Zulki’s PCB Nuggets: Consider Low-Temp Solder for PCB Microelectronics Assembly

As I’ve often said, growing numbers of OEMs are producing small, more portable devices, especially medical electronics OEMs that are pioneering insertable and ingestible medical devices. With these newer, smaller devices coming on the market, EMS providers and contract manufacturers (CMs) must adjust their PCB assembly technologies to comply with these demands. Thermal profiling and the use of the correct solder paste, in this instance, become an even more critical step than before in conventional SMT and the newer microelectronics assembly.

In today’s day and age, when many products are shrinking, PCB, component, and packaging sizes matter considerably. That’s why caution has to be taken when products like these are subjected to high-temperature reflow cycles. It’s important to consider low-temperature profiling. It is more effective if you can reflow components and solder pastes at lower temperatures and with lower thermal profiles, if possible.

But first, it’s a good idea to revisit various reflow temperature ranges. Lead-free solder, like SAC305, is 235–245°C peak temperatures; leaded solder is about 180–185°C. Meanwhile, you have to take into consideration chemical mixtures in low-temperature lead-free solder pastes. The most common compositions are tin/bismuth and tin/bismuth/silver. Peak reflow temperatures associated with tin/bismuth solder paste range from 165–195°C compared to tin/lead solders at 220–240°C.

A key benefit is the peak reflow temperature of between 165–195°C that provides a Delta of 50° C between SAC305 and low-temperature bismuth-based solder, thereby subjecting the boards and components to lower temperature cycles and keeping the temperatures in more of a “safe zone.”

To review, some of the issues that can crop up when applying higher-temperature thermal profiles and reflow during the assembly of PCBs include:

  • Board warpage
  • Chances of creating larger voids and head-in-pillow are higher in BGA and QFN packaging
  • Greater chance of board delamination

Now, let’s take a look at this low-temperature bismuth-based solder paste and its use for a thermal profile. It can reduce peak reflow temperature requirements to as low as 165–180°C. Also, sizes of voids and head in pillow flaws on bottom terminated devices are reduced since there is a 50°C Delta between a low-temperature peak profile versus the SAC305 peak profile. Plus, low-temperature solder paste allows considerably less thermal exposure. The less a board is exposed in terms of high temperature, the better it is for the boards and components, which increases long-term reliability.

For example, the quality of an FR-4 or Rogers board can experience deterioration and possible delamination when it unnecessarily undergoes three to four thermal cycles instead of two, which can be sufficient. Those extra thermal cycles can possibly deteriorate component longevity and the epoxy resin inside the FR-4 material.

There are also negative sides to using low-temperature bismuth-based solder paste. There’s a tendency/possibility of cracking during cool down. To alleviate that problem, low-temperature solder (LTS) can be cooled down at a slower gradient. You cannot rapidly cool it down because it becomes brittle and can break. This means you have to carefully create the thermal profile from peak to cool off period. You have to make sure the temperature is not dramatically dropped, but instead gradually.

It takes more slump when you’re creating the thermal profile because tin-bismuth alloy, such as Sn42Bi58, creates more wetting when the slump is calculated properly and is done accurately. In SAC305, it is more forgiving when you are increasing or decreasing the temperature. But LTS is not as forgiving, and you have to create the profile and maintain the profile in a much better-controlled way.

Less Thermal Shock
By applying low-temperature solder paste, the board sustains less of a thermal shock compared to SAC305, which is around 235–245°C versus 160–170°C. This is a huge Delta. In the long run, medical device PCBs, for instance, perform considerably better, more reliable, and much longer.

Let’s go a little deeper into LTS and ask, “Why is this specifically needed?” Take medical electronics OEMs as an example. The new products they are launching together with the chips/devices being used cannot handle high temperatures for either the wire bonding or the packaging involved or otherwise.

But also consider that some OEMs have gradient melting point limitations for their terminations at considerably lower temperatures. In cases like that, care has to be taken about the type of low-temperature solder paste to be used. That’s why it is important that the EMS provider or CM work closely with the customer to coordinate and approve these critical thermal profiling points before running the product through SMT lines.

Another key point to consider is that solder paste companies offer different compositions involving tin, silver, copper, and bismuth for the thermal cycle. Here’s where the EMS provider and OEM customer need to closely collaborate about solder paste composition and characteristics, carefully reviewing the datasheets for these low-temperature solder pastes from their manufacturers.

Drop shock is one other aspect to investigate closely. This involves testing to determine how these assemblies are going to hold up when they are dropped from a certain distance to the ground or applying a certain amount of force to make sure if there are no cracks in the assembly, thereby making them reliable.

These solder joints are tested by thermal shocking, drop shock, and mechanical strength for vibrations. If these are projects that are used in a hazardous environment, you have to make sure that they can withstand mechanical strength once the product is fully assembled.

The OEM, EMS company, or CM needs to ensure that although the composition of bismuth is more in the alloy for low-temperature profiles and low-temperature solder paste, the joint created and assembly level or the microelectronic level for chip on board are still as sturdy as lead-free or leaded assemblies. Product usage is not going to change because the solder paste or thermal profile is changed.

In conclusion, there are certain considerations that need to be investigated before a low-temperature thermal profile paste can be selected for both SMT and microelectronic assemblies.

Zulki Khan is the president and founder of NexLogic Technologies Inc.

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2020

Zulki’s PCB Nuggets: Consider Low-Temp Solder for PCB Microelectronics Assembly

11-11-2020

With newer, smaller devices coming on the market, EMS providers and contract manufacturers (CMs) must adjust their PCB assembly technologies to comply with these demands. Zulki Khan details how thermal profiling and the use of the correct solder paste become an even more critical step than before in conventional SMT and the newer microelectronics assembly.

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Zulki’s PCB Nuggets: What’s Different Between C2 and C4 for PCB Microelectronics Assembly?

10-21-2020

In Zulki Khan's last column he talked about flip-chip ball grid array (BGA), or FCBGA, making its grand entrance into PCB microelectronics assembly. But that subject requires a lot more digging to get the full story for OEMs planning highly advanced products that demand PCB microelectronics assembly. In that regard, C4 and C2 bumps for flip-chip assemblies are among the top techniques that require close attention.

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Zulki’s PCB Nuggets: FCBGA Packaging Enters PCB Microelectronics Assembly

09-09-2020

The demand for smaller circuitry and packaging, as well as ever-shrinking PCB real estate, have continually pushed PCB assembly and manufacturing protocols. Part of these technological advances involves a combination of flip-chip and BGA (FCBGA) packaging. Zulki Khan explains the importance of FCBGAs.

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Zulki’s PCB Nuggets: DOEs on Call for New Wearable Medical Devices

08-05-2020

Zulki Khan explores how biosensors for human-machine interfaces (HMIs) and new, flexible electrodes are leading the way, are among the most recent developments and promise more sophisticated medical wearable devices for health monitoring.

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Zulki’s PCB Nuggets: Soft Electronics Pose PCB Microelectronics Assembly Challenges

07-08-2020

Zulki Khan explains how PCBs have moved from traditional large rigid boards to considerably smaller rigid and combinations of rigid and flex circuit boards, even to the point that bare chips and wire bonding are used during the PCB microelectronics assembly of these tiny boards.

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Zulki’s PCB Nuggets: Medical Miniaturization and PCB Microelectronics Assembly

06-24-2020

Medical electronics continue to be a gamechanger, with miniaturization being foremost today in the minds of medical OEMs. Zulki Khan discusses how there is a growing demand for even greater device and component miniaturization that plays a major role in the PCB microelectronics assembly of these medical devices today.

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Zulki’s PCB Nuggets: Add Hi-rel to ISO 13485 for More Robust Ventilator PCBs

05-13-2020

It's important to meet FDA and ISO 13485 standard quality and reliability requirements for ventilators and other medical equipment. Zulki Khan explains how there’s still more that ventilator OEMs need to put into practice, specifically in the high-reliability or “high-rel” area to further add to ISO 13485.

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Zulki’s PCB Nuggets: Urgent Call for Ventilators—PCB Technology at the Ready

04-15-2020

An urgent call is out to medical equipment makers that thousands—even millions—of ventilators are in the greatest demand in our history due to the worldwide COVID-19 outbreak. Zulki Khan explains how new ventilator makers—as well as traditional ones—must weigh a number of key PCB design, assembly, and manufacturing factors.

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Zulki’s PCB Nuggets: Putting the Heat on for Thermal Profiling

03-11-2020

A unique thermal profile is designed for each PCB job undergoing conventional SMT assembly, as virtually every PCB assembly professional knows. But what about a PCB assembly project involving both conventional rigid board and an extraordinarily small rigid or rigid-flex circuit undergoing microelectronics assembly? Zulki Khan covers PCB hybrid assembly, which requires two separate, unique, and distinctly different thermal profiles.

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Zulki’s PCB Nuggets: 7 Steps to Successful Assembly for Medical Devices Using Microelectronics

02-12-2020

Seven major steps need to be taken to achieve successful SMT and microelectronics assembly for medical electronic devices. Zulki Khan explains how these key steps take on special significance for newly emerging implantable and ingestible medical devices and result in medical devices that are robust, smaller, highly reliable, more powerful, and lighter.

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2019

Zulki’s PCB Nuggets: Vital Details for Implantable Medical Devices

12-04-2019

In addition to smart pills and smart cameras, which Zulki Khan covered in a previous column, another segment of the medical electronics devices market is rapidly growing, as well: implantable medical devices, which medical personnel surgically or otherwise insert into various parts of the human body. Zulki explains the extra measures required for these devices.

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Zulki's PCB Nuggets: Multi-tier Wire Bonding—Diving Into PCB Microelectronics

11-07-2019

As the name implies, multi-tier wire bonding involves several levels of wire bonding beyond the single level of wire bonding, which is traditionally used in semiconductor and/or PCB microelectronics assembly. Here, you have two, three, and four levels of wire bonding, in some cases, called stacked wire bonding. Also, multi-tier wire bonding offers OEMs a solution when the number of inputs/outputs (I/Os) are far beyond the traditional ones that are used in the single wire-bonding application.

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Zulki’s PCB Nuggets: Smart Pills & Cameras—The Next Frontier for PCB Microelectronics

10-23-2019

"Take two aspirin and call me in the morning," is the proverbial, jovial, and often-cited elixir that doctors have prescribed over the years for whatever ails you. Today, medical electronics are adopting the same concept but with new technologies. Now, the phrase, "Take two aspirin," takes on new meaning, as medical electronics move into new frontiers of inspecting a human’s gastrointestinal tract with new, revolutionary ingestible smart pills and "pill cams."

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Zulki's PCB Nuggets: A Better Grasp of Glob Top Epoxy Factors

09-25-2019

In my last column, I cited important aspects of glob top epoxies, calling attention to the fact there are different epoxy manufacturers. In this column, I will continue to emphasize six other important factors of glob top epoxies.

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Zulki’s PCB Nuggets: Get a Handle on Glob Top Epoxies

09-12-2019

Most often, glob top is the prevalent method EMS providers use today. However, the most important point to be made about glob top is the fact that multiple manufacturers are producing different glob top epoxies. And within each manufacturer, there are numerous types of epoxies being produced. Another key point is that EMS providers and contract manufacturers generally are the ones deciding on the kind of epoxy to use. This column will further describe how you can get a handle on glob top epoxies.

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Zulki’s PCB Nuggets: Protect the Die and Wire Bonding for Effective PCB Microelectronics Assembly

07-31-2019

Protecting bare dies on a PCB or substrate is a major process of microelectronics assembly. As we’ve said before, microelectronics assembly and manufacturing work in tandem with traditional SMT manufacturing for complete PCB hybrid manufacturing of today’s smaller form factor products, including IoT, wearables, and portable devices.

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Zulki’s PCB Nuggets: PCB Microelectronics—Inspection and Calibration

07-18-2019

Microelectronics manufacturing is the companion of SMT manufacturing and forms PCB hybrid manufacturing. Tools for SMT manufacturing have been around for a long time and have proven their value. Now, with microelectronics, new and different types of high-powered laser microscopes are populating the microelectronics assembly and manufacturing area to provide highly effective inspection and calibration.

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Zulki’s PCB Nuggets: Three Die Attach Methods for Microelectronics Manufacturing

06-27-2019

Die attach technology is increasingly being applied in PCB hybrid manufacturing (i.e., combining traditional SMT manufacturing with microelectronics) to comply with the requirements of small PCBs, especially rigid, flex, and combination rigid-flex circuit boards. These smaller boards are used in a variety of IoT, wearable, and portable applications.

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Zulki’s PCB Nuggets: Consider the Integrity of Wire Bonding

06-12-2019

While reliability and integrity can be regarded as synonymous as far as PCB manufacturing with microelectronics assemblies is concerned, the integrity of wire bonding—the methodology of interconnecting the wire to the bond pad—takes on other reliability-associated process qualities. Here are three factors that need to be implemented to create the integrity of wire bonding.

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Zulki’s PCB Nuggets: Avoid PCB Wire-bond Loop Failures

05-30-2019

Today, hybrid PCB manufacturing is making greater inroads into our industry, which is the marriage of traditional SMT manufacturing together with microelectronics and wire bonding. In many cases, the OEM working with EMS providers doesn’t fully understand the nuances of effective wire bonding and related failures.

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2014

Tighter Scrutiny Needed for PCB Cleaning Agents

05-13-2014

PCB cleanliness on the assembly floor is now getting more attention, due to tiny residues and contaminants being left on assemblies after new, advanced assembly processes. Cleaning methodologies, testing, analysis, and special chemistries are being taken to a new level to assure customers of ultraclean boards to avoid costly latent issues.

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Zulki's PCB Nuggets: Tighter Scrutiny Needed for PCB Cleaning Agents

05-13-2014

PCB cleanliness on the assembly floor is now getting more attention, due to tiny residues and contaminants being left on assemblies after new, advanced assembly processes. Cleaning methodologies, testing, analysis, and special chemistries are being taken to a new level to assure customers of ultraclean boards to avoid costly latent issues.

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Uncovering Assembly Problems of High-Speed PCBs

03-12-2014

The high-speed board may be perfect when it comes to BGA assembly. All the balls properly collapse; all the thermal profiles are accurately determined and performed. All soak temperatures, pre-heat, soak, and cool-off periods fall within manufacturer limits and ranges. Yet, this high-speed board fails at high speed at the time of system functional level testing in the system.

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Zulki's PCB Nuggets: Uncovering Assembly Problems of High-Speed PCBs

03-12-2014

The high-speed board may be perfect when it comes to BGA assembly. All the balls properly collapse; all the thermal profiles are accurately determined and performed. All soak temperatures, pre-heat, soak, and cool-off periods fall within manufacturer limits and ranges. Yet, this high-speed board fails at high speed at the time of system functional level testing in the system.

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EMS Discovers Mature IC Technologies

01-14-2014

Columnist Zulki Khan asks, "Did you know that really new, up-to-the-moment PCB technologies are nesting on the doorstep of PCB assemblers?" In fact, he says some of these technologies are very mature, but they're completely new to the assembly side of things.

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Zulki's PCB Nuggets: EMS Discovers Mature IC Technologies

01-14-2014

Columnist Zulki Khan asks, "Did you know that really new, up-to-the-moment PCB technologies are nesting on the doorstep of PCB assemblers?" In fact, he says some of these technologies are very mature, but they're completely new to the assembly side of things.

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2013

Another Look at AOI

11-13-2013

PCB inspection is taking on greater significance as boards and packaging become increasingly smaller, with greater functionality. Automated optical inspection (AOI) and its backup associate, X-ray, team up to catch a variety of board assembly problems. But it's AOI that's at the forefront of this process.

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Zulki's PCB Nuggets: Another Look at AOI

11-13-2013

PCB inspection is taking on greater significance as boards and packaging become increasingly smaller, with greater functionality. Automated optical inspection (AOI) and its backup associate, X-ray, team up to catch a variety of board assembly problems. But it's AOI that's at the forefront of this process.

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Zulki's PCB Nuggets: ECOs Reviewed - The Importance of Accuracy

09-11-2013

Designers can perfectly layout a design and, in theory, follow written specifications to the letter, but when one factors in the practicality of that design, virtually everything associated with it has its limitations--ranging from the material used to make the board to assembly, machine tolerances, and process limitations.

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