Multi-layer board showing a design using critical through-hole and SMD technology. Modern through-hole components and devices add capabilities to the end product. There is a big increase in machine and patient interaction, requiring large, easy-to-read displays and switches. Many of these cannot withstand typical solder processing temperatures.

Modern medical monitoring devices require extremely complex internal electronic assemblies. The availability of highly miniaturized, low-power RF devices and specialized communication protocols open new paths to how physicians are monitoring and managing chronic conditions--and even delivering medication doses.

In hospitals critical 2nd tier equipment includes monitoring heart rates, oxygen levels and a realm of both medical delivery levels and bodily function levels related to patients.

Some of the biggest innovations are seen in outpatient monitoring and drug delivery. Where patients used to require continual visits to a hospital, clinic or to their doctor for monitoring of critical conditions, modern technology now allows many to use home monitoring devices, upload data to their doctors and receive new programming over phone or wireless methods.

The FCC allocated the Medical Implant Communications Service of band frequencies (402-405-Mhz) for implanted medical devices to communicate with external devices several years ago.

On the drug delivery level, new monitoring equipment allows patients to self dose, with failsafe electronics within the monitors that prevent overdose or dosing more frequently than allowed. It is a changing world that makes medical processes more accurate, easier for both the physician and patient, and reduces the overall cost of care.

All of this technology relies on some type of monitoring system to control the end result of the medical process. The actual electronic assembly and packaging that goes into these monitoring systems is the critical heart of all these systems. If it isn’t perfect on the assembly level, it isn’t going to work consistently in the field.

Electronic Assembly Challenges

Critical medical electronic assemblies are seeing the same challenges as electronics in all areas; a demand for higher capabilities with guaranteed accuracy in continually shrinking real estate. To accommodate the multitude of new demands means miniaturized components and extremely high component density; often using double sided and stacked board assembly designs.

If every component was surface mounted, this would still be a challenge but it would be easier to meet. However, this is not the case. Like all electronic assemblies, “old meets new.” SMT must co-exist with through-hole. Devices such as power supplies, connectors of all sizes and configurations, switches, relays, heat sink thermal solutions and the latest in through hole LEDs must function side by side with SMDs. There is no “one size fits all” in this area; the selection is vast and based on the needs of each exclusive product design.

While some mixed technology assemblies can be soldered with wave solder equipment using specially designed carriers and masking, many, particularly stacked and double side SMT assemblies simply can’t be processed this way. Even though the assembly may incorporate only 5 to 10 percent through hole devices, processing is critical to the overall functionality of the end product.

If a board assembly can’t run through the wave soldering process, they are typically hand soldered. The issue here is that with some very dense component designs, particularly those involving RF components where a slip can cause a false antenna, this can be a very tricky issue for even the best operator to handle. It opens a path for human error and increases the potential for failures that require rework, or at the worst, scrap.

Also from a purely productivity point of view, stopping an automated production line to hand solder specific components is very time consuming. Both added time and wasted product mean less profit for the company. For an EMS specializing in processing high quality medical products for OEM customers, this is not an option.

Medical EMS facilities are actually seeing an increase in through hole technology. In general, almost all of the modern board designs use through-hole components and devices because of the added capabilities they bring to the end product. Therefore, the challenge to the EMS is how to do as much automation of the through-hole process as possible to eliminate the potential processing drawbacks that loom over the assembly.

Automating Through Hole

When looking at the gap between what is wave solderable and what needs to be hand-soldered, new automated selective soldering systems are beginning to fill this in-between step. There are several types of selective soldering machines on the market today, but the most competent to fill the needs of any volume of medical EMS production are those based on the mini-wave technology.

This was the type of system first tested at E.I. Microcircuits. Several issues were considered before making the move to a specific system; it needed easy programmability with options for inline and offline programming and most of all, the system needed to be able to integrate into the automated production line. That is because, while a stand-alone system would potentially help reduce some cost and labor and increase quality, it would still require substantial operator interface that a fully automated system with SMEMA line interface would not need. This would keep the automated line flow stable. Because the equipment met all the criteria, the selective soldering systems chosen for use on the E.I. production lines were from RPS Automation, based in Washington state.

These selective solder systems integrated into the E.I. Microcircuit production lines easily. The process conveyor is set parallel to the solder nozzle. An XYZ axis motion articulates the mini-solder wave nozzle beneath the circuit boards. Using this technology, the system effectively performs precise dip, drag, and mini-wave soldering. The flux system is also mounted to the XYZ table and can be configured with spray, drop jet, or ultrasonic nozzles and enables easy change-over for different PCBs, components, solder, nozzles, and odd-form assemblies. Nitrogen provides superior soldering performance by assisting the thermal capability, improving the surface tension of the solder and controlling wave stability. The solder pots are small (35-75 lbs typical) and wave solder nozzles range from 2-8 mm diameter in round configurations and from100 to150 mm in rectangular nozzle configurations.

The temperature of the solder and dwell time is typically the most critical area for selective solder processing. Most operators try to run the solder pot at 490 degrees F because of the type of FR4 Tg of 140, while taking into consideration panel and board size.

Programming on all machines is recipe based and can also be accomplished through the basic program system, using scanned images, imported Gerber data or even a camera based jog-to-teach method. Solder points are selected and nozzle configuration set to generate a selective soldering program in minutes.

Statistical data can be tracked and all control settings can be saved in a “program” that is loaded each time the associated product is manufactured— delivering repeatable results from production lot to production lot.

Critical Applications Examples

One particular project required several through-hole components to be hand soldered. This project had a heavy ground plane on the inner layers of the PWB and the operator could not achieve even 50% barrel fill. By introducing the selective solder machine with a constant molten solder supply, we were able to utilize its heat transfer capabilities and achieve nearly perfect solder joints.

As a result the operation went from an almost100% rejection rate to almost defect-free every time. This scenario has become even more prevalent when customers switch to RoHS standards of Pb-free.

Elevated melting temperatures of the Pb-free metals in some applications make hand soldering almost impossible due to the inability to transfer enough high temperature to specific areas.

Even when it can be done by conventional methods, there is the potential to damage surrounding components with the high temperature required to solder specific leads. By utilizing the selective solder machine’s molten solder supply in conjunction with preheat, even using Pb-free solder, large components and heavy ground planes can now be soldered in a consistent, automated fashion.

Another good example is a blood pressure monitor that required wave solder processing due to the volume of components and an LCD display.  At the same time, production faced the issue of being unable to palletize for the wave process due to particular critical SMT components used in the board design. We tried masking the “keep out” areas and wound up dealing with the results that required extensive rework after the wave operation.

By moving this specific assembly challenge to automated selective soldering, with its ability to move in and around SMT components, it solved the problem. Masking was no longer required. Rework virtually disappeared. The selective soldering process allowed us to provide consistent solder quality and in much less time than either hand soldering or the time it took to perform solder touchup on these assemblies after the wave soldering process had been used.

Before selective soldering this project had a DPMO level (defects per million opportunities) of 1,957 per unit. After selective soldering, the DPMO level dropped to 6 with none pertaining to the hard to solder through-hole parts. We saved time and rework cost, reduced the defect rate and eliminated the masking step altogether.

In the overall picture, selective soldering is not always about zero defects. In one project, the customer’s product design layout could not be modified for better assembly. Due to this, both the initial defect rate and assembly time were very high. With the selective soldering machine, the overall assembly process (i.e.: the time to obtain a perfect end product) was reduced by a productive 23 minutes—even with a small amount of touch-up.

This product was part of a drug delivery system and it’s complexity had required detailed hand soldering. The typical hand solder operation tact time was 30 minutes per unit and had a defect rate of .57 DPU (defects per unit). When the process was set up on selective soldering, the soldering application was completed in 7 minutes. This was less than 1/3rd of the processing time it took for the manual soldering operation.

The 7 minutes also included the time for minor touchup that was be needed due to the problematic design layout. The end result was a dramatic improvement in the overall process throughput, while keeping the customer’s design layout intact. The process time reduction enabled both monetary savings and reduced delivery times for the customer.

The Bottom Line

Anytime a process step can be automated, it can be controlled. A manual process such as hand soldering, can be controlled to a certain extent, but it’s much more difficult and never 100%.Removing the “human factor” provides a controllable process. Once set, an automated selective soldering machine will perform consistently per the program applied to the specific process.

Like everyone, when E.I. Microcircuits started in business, our utopian goal was to change over to total surface mount. It was the coming technology. Over the years, it has been proven that not only are through hole components not going away, they are also adding capabilities and options each year that make them highly desirable in a wide range of medical electronics.

Many components such as connectors, capacitors, resistors, and digital displays are still through-hole due to power requirements or the nature of the design. The medical industry is trying to make things as small as possible and that means top and bottom side surface mount. This makes soldering through-hole components that much more difficult as space constraints require the precision and dexterity of the operator doing hand soldering. The selective solder machine comes to the rescue and can tackle projects such as high pin counts, deep and hard to reach solder points, Pb-free and larger volume.

The medical EMS world is seeing a big increase in machine and patient interaction. This means there are large, easy-to-read displays and switches, many of which cannot withstand typical solder processing temperatures. Selective soldering can solder these high pin count components with no processing heat damage that a wave or reflow process could induce. Also, automated selective soldering will do this highly repetitive work consistently every time, as programmed.

Good hand-solderers who can solder class 2 and class 3 joints are a difficult commodity to find today and take a long time to train. Selective soldering machines help ease the load and produces class 2 and 3 solder joints in a consistent manner. In any industry, but especially the medical industry, reliability is paramount when it comes to health related machines. Selective soldering is as good as any other soldering practice out there, and quite often better. Hand soldering will never go away, but automated machine soldering will produce quality consistent results day after day.

Two years ago the company invested in its first selective soldering system. Today, there are three inline machines running both RoHS and leaded operations. Programmable selective soldering equipment gets in-between tall components in densely populated formats giving quality joints. It is a controllable and repeatable process that solves the issue of bringing SMT and through-hole together in a way that provides both sustainable line speed and repeatable quality.

Quite a few of the PWBs that E.I. runs have large copper ground planes and devices which are very difficult to hand solder. The selective solder system provides a constant heat that a wave can give, making these much easier to process. Since the production volumes are expected to increase in these applications, a high percentage of all these critical mixed technology assemblies that formerly needed hand soldering can now be handled by the automated selective soldering systems.

Although selective soldering does not replace 100% of the hand soldering, the medical production lines at E.I. Microcircuits have the automated selective soldering systems running full time in-line. These machines have taken over a minimum of 60% of what formerly needed to be pulled offline for manual soldering.