Medical Device Link.

SOLDERING

(click to enlarge) This multilayer board shows a design using critical through-hole and SMD technology. Many of these boards 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 have opened new paths for physicians to monitor and manage chronic conditions—and even deliver medication doses.

Some of the biggest advances in the medical electronics field have been in the areas of outpatient monitoring and drug delivery. The growth of these devices was aided in 1999 when the FCC allocated the Medical Implant Communications Service of band frequencies (402–405 Mhz), enabling implanted medical devices to communicate with external devices. In many cases, patients who used to regularly visit a hospital, clinic, or doctor’s office for monitoring of critical conditions can now use home monitoring devices, upload data to their doctors, and receive new programming over phone or wireless methods.

On the drug-delivery side, new monitoring equipment is letting patients self-dose. Failsafe electronics inside the equipment prevent users from overdosing or dosing more frequently than allowed. The actual electronic assembly and packaging that go into these types of monitoring systems are critical. If it isn’t perfect on the assembly level, the system won’t work consistently in the field.

Electronic Assembly Challenges

Like electronic assemblies in all industries, critical medical electronic assemblies must meet the demand for higher capabilities with guaranteed accuracy in continually shrinking real estate. Accommodating these demands requires the use of miniaturized components as well as extremely high component density—often using double-sided and stacked-board assembly designs. This makes soldering through-hole components more difficult because space constraints require the precision and dexterity of the operator doing hand soldering.

Another challenge is that surface-mount technology (SMT) must coexist with through-hole devices and components in assemblies. Electronic manufacturing services (EMS) providers are starting to accept that through-hole technology can’t be totally replaced by SMT. In fact, through-hole components are adding capabilities and options each year that make them highly desirable in a wide range of medical electronics. Components such as connectors, capacitors, resistors, and digital displays are still through-hole due to power requirements or the nature of the design.

The Drawbacks of Soldering

(click to enlarge) Even when through-hole devices constitute only 5-10% of the assembly, thorough processing is critical to the overall functionality of the end product.

While some mixed-technology assemblies can be soldered with wave solder equipment using specially designed carriers and masking, many assemblies such as stacked and double-sided SMT assemblies, are too complicated to be processed this way. This is true even if through-hole devices comprise only 5%-10% percent of the assembly.

If a board assembly can’t run through the wave soldering process, it is typically hand soldered. However, very dense component designs, particularly those involving RF components where a slip can cause a false antenna, can be a very tricky issue for even the best operator to handle. Such designs open a path for human error and increase the potential for failures that require rework or, in the worst case, scrap. Additionally, good hand-solderers who can solder class 2 and class 3 joints are a difficult commodity to find today, and they take a long time to train.

Another disadvantage is that 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 provider that specializes in processing high-quality medical products for OEM customers, this is not an option. Therefore, they strive to automate as much of the through-hole process as possible to eliminate the hand soldering drawbacks that loom over the assembly.

New automated selective soldering systems have started to reduce the gap between assemblies that can be wave soldered and assemblies that must be hand soldered. Soldering machines based on mini-wave technology are suitable for any volume of medical EMS production.

How Selective Soldering Systems Work

Selective soldering machines offer easy programmability with options for inline and offline programming. More importantly, the systems’s SMEMA interface enables it to be easily integrated into the overall flow of the automated production line. This eliminates the need for substantial operator interface that stand-alone machines require.

(click to enlarge) Selective solder pots are small (35-75 lb typical). Wave solder nozzles range from 2 to 8 mm diam in round configurations and from 100 to 150 mm in rectangular nozzle configurations.

Once integrated into a line, a selective soldering system lines easily. Its process conveyor is set parallel to the solder nozzle. An x-y-z 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 x-y-z table and can be configured with spray, drop jet, or ultrasonic nozzles and enables easy changeover for different printed circuit boards, components, solder, nozzles, and odd-form assemblies.

Nitrogen provides excellent 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 lb typical), and the wave solder nozzles range from 2 to 8 mm diam in round configurations and from 100 to 150 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°F when using FR4 Tg140 PCBs, while taking into consideration panel and board size. During traditional soldering with a wave, there is a tremendous thermal mass of solder acting on the entire surface of a relatively modest mass of PCB material. However, during selective soldering, a tiny amount of solder is presented to the PCB. This means that selective soldering can be conducted at a higher temperature than wave soldering without damaging the PCB.

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 the nozzle configuration is 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. This enables repeatable results from production lot to production lot.

Moreover, programmable selective soldering equipment gets in between tall components in densely populated formats, helping to ensure quality joints. It is a controllable process that solves the issue of bringing SMT and through- hole together in a way that provides both sustainable line speed and repeatable quality.

Critical Applications Examples

(click to enlarge) Programming on all selective soldering machines is recipe based. Solder points are selected and nozzle configuration set to generate a program within minutes.

Some projects require several through-hole components to be hand soldered. For example, a project that has a heavy ground plane on the inner layers of the printed wiring board would prevent an operator from achieving even 50% barrel fill. A selective soldering’s system’s heat-transfer capabilities can be used to achieve nearly perfect solder joints. As a result the operation becomes almost defect-free every time.

The system can also be useful when an OEM is trying to meet RoHS standards and reduce its use of lead. In some applications, the elevated melting temperatures of lead-free metals makes hand soldering almost impossible because of the inability to transfer enough heat to specific areas. Even when this heat transfer can be done by conventional methods, there is the potential to damage surrounding components with the high temperature required to solder specific leads. By using a selective soldering machine’s molten solder supply in conjunction with preheat—even using lead-free solder—large components and heavy ground planes can be soldered in a consistent, automated fashion.

Another good application of selective soldering would be a blood pressure monitor that has critical SMT components and an LCD display. Although wave processing is necessary for this type of product, the SMT components prevent the manufacturer from palletizing the wave process. Even if keep-out areas are marked, extensive rework will still be required after the wave operation. Masking and rework can be avoided by using an automated selective soldering system, which can move in and around SMT components. This also reduces the defects-per-million opportunities level.

(click to enlarge) The inline selective soldering system effectively performs precise dip, drag, and mini-wave soldering.

But, in the overall picture, selective soldering is not always about zero defects. Some device layouts, like those found in critical systems such as drug-delivery devices, cannot be modified for easier assembly because of design issues. Although hand soldering must still be used on these applications, the use of selective soldering can reduce as much as 2/3 of the hand solder operation required. The combination of the methods keeps the original design intact, but reduces the assembly times and amount of defects.

Conclusion

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 without the processing heat damage that a wave or reflow process could induce. Also, automated selective soldering can do this highly repetitive work consistently every time, as programmed.

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 100% control is impossible. Removing the human factor provides a controllable process. Once set, an automated selective soldering machine can consistently perform the program that is applied to a specific process.