities for improvement. After observing the operations out on the floor this morning, I think we can get the uptime to greater than 40%.” Pete shot back, “You’re kidding, only 40%?”
“I’ve only seen two operations that have greater than 45% uptime, and I’ve been to over 150 facilities worldwide,” answered The Professor.
“Where do we start?,” asked John.
“How about lunch?” beamed The Professor.
“We just had lunch!” Pete groaned.
“No, no Pete,” The Professor chuckled, “I mean how lunch is handled out on the line. Lunch costs the company more than 1½ hours of production in an eight hour shift. That’s nearly 20% of the entire shift.”
Now John was a little agitated. “Professor, lunch is only 30 minutes. We purposely have a short lunch period to avoid the line being down for a long time,” John said with a note of annoyance.
“John, this is true, but I watched what the operators did. Lunch is supposed to start at 12 noon, but the operators turn the line off at 11:40 a.m. They don’t get back to the line until 12:40 p.m. and it takes them more than 30 minutes to get the line running again. Today, the line was not running until 1:15 p.m. It was down for 1 hour and 35 minutes,” stated The Professor.
John thought again, “Yes, I should really be fired.”
Will John keep his job? What restaurant will Patty choose for dinner? What should be done about lunch? Where are all of the other hours lost? Stay tuned for the answers to these and other questions.
I teach a course at Dartmouth on manufacturing processes: ENGM 185. In this course, I use many of the chapters from “The Adventures of Patty and the Professor.” This book started as a series of posts on this blog and the posts ended up being gathered into the book. It’s hard to believe that the first post was nearly 10 years ago.
I think most students that have read “The Adventures of Patty and the Professor” have a sense that the vignettes in the book are exaggerated, even though I point out that I have attempted to make them as close to real events as possible. Recently, one of my grad students, Amritansh (Amro) Varshney, had a chance to see some of the real world of manufacturing. After Amro returned to Dartmouth, we chatted and he shared that not only do the stories convey the sense of how poor some manufacturing operations are run, but, in some cases, the realities are worse!
In light of this epiphany, I decided to repost some of the original episodes from the book for a new generation of readers. As you share Patty and the Professor’s experiences, remember they are strongly based on real events. I hope you enjoy the “Adventures!”
Business was good at ACME. Even in these challenging times, the company’s three assembly lines could not keep up with demand. John, the manager of the assembly lines, decided to request the funds for an additional assembly line. A member of his team, Patty, suggested he might want to consult “The Professor,*” before getting a new line. The Professor taught a course on line balancing that Patty took at the SMTAI conference last summer. Line balancing is an important part of optimizing productivity in electronics assembly. A balanced line ensures that the component placement process, usually the “constraint,” is the fastest possible by assuring that each placement machine spends the same amount of time placing components. If any machine is waiting for the others, assembly time is being wasted. In a sense, line balancing is an application of Goldratt’sTheory of Constraints. John remembered that when Patty applied what she learned from The Professor, throughput increased 25%. Unfortunately, Patty did not attend The Professor’s other class on “Increasing Line Uptime.”
John decided to have a chat with Patty about The Professor. “Patty, why do you think I should consult with The Professor, about getting a new line?”
“Well John, perhaps with some effort to improve our uptime, we wouldn’t have to buy another line,” said Patty.
“Patty, that’s a good point,” said John.
Patty contacted The Professor and he agreed to fit ACME into his busy schedule. Upon his arrival, The Professor was given a tour. As part of the tour he was shown the process that ACME used to minimize changeover time between jobs. The Professor appeared to be impressed. After the tour, The Professor asked if a brief meeting could be held with the engineers and managers to discuss the situation.
“What is the average line uptime?” The Professor asked the assemblage. There was some hemming and hawing, finally Pete, the senior process engineer replied, “I’d say at least 95%, we work our fannies off out there.” There was a murmur of agreement from the 9 or 10 people in the room. Finally John spoke up, “Professor, what is your definition of uptime?” The Professor responded, “Simply the percent of time an assembly line is running.” Pete again responded that 95% was the right number.
The Professor asked for some production metrics and performed some calculations on his laptop. In a few moments he commented, “From the data you gave me, I estimate that your average line uptime is about 10%.” Upon hearing this, Pete became red in the face, especially after Patty whispered in his ear, “I told you so.” The noise in the room became so loud that John was concerned he might have a riot on his hands. The Professor asked to speak and John, in a booming voice, asked for calm.
“Let’s not become angry, perhaps my calculations are off. Why don’t we measure the uptime for a few weeks to be certain.”
“How do we do that?” asked Pete, his face still crimson.
“Each day one process engineer will go out to the lines every 30 minutes. If the line is running, he will put a 1 in an Excel® spreadsheet cell, if the line is not running a 0 will be entered,” responded the professor.” It was agreed that this will be done and The Professor will be back in two weeks.
Will Pete’s red face return to normal? Will the line uptime be 95%? Will Patty and Pete ever be on speaking terms again? Stay tuned on May 27 for the next episode.
Cheers,
Dr. Ron
* The Professor, as he is affectionately called by his many students, is a kindly older man who works at a famous university. Few know his real name. The Professor is an expert in process optimization.
Soldering copper to copper with a tin-based solder, such as tin-lead eutectic solder or a common lead-free solder like SAC 305, requires only the liquid solder and copper to form the tin-copper intermetallic bond. This simplicity, with one small catch, was brought home to me by some colleagues at Speedline Technologies. They took a PWB with through-hole components mounted and ran it through a wave-soldering machine without using any flux. The result was comical. The PWB weighed about 10 pounds as it had huge solder ice cycles hanging off of it. Oxides that form on the copper created this mess. Running the board though again with a nitrogen blanket produced a beautifully wave-soldered board that could be ready to ship. So in reality, either a flux or nitrogen, preferably both are needed for successful wave soldering in addition to the solder and copper; however, it is still relatively simple.
Have sympathy for the solder scientists of the late 1970s and early 1980s. SMT was an emerging technology and the world wanted to buy solder paste; however, the only experience many solder scientists had was with wave soldering. In wave soldering, the flux’s main job is to remove the oxides from the PWB pads and components. The solder is in a molten state and its oxidation is not a main concern. In the soldering process, the solder only touches the board for a few seconds and the board only experiences the high temperatures during this brief period.
I imagine some early solder pastes consisted of solder powder with fluxes similar to those used in wave soldering. If so, they probably didn’t work too well. Consider the dramatic differences that solder paste experiences as compared to solder in wave soldering. The “flux” in solder paste has to remove oxides from the PWB pads, component leads, and solder particles, but it also has to protect all of these surfaces from re-oxidation for several minutes while in the reflow oven. To achieve this protection, the “flux” has to contain materials that act as an oxygen barrier. The most common materials used in no-clean solder pastes are rosins/resins. Rosins, or resins which are modified or synthetic rosins, are generally medium to high molecular weight organic compounds of 80-90% abietic acid. They are typically found in coniferous trees. Rosins/resins are tacky in nature, and provide some fluxing activity and oxidation resistance during the reflow process.
The reason I wrote “flux” in the above paragraph is that what most people call the flux in solder paste is a complex combination of materials. These “fluxes” will consist of:
Rosins/resins: for oxygen barrier and some fluxing activity
Rheological additives: to give the best printing properties, e.g., good response-to-pause, good transfer efficiency, excellent slump resistance, good tack, etc
Solvents: to dissolve the other materials
Activators: to perform the main fluxing action (removing oxides)
Figure. Solder pastes are one of the most highly engineered materials.
Modern solder pastes must have good oxygen barrier capability. In most reflow profiles, the solder paste is at temperatures above 150ºC for more than several minutes. During this time, an oxygen barrier is needed to protect both the solder particles and the surfaces of the pads and leads.
A common example of an insufficient solder barrier is the graping defect or its relative, the head-in-pillow defect. If you are experiencing one of these defects, a solder paste with better oxygen barrier properties is bound to help.
Before reflow, the solder paste must print well, possess good response-to-pause, not shear thin, resist cold slump, and have good “tack” to support the components after placement. During reflow, in addition to the oxygen barrier challenge, the solder paste must not exhibit hot slump, should “Avoid the Void,” not create the “head-in-pillow” (HIP) defect, work with all common PWP pad finishes, and produce reliable solder joints in thermal cycling, drop shock, and vibration environments. Whew! What a complex challenge.
As a result I would argue that solder paste is a candidate to be the most highly engineered material in the world… and it certainly is NOT a commodity.
In light of this epiphany, I decided to repost some of the original episodes from the book for a new generation of readers. As you share Patty and the Professor’s experiences, remember they are strongly based on real events. I hope you enjoy the “Adventures!”