Minimizing Graping

Folks,

This post is an excerpt on graping from Indium Corporation’s The Printed Circuits Assemblers Guide to Solder Defects.

Introduction

The growth of personal electronic devices continues to drive the need for ever-smaller active and passive electrical components. This miniaturization trend, together with the demands for RoHS-compliant Pb-free assembly, has created more challenges, including the graping effect.

As a solder paste deposit decreases in size, the relative surface area of exposed solder particles increases, and the amount of available flux to remove surface oxides decreases. Compounding this is the additional heat necessary to reflow most Pb-free solders, resulting in a formula conducive to producing the graping phenomenon. During the heating process, as the flux viscosity decreases and begins to spread downward and outward, the solder particles are exposed at the top of the solder paste deposit. If there is no flux in proximity, these solder particles may become oxidized when the solder paste enters the ramp or soak stage of reflow. These oxides will inhibit the full coalescence of the particles into a uniform solder joint when the solder is liquidus. The unreflowed particles often exhibit the appearance of a cluster of grapes, as can be seen in Figure 1.

Figure 1. The graping effect.

Stencil Printing

The area ratio (AR) is a critical metric in successful stencil printing. It is defined as the area of the stencil aperture opening divided by the area of the aperture sidewalls. Figure 2 shows a schematic for square/rectangular and circular apertures. A simple calculation shows that the AR is simplified to the diameter (D) of the circle divided by four times the stencil thickness (t) or AR=D/4t. Somewhat surprisingly, the result is the same for square apertures, with D now equal to the sides of the square. For the AR of a rectangular aperture, the formula is a little more complicated: ab/2(a+b)t, where a and b are the sides of the rectangle.

Figure 2. Aperture schematics for rectangular and circular apertures.

It is widely accepted in the industry that in order to get good stencil printing, the AR must be greater than 0.66. Experience has shown that if the AR <0.66, the transfer efficiency could be low and erratic, although this has gotten better with advances in solder paste technology.

Transfer Efficiency

Transfer efficiency, another important stencil printing metric, is defined as the volume of the solder paste deposit divided by the volume of the aperture. To accommodate fine-feature stencil printing, it is not uncommon to look at solder paste that incorporates finer powder in order to optimize the printing process. However, as the size of the powder particles within the solder paste decreases, the relative amount of surface area exposed increases. With this increase in surface area, an increase in total surface oxides is also introduced. This increase in surface oxides requires the flux chemicals to work even harder at removing the oxides and protecting the surfaces of the powder, component, and board metallizations during the entire reflow process.

On a 3mil-thick stencil, the AR for a 6mil square aperture is the same as the AR for a 6mil circular aperture: 0.50. However, when comparing the two, the volume of the square solder paste deposit is greater (~108 cubic mil) than the circular deposit (85 cubic mil). The additional solder paste volume provided by the square aperture may help reduce graping. Of greater importance, though, is the increased transfer efficiency provided by the square aperture. The square aperture design provides more consistent transfer efficiency, further reducing the potential for graping as inconsistent deposits could mean less volume.

Results from solder masking experiments have shown that the graping effect is less prevalent for the solder mask defined (SMD) pads. It is believed that the solder mask provides a barrier (dam), restricting the spread of the flux during the heating process, and increases the potential availability of the flux to remove oxides and protect from further oxidation. The solder mask can also act as a barrier to protect the solder paste powder particles in close proximity from further oxidation.

Water-Soluble vs. No-Clean

No-clean flux chemistries are generally rosin/resin-based (hereafter referred to only as resin) formulas. Because resins are not very soluble in the solvents used in water-soluble flux chemistries, they are typically replaced with large molecular compounds, such as polymers, in water-soluble fluxes. The activator(s) within the flux chemistry removes the current oxides on the joining surfaces, as well as the solder paste powder particles within the solder paste itself. Further oxidation/re-oxidation does occur during the heating stage. Whereas the resins in no-clean fluxes are excellent oxidation barriers and protect against re-oxidation, the lack of resins in water-soluble chemistries cause them to fall short in terms of providing oxidation resistance.

Hence, for the same reflow profiles—though water-soluble chemistries are generally more active—the lower oxidation resistance of water-soluble chemistries makes them more sensitive in long and/or hot profiles, increasing the potential for graping defects.

Ramp-To-Peak vs. Soak

For many years, the “soak type” reflow profile was quite prevalent. Over time, however, focus has shifted to ramp-to-peak (RTP) as the preferred reflow profile. Contributing to this shift is the higher reflow process temperatures associated with Pb-free solders, as well as the need to diminish the total heat exposure of the smaller paste deposits and temperature-sensitive components and board laminate. Another benefit of the soak profile is its utilization to reduce voiding. However, it is not as effective with Pb-free solders, due to the increased surface tension of Pb-free solders and the higher temperature used to reflow them.

To minimize graping, a reduced oven time is better, provided you use the same time-above-liquidus (TAL) and peak temperature, see Figure 3. The soak profile typically produces more graping than an RTP profile. The graping effect is exacerbated as the total time in the oven increases. Decreasing the total heat dramatically decreases the graping effect. A ramp rate (from ambient to peak) of 1°C/second is commonly recommended, which equates to approximately 3 minutes, 40 seconds to a peak temperature of 245°C.

Figure 3. Typical reflow Pb-free profiles.

Conclusions

To reduce the graping effect, it is vital to ensure an optimal printing and reflow process. Using the guidelines provided for the area ratio and good process/equipment setup will ensure good transfer efficiency. Though the area ratio for circular and square aperture designs may be equal, the potential for graping increases with circular aperture designs due to decreased paste volume and decreased transfer efficiency.

From a reflow standpoint, decreasing the total heat input will decrease the likelihood of the effect. Using an RTP-type profile with a ramp rate of ~1°C/second is suggested.

Material factors also influence the outcome. The observance of graping increases as the solder paste particle size decreases and the area of surface oxides increases. Water-soluble solder paste chemistries do not provide the oxidation barrier that resins do for no-clean chemistries and are more prone to the graping effect.

Cheers,

Dr. Ron

It is hard to believe that in July we will celebrate the 9th anniversary of the advent of RoHS. So the timing seemed right when I was recently asked to speak at the Boston SMTA Chapter on The Status of Lead-Free 2015: A Perspective.

An overview of the entire 75-minute presentation would be a bit long, so I am going to discuss three of the “questions” that I covered.

1. Q: We are now almost nine years into RoHS’s ban on lead in solder. How has lead-free assembly worked out?

1. Q: In light of sourcing difficulties, is there an industry consensus regarding lead-free conversion for military, medical, aerospace etc. assemblers that will continue to be exempt?

A: The main issue is getting components with tin-lead leads, especially BGA balls. Many assemblers are reballing BGAs, which has become a mature technology, although with an added cost. As years go by and there becomes more confidence in medium to long term lead-free reliability, some exemptees may switch to lead-free. However, I think mission critical applications with 40-year reliability requirements must be extremely cautious to make the switch. There may be subtle reliability issues that may show up in 40 years, that are not found in accelerated testing. One concern is aging. Even at room temperature, solders are at over 50% of their melting temperature on the absolute scale (300K/573K = 0.52). So aging can occur at room temperature. Some research suggests that lead-free alloys may be more affected by aging than tin-lead alloys.

1. Q: It has been said that you claim that lead-free assembly has some advantages. Can this be true?

A: Guilty as charged. Lead-free solder does not flow and spread as well as tin-lead solder. This property can result in poor hole fill in wave soldering and some other assembly challenges. However, this poor wetting and spreading means that pads can be spaced closer on a PWB without the concern of shorting as seen in the image below. Your mobile phone would likely be bigger if assembled with tin-lead solder.

Cheers,

Dr. Ron

Photo courtesy of Vahid Goudarzi.

The Importance of Oxygen Barrier in Solder Pastes

Folks,

Pity 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 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 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 in the reflow oven. To achieve this protection, the “flux” has to contain materials that act as oxygen barriers. The most common oxygen barrier 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, they provide some fluxing activity, and provide the critical oxidation resistance during the reflow process.

The reason I wrote “flux” in quotation marks in the above paragraph is that what most people call the flux in solder paste is actually a complex combination of materials. These “fluxes” 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).

Because of these complexities, and the material’s multi-functionality, they are sometimes referred to as, “flux-vehicles.”

Modern solder pastes must have good oxygen barrier capability. In most reflow profiles, the solder paste is at temperatures above 150°C for several minutes. During this time an oxygen barrier is needed to protect both the solder particles and the surfaces of the pads and leads.

The graping defect. A common example of cases where the solder barrier was insufficient is seen in 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.

Cheers,

Dr. Ron

On Stats and Solders

Folks,

Everyday, we are exposed to the results of surveys and polls. A typical example might be that President Obama is leading Mitt Romney in a poll by 48% to 45%, but the results are not statistically significant. A reasonable question might be, “What does it mean to be statistically significant?”

To determine statistical significance, typically, the statistician will use the criteria that if there is only a 5% or less chance that the conclusion would be wrong, it is considered statistically significant. So, when another poll would state that President Obama leads by 49% to 44% and it is statistically significant, there is, statistically, less than a 5% chance that the conclusion is wrong. The 5% criteria is not cast in concrete. Sometimes 10%, 1%, or even 0.1% might be used. However, tradition has given us 5% as the default value for “statistically significance.” It is also helpful to understand that, the more data points in the sample, the more likely the results will be statistically significant.

But if some data are statistically significant, is it always “practically” significant? As an example, let’s say that you really like chocolate. Your favorite brand is in a taste test and it scores 9.6 out of 10, whereas a new chocolate scores 9.7/10 and the results are statistically significant. On the downside, the new chocolate costs 5 times as much. Is it worth the extra money to convert to the new chocolate? In this case, we have to ask, is the difference practically significant. The answer is, in all likelihood, no. Such a difference as 0.1 point out of 10 is very small, and taste is also subjective. Here, the result might not be practically significant. The subjectivity of a taste test may mean that you either can’t tell the difference or that you still like your favorite chocolate the best.

Let’s consider another less subjective example. Suppose that, in a certain application, solder voiding is a critical concern. So, you measure the voiding of two solder pastes. After collecting hundreds of data points, you find that the average voiding of one solder paste is 8% and that of the other is 7%. Analysis with Mintab software tells you that the difference is statistically significant. But is the difference practically significant? Probably not.
How do you determine practical significance? Typically it would be by experimentation or in some cases by experience. In our example of solder voiding, suppose experiments showed that, as long as the voiding average is below 30%, there will be no concerns. In light of this, engineering may have set a specification that voiding must not be greater than 25% on average. (All this discussion assumes that the spread or standard deviation of the data is not large, but this subject is the topic of another discussion.) In this case, the difference between 7% and 8% voiding may be statistically significant, but not practically significant. A prudent engineer may select the 8% paste if it had other desirable features, such as better response to pause, or resistance to graping, or improved head-in-pillow defect.

Always ask yourself, is the difference both statistical and practical?

The image shows solder joint graping, which is often more of a concern than voiding.

Cheers,
Dr. Ron

Revelations at ACI

Folks,

I’m taking a few moments from Wassail Weekend, held annually in my village, Woodstock, VT (“The prettiest small town in America”), to write a post about the recent workshops at ACI.

Indium colleague Ed Briggs and I gave a three-hour presentation on “Lead-Free Assembly for High Yields and Reliability.” I think Ed’s analyses of “graping” and the “head-in-pillow” defect are the best around.

There was quite a bit of discussion on the challenges faced by solder paste flux in the new world of lead-free solder paste and miniaturized components (i.e., very small solder paste deposits.) One of the hottest topics was nitrogen and lead-free SMT assembly. There seemed to be uniform agreement that solder paste users should be able to demand that their lead-free solder paste perform well with any PWB pad finish (e.g., OSP, immersion silver, electroless nickel-gold, etc.) without the use of nitrogen. Not only does using nitrogen cost money, but it will usually make tombstoning worse. However, in the opinion of most people, nitrogen is a must for wave soldering and, since it minimizes dross development, it likely pays for itself.

After Ed and I finished, Fred Dimock, of BTU, gave one of the best talks I have ever experienced on reflow soldering. He discussed thermal profiling in detail, including the importance of assuring that thermocouples are not oxidized (when oxidized they lose accuracy). He also discussed a reflow oven design that minimizes temperature overshoot during heating, and undershoot when the heater is off. Understanding these topics is critical with the tight temperature control that many lead-free assemblers face.

Fred Verdi of ACI finished the meeting with an excellent presentation on “Pb-free Electronics for Aerospace and Defense.” Fred’s talk discussed the work that went into the “Manhattan Project.” A free download of the entire project report is available.

There appears to be agreement that acceptable lead-free reliability has been established for consumer products with lifetimes of five years or so, but not for military/aerospace electronics where lifetimes can be up to 40 years and under harsh service conditions. These vast product lifetime and consequences of failure differences are depicted in Fred’s chart (see the pdf link). Commercial products are in quadrant A and military/aerospace products in quadrant D.

One of the greatest risks faced by quadrant D products is tin whiskers. Fred spent quite a bit of time discussing this interesting phenomenon. One of the challenges of this risk is that there is no way to accelerate it, so you can’t do an equivalent test to accelerated thermal cycling or drop shock. Fred mentioned that there have now been verified tin whisker fails, the Toyota accelerator mechanism being one.

In addition to tin whiskers, lead-free reliability for quadrant D products (with a service life of up to 40 years) in thermal cycle and other areas remains a concern.  I mention that tin pest was not on the list of issues for this quadrant.

Fred and the Manhattan Project Team have identified many “gaps” that need to be addressed to determine and mitigate the risk of lead-free assembly for quadrant D products.  They plan to start this approximately \$100 million program in 2013.

For those that missed this free workshop, another is planned in about six months.

Cheers,

Dr. Ron

Graping in Shenzhen

Folks,

After a bit of a break, the adventures of Patty, Rob and The Professor continue.

The plane droned on as it made its slow march from Detroit to Tokyo. Patty looked down at Rob as he slumbered peacefully. She caught a glimpse of The Professor, he looked at both of them from across the aisle with a satisfied smile. The proud mentor looking at his protégés.

This was the first time in a while when Patty didn’t feel totally stressed. She had resisted going to China only three weeks before her wedding, but senior management insisted. She would arrive home only 5 days before the big day. She and Rob had their first real fight, she got angry with him because he wasn’t appreciating the pressure that she felt. However, with one long weekend with their moms, she was able to get most of the tasks done and finally felt relaxed that the wedding plans were in good shape.

She had to chuckle at Rob. He was all nervous being with The Professor by himself. The plans that they had made had Rob and The Professor focusing on productivity improvements at ACME’s new acquired plants in China. While they were working on these tasks, Patty would handle some process materials related issues. The rest on the trip went smoothly and after a night’s rest they were off to the first of ACME’s new factories.

This one was located in Shenzen. Our trio was ushered in to see the site GM, Peng Zhou, a native of the area. He addressed them in good English. When Rob and Patty answered in better Mandarin, he seemed shocked. When The Professor answered him in flawless Cantonese he and Patty and Rob were stunned.

“Perhaps we should all speak in Mandarin, since we speak it well,” said Rob.

Rob and The Professor went off to audit a few assembly lines, while Peng accompanied Patty to visit an assembly line that was having a quality problem.

(Dialogue translated from Mandarin)

“I’m very impressed with how well you all speak Mandarin,” said Peng. “Where did you learn it?”

“Thank you,” replied Patty. “Rob and I studied Mandarin in college and we did an internship in China,” she went on.

“Very impressive,” Peng commented. “But I have to tell you, I’ve never heard any American speak Cantonese at all, let alone as well as The Professor does. It’s like he was born here,” he went on.

“He never ceases to amaze me,” Patty responded. Patty and Peng finally arrived at the assembly line. Patty was introduced to the line engineer, Elvis Chang. She chuckled inside, this was the third Asian person her age she had met that had chosen “Elvis” as an English nickname. Elvis was relieved that Patty spoke Mandarin. They went to a stereo microscope and looked at some of the assembled PCBs that had quality issues. Patty was quick to pick out the problem: graping. She looked at the stencil and the pad sizes on the PCB. She performed a few calculations and appeared satisfied that she had the answer.

Patty suggested that, if Elvis would like, she could give a brief presentation on what she thought the problem was.

“Patty, that’s a great idea, but it might be best to wait until after lunch,” Elvis suggested.

Elvis, Patty, and a few other young engineers went together for lunch. They seemed to be fascinated with Patty, especially her ability to speak Mandarin. They all spoke some English and were studying it as they recognized a promotion to a senior level required English fluency. One of them pointed out that she had read that about 250 million Chinese people are studying English, while only 20,000 Americans are studying Chinese. Patty enjoyed Chinese food and was happy to find Sea Cucumber on the menu. One of her friends said it was the only Chinese food he couldn’t eat. She tried it and liked it.

After lunch, Patty asked for a few hours to prepare her presentation. Her main points are summarized below:

1. The aperture size for the pads that experience graping is 8 mils in diameter for the 0.004″ thick stencil.

2. The resulting area ratio (D/4t, D= diameter, t = stencil thickness) for this aperture is 0.50, less than the recommended 0.66.

3. The very small solder paste deposit doesn’t not have enough flux to avoid oxidation of the solder particles in reflow. The resulting defect looks like a bunch of grapes so it is called graping.

4. Likely solutions:

a. Use a square aperture. An 0.008” square aperture provides 27.3% more volume, and it has better transfer efficiency. (Transfer efficiency is the volume of the solder paste deposit divided by the volume of the aperture times 100.) The result would be >30% more solder paste. The more solder paste, the less likely to experience graping

b. The solder paste they were using was not best of breed re: graping resistance. She recommended another one, which she knew performed well in all respects — and minimized graping. This solder paste’s flux was robust and designed to minimize defects like graping. Her presentation was received very well. Fortunately some of this excellent solder paste she recommended was being used for another job in the plant. So with approval from Peng, the team switched to this paste.

After the meeting, Patty thought about how much one of the technical engineers from one of her favorite solder paste suppliers had helped her to understand graping and how to minimize it. His name is Ed Briggs and she had just attended SMTA Toronto where he gave a paper on graping. Much of the information in her presentation came from the paper and his other writings on graping.

Epilogue: Three weeks later, the graping had disappeared from Elvis’s assembly line. They didn’t even need to adopt a stencil with square apertures; the solder paste change itself was enough.

Cheers,

Dr. Ron