SMT assembly is an optimization process. There is no single stencil printing process for all PWB designs. The stencil printing parameters of stencil design, squeegee speed, snap off speed, stencil wipe frequency, and solder paste for assembling all PWBs will not be the same; just as there is no single reflow oven profile for all PWBs. Fortunately, most solder paste specifications give good boundaries for all of these parameters, but typically some trial and error experiments will be needed when assembling a new PWB design that is not similar to past assemblies.
The need for optimization is most obvious when trying to minimize defects. As an example, minimizing graping is often facilitated by using a ramp to peak reflow profile. However, the ramp to peak profile may acerbate voiding. See Figure 1.
Figure 1. The ramp to peak reflow profile may minimize graping, but acerbate voiding.
Thankfully your SMT soldering materials and equipment suppliers deal with these optimization issues on a daily basis. So if you are ever stuck with some challenging SMT assembly process, contact these solder materials and equipment experts first.
Here are the answers to theSMT IQ Test of a short while ago.
What does the “A” in SAC305 stand for? ANSWER: SAC stands for tin (Sn), silver (Ag), and copper (Cu). The “305” indicates 3.0 percent by weight silver, 0.5% copper, and the balance (96.5%) tin.
The belt speed on a reflow oven is 2 cm/s. The PCB with spacing is 36 cm. What is the maximum time that the placement machines must finish placing the components on the PCB to keep up with the reflow oven? ANSWER: Time (s) = product length (cm)/belt speed (cm/s) = 36 cm/2 cm/s = 18 sec.
In mils, what is a typical stencil thickness? ANSWER: In range of 4 to 8 mils.
BTCs are one of the most common components today; a subset of BTCs is the QFN package.
What does BTC stand for? ANSWER: Bottom terminated component
What does QFN stand for? ANSWER: Quad Flat Pack No Leads.
What is the melting temperature of tin-lead eutectic solder? ANSWER: 183° C.
In mm, what is the finest lead spacing for a PQFP? ANSWER: Most common is 0.4 mm. A few have 0.3 mm, but these smaller spacings are hard to process.
Are solder pastes thixotropic or dilatant? ANSWER: Thixotropic; the viscosity of solder paste drops when it is sheared (i.e forced through a stencil). Dilatant materials stiffen when sheared.
In stencil printing, what is response to pause? ANSWER: When stencil printing is paused, the viscosity of the solder paste can increase; this situation would be considered a poor response to pause. Pastes that have stable viscosities during pausing are considered to have good response to pause.
For a circular stencil aperture for BGAs or CSPs, what is the minimum area ratio that is acceptable? ANSWER: Typically greater than 0.66, although some solder pastes can print well a little lower than this.
What are the approximate dimensions of a 0201 passive in mils? ANSWER: Approximately 20 by 10 mils.
Mary had worked at a small SMT “mom and pop” shop for 12 years. Business was always good and she moved up to CFO of the 60 person company. Revenue had been over $12 million for a few years with profits north of $1 million each year. She marveled how well Fred, the owner, managed the small firm. As CFO, she was well aware of the strong financial strength of the company.
Mary was stunned when 18 months ago, Fred said he wanted to retire in less than two years, and he wanted her to “buy him out.” Fred was fit and spunky, but 75 years old was now in the rear view mirror.
Mary was more than stunned by the price Fred wanted; it was way, way too low. She even “complained” about this. But, Fred considered her more as a daughter and insisted on the low price. However, one of the concerns they both had was that Fred was really also the chief engineer. They had many loyal workers, as Fred paid 50% over the local rate and provided great benefits, but no one could fill in for Fred in the technical aspects of running the shop.
Fred had been trying to coach Mary for the past 18 months so that she would understand the technical aspects of SMT assembly better. Mary was a fast learner, but with only 6 months left before Fred’s retirement, they both agreed they needed to hire a chief engineer.
So, Fred developed an SMT IQ Test for the candidates. If they could not get at least 80%, they would not be considered. Fred argued that if you were really good enough, you had to know 80% of these questions. Here they are:
What does the “A” in SAC305 stand for?
The belt speed on a reflow oven is 2 cm/s. The PCB with spacing is 36 cm. What is the maximum time that the placement machines must finish placing the components on the PCB to keep up with the reflow oven?
In mils, what is a typical stencil thickness?
BTCs are one of the most common components today. A subset of BTCs is the QFN package.
What does BTC stand for?
What does QFN stand for?
What is the melting temperature of tin-lead eutectic solder?
In mm, what is the finest lead spacing for a PQFP?
Are solder pastes thixotropic or dilatant?
In stencil printing, what is response to pause?
For a circular stencil aperture for BGAs or CSPs, what is the minimum area ratio that is acceptable?
What are the approximate dimensions of a 0201 passive in mils?
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.
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.
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!
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
“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
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
“I’m ,glad you approve,”
Patty said wryly. “I just finished it. Do you want to take a look at it?”
Patty printed out a few
copies and handed one to Pete. They both looked at it for a few minutes, in
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
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?
BGA Ball Matting
Which is a closest to typical stencil thickness?
Which is closest to a typical lead spacing for a plastic quad flat
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
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
“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?
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.
My good friends, Phil Zarrow and Jim Hall, in their audio series “Board Talk,” were recently asked about the “Five Ball Rule”. In the comments section for this session, one listener asked if this rule, created in the 1990s, was still valid. After all, the 1990s was the era of 0603 and 0402 passives; 01005 and even 008004 passives have arrived.
First, let’s consider what a “rule” is verses a “law.” As an example of a law, consider Newton’s Laws of Motion. At everyday speeds, these laws are shown to be accurate to within our capability to measure. As we will recall from Physics 101, these laws were superseded by Einstein’s Theory of Relativity, at speeds close to those of the speed of light. However, in our everyday world, Newton’s Laws are well … laws. They are, for practical purposes, exact.
What is a “rule” then? A rule is an expression that approximately fits some empirical data or the experience of experts. Moore’s Law is actually a rule, as it is not precise. The doubling of transistor density has varied from every 18 months to every two years. That’s why I call it a rule, a very useful rule indeed!
The “Five Ball Rule” is clearly a rule. It was likely developed a generation ago by some of the first SMT pioneers. It may be backed up by experiment, but I think it was likely more a consensus of SMT industry authorities from the 1980s and 1990s.
What is the “Five Ball Rule?” It states that the solder paste’s largest solder particle diameter should be such that at least five of these particle diameters would span the width of a rectangular stencil aperture (Figure 1).
Figure 1. The Five Ball Rule
When this rule was developed, stencil apertures were much coarser than today, and the finest solder powder was a Type 3, with Type 4 on the horizon. While it is true that stencil aperture widths are much finer today, solder pastes of Type 4.5, 5, and even 6 are now in use.
The particle sizes of different “Type” solder pastes are shown in Figure 2. Note that, for Type 4 powder, 80% by weight of the particle diameters are between 20 and 38 microns. 38 microns is considered the “largest particle.” So, from Figure 2, for Type 5 powder, the “largest particle” is 25 microns. For the sake of the Five Ball Rule, the “largest particle,” for each powder type are those shown in Figure 2.
Figure 2. Solder Powder Sizes.
So, is the Five Ball Rule still valid? It would be hard to argue that it is not. Hundreds of experiments have been performed using the Five Ball Rule, combined with the aperture ratio being >1.5 for rectangular apertures or the area ratio being > 0.66 for square or circular apertures, with successful results.
StencilCoach software now includes the newer (finer) solder powder sizes to 1) tell the user the fineness of solder paste powder for the Five Ball Rule, as well as 2) help with calculating aperture or area ratio. By the way, some have suggested that, for a square or circular aperture, an “Eight Ball Rule” is more appropriate. So, StencilCoach uses the Eight Ball Rule for such apertures.
In the past, it was usually pretty easy to find chips in both surface mount and through-hole packages. Somewhere in the past decade or so, component manufacturers stopped introducing through-hole versions of their newest chips as standard practice. In many cases, new components can only be found in tiny QFN (quad flatpack, no leads), or wafer-scale BGA (ball grid array) packages.
The maker community, never shying away from a good hack, found ways to work with many of these parts while still hand building. There are very few components used in the pro-design world that are still unusable by a creative DIY maker.
But what happens when a maker has a great design and wants to mass-produce it?
Sometimes the techniques that make things work when hand soldering, will completely break a machine assembly process. To cure that ailment, I’ve compiled five common traps to avoid when moving from hand to robotic assembly.
5. Consider moisture sensitivity. It may not seem logical, but plastic does absorb moisture. And, it doesn’t have to be dropped in the sink for it to happen. Just sitting around exposed to the air, plastic chips will absorb humidity. In a reflow oven, these parts can end up acting a bit like popcorn. The moisture turns to steam, and if it can’t outgas fast enough, may split the chips open. Often, the damage isn’t visible to the naked eye, but with show up as an unreliable product in the field.
When we DIY folks hand-build boards, we tend to open the component packages and then just let the parts lie around without giving thought to proper storage. If you are going to send your project off to be machine assembled, you can do two things with moisture sensitive parts.
First, order the parts when needed, not before, and keep the packages sealed. Alternately, you can send in parts that have been exposed to the air; if you inform your assembly house that the parts are moisture sensitive, and ask that they be baked prior to assembly. Prebaking will remove the moisture safely.
4. Don’t skimp on solder mask. Some board fabricators offer reduced prices if you order your boards without soldermask or silkscreen. That’s not a problem when you’re hand building — you can regulate the amount of solder by eyeball.
However, when a stencil is used to apply solder paste and the board is run through a reflow oven, the solder will spread back on the exposed copper traces. This may leave your parts without enough solder on the pins to create a reliable connection.
Solder mask may add a bit of cost up front, but will increase reliability and reduce cost in the long run. Creative choice of solder mask color can also add some personality to your boards.
3. Silkscreen is important too. Lack of sIlkscreen isn’t a reliability issue, but it can make accuracy of assembly more difficult to achieve. In a perfect word, the CAD files would tell the assembly machines exactly where each part is supposed to go and what angle and orientation it needs.
Unfortunately, we don’t live in a perfect world (who knew?). It’s far too common to have footprints with errors in them, or components with ambiguous marking, to depend on the CAD files alone. Clear silkscreen will help to ensure that any errors in the data are caught visually.
If you don’t want to clutter your PCB with reference designators and polarity markings, put the designators and any other important markings in the document layer in your layout software. Then, tell your assembly house to look on that layer for the information.
2. No need to fear surface mount. One of the easiest ways to ensure that a board can be hand-built is to stick with through-hole parts. But doing so puts many limits on a design, and rules out a lot of new technologies.
Little breakout boards — a small surface mount chip pre-mounted on a PCB, with hand-solderable headers — are available for a lot of new parts, but not all. That’s helpful, but they take up a lot of extra board real estate and cost more that the part alone.
If you’re hand building a prototype, or a small number of boards for your own use, go ahead use a breakout board. But, when it’s time to get a thousand built up to sell, re-layout your PC board to use the chip without the breakout board. Just don’t forget the bypass capacitors or any other required support components.
As a bonus, many breakout boards are open source, so you may be able study and use a proven schematic and layout for that part of your design.
1. No open vias in pads. QFNs and BGAs have pins/pads under the part, often completely inaccessible. That’s fine for a reflow oven, but what if you’re soldering it by hand?
A common hand-soldering practice is to put large vias in the pad. Fix the part onto the board with tape. Then, turn the board over and stick solder and a small tipped soldering iron through the via. By doing this, you can hand solder almost any leadless surface mount part.
You can probably guess that I’m going to tell you open vias in pads will not work with automated assembly. The solder will flow down the via and end up on the back side of the board. You may end up with shorts on the back side, and parts that fall off of the front side, or just don’t connect with all their pads.
If you use the open via hand solder technique, you’ll need to re-layout your board without any open vias in the pads before sending it for manufacture.
0. Go for it! It wasn’t that many years ago when the tools and services necessary to get an electronic product manufactured were so complex and expensive as to pretty much make it impossible for DIYers to turn a hobby project into a small business. Times have changed, and with those changes, the hardware startup is back — and within just about anyone’s reach.
Duane Benson Breaker, breaker, one nine, clear the line, we’ve got boards to build
Patty was just dropped off at O’Hare airport after finishing a 3 day workshop on Lean Six Sigma statistics, design of experiments, and statistical process control. Interestingly, the students were lawyers. In recent years more and more service-based organizations were adopting lean Six Sigma and it was a long time since Patty had taught such a workshop to engineers. She noted that although the lawyer’s math skills were a bit rusty, they were very good listeners and picked up the math behind lean Six Sigma topics very quickly.
After paying the cab driver, she entered the terminal and went to see an agent. She was early enough to get an early flight home, so she had called the people at the online ticket agency during the cab ride. They said the change fee would be over $300, she felt that was just too much to pay. She was delighted to see that it was only $75 at the terminal.
She looked at her paper boarding pass and saw that she had more than two hours, just enough time for a relaxed lunch at Wolfgang Puck while she read USA Today. Patty was the only person her age that she knew who enjoyed reading a paper newspaper, she guessed that she picked the habit up from her dad.
The two hours went by quickly and she was standing in line waiting to board the flight to Boston’s Logan Airport. She had now been at Ivy U for a few years and traveled much less than when she worked at ACME. She had forgotten how stressful and unpleasant traveling was. As she stood in line, the man in front of her put his smartphone on the scanner and the scanner could not read the QC code. He and the agent fumbled for a while before they got it to work. This was another place where, in her opinion, paper was still king.
Patty got on board and settled into her middle row seat. She groaned a little bit at how uncomfortable and cramped it was. Patty was reminded of what her dad used to say in situations like this; “I know it is a bit uncomfortable, but just think what the 49ers went through to get to California,” he would tease.
After takeoff, she turned on her laptop. She absolutely had to send some emails, so she signed on to the onboard WiFi. She got sticker shock when she saw that it cost $18.95! Even though Ivy U would pay for it, the high price galled her.
After she finished the emails, a wave of fatigue swept over her and she needed a break. She chuckled to herself when she thought of a recent event. She had taken two of her best teaching assistants (TAs) to lunch and the conversation somehow came to discussing people who hid Jews from the Nazi’s in World War II. Patty mentioned to her two young protégés about an excellent book and movie she read and saw as a teenager, The Hiding Place. The story is about Corrie Ten Boom and her family and how they hid, and hence saved, many Jews from the Nazis in Holland during WWII. Although the movie was made before she was born, it was shown at Patty’s church every few years, for the new sets of youngsters who came along. Patty mentioned to her two superstar TAs that the film was produced by Billy Graham’s organization.
“Who is Billy Graham?” they both asked in unison.
Patty struggled to keep her composure as she explained who he was. How could they not know this? She decided to examine the situation a bit further.
Patty started humming a few bars of Orbison’s most popular song.
“Oh, Pretty Woman,” the boys said in unison.
Patty thought to herself, “Each of these young lads are the best student in every class that they take and yet they don’t know these ‘celebrities’?”
The next day Patty arrived at her office early to meet with Rob and Pete to discuss how the presentations that they were making for Mike Madigan on voiding were coming. Patty had arrived so late the night before, that Rob was already asleep. She did not see him in the morning as it was her turn to get the boys ready for school and he was off early to get in his 90 minutes of exercising. So, they had no chance to discuss the progress of the presentation.
“Pete, your presentation of BGA voiding is terrific. How is my hubby doing on BTC voiding?” she chuckled as she looked at Rob.
“I feel like I’m going to get yelled at ’cause I didn’t do my homework,” Rob said sheepishly.
“Yikes! We only have a few days,” Patty responded. “And I have yet to do my part on using solder preforms to minimize voiding,” she went on.
“I’m only teasing. I have quite a bit of info,” Rob said.
“We have been out of the mainstream for a while and one thing is for sure, voiding is the number one issue among assemblers today. So many people are assembling QFNs and are struggling with voiding. Voiding with some solder pastes can be over 50% of the area,” Rob went on.
“Wow! With 50% voids, think of how poorly the heat is being transfer away for the BTCs,” she looked at Rob and chuckled. “Remember, ‘BTC’ not ‘QFN,’ Patty went on.
“Yes ma’am,” Rob jokingly replied.
“Can you imagine the effect on reliability and field issues with so little heat being removed? The ICs inside the BTCs must be frying” Pete added.
“Voiding at this level has got to be really costly,” Patty mused.
“One of the things that really helped me was that I found quite a few experiments on voiding,” Rob added.
“What were some of the key points?” Pete asked.
“Well, as you might expect, the solder paste is typically the most critical part of the process. Some pastes have voiding lower than 10% with others above 50%,” Rob replied.
“What about the process?” Patty asked.
“Well, the reflow profile can be very important, as is controlling the PWBs and components. But, with the best pastes, it has been found that you can control the voiding content even if you can’t change the reflow profile and the PWBs and components have some issues,” Rob responded.
“Look at the x-rays of poor and good voiding between two pastes,” Rob said.
“What a difference,” Patty and Pete said in unison.
“What about the stencil design and venting?” Pete asked.
“Chris said that stencil design for venting is not as critical as once thought, although a window pane design is usually used,” Rob replied.
Figure 1. The window pane design for the stencil is used to permit venting.
“So it sounds like starting with the best solder paste solves 90% of the problem and adjusting the process, say with the right reflow profile, helps refine the result,” Patty summed up.
With this Rob went off to put the finishing touches on his PowerPoint® slides for his part of the presentation, while Patty started working on her part of the presentation on using solder preforms to reduce voiding.
Two weeks later.
Patty’s mom and dad came for a visit on a Sunday. Her mom had graciously offered to bring a complete Sunday dinner. Patty, Rob and the boys were grateful for the delicious meal. As they began to eat, Patty shared the story of her best students not knowing Billy Graham, et al.
“But, what was even more surprising was that I ended up asking 10 or 20 more students and only one had ever heard of any of these four ‘famous’ people,” Patty sighed.
“It’s your age,” Patty’s mom replied.
Thirty years old was not that far in the rear view mirror for Patty and she really didn’t consider herself old.
“These youngsters were born in the late 1990s, a generation after these people were prominent,” her mom went on.
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.