High temperature resistant

Saturday, 05 October, 2002



The trend to produce printed circuit boards (PCBs) using surface mounting technology (SMT) is becoming increasingly prevalent.

One requirement for this technology is the provision of the majority of components built in a suitable way: surface mountable, high temperature resistant, small and optimally packaged. Many components have already been converted and are now available. However, owing to their size or high demands on their functionality, for performance and connector components and similar the wired type is still predominantly used.

Generally they cannot be incorporated directly into the high-temperature reflow process, instead necessitating an extra job. From the viewpoint of the PCB manufacturers, this is a restraint, since the costs of dealing with these items in separate jobs are not inconsiderable.

As a manufacturer of various PCB mounted components, Phoenix Contact is pursuing a new trend, the so-called through-hole technology (THT) or 'pin in paste' process for high temperature applications.

'Pin in paste' process

'Pin in paste' is a process that supplements SMD mounting and reflow soldering and eliminates the previously necessary wave soldering step for THT components.

The advantage is that SMD components and wired connector components (THT) can be placed on the SMT board and collectively soldered in the reflow process.

For THT components to be used in the 'pin in paste' process, they must be adapted for the reflow process. Additionally, a few rules have to be observed concerning the PCB layout.

Requirements for THT components

PCB mounting components that withstand high temperatures are ideal. Packaged suitably for the application, these are placed directly into SMT system pick and place machines and finally soldered in the oven.

At the same time these components should meet all the requirements for electrical values, mechanical stability or color that applied for the standard component being replaced.

High-temperature resilient plastics

The material must be able to withstand the now common temperatures in the reflow process - a few minutes at 185°C as well as short exposures lasting a few seconds at 230 to 260°C.

Because of the imminent EU guidelines concerning the use of lead-free solder in electronics production, these plastics will have to meet higher temperature resilience requirements.

Future lead-free solder pastes for reflow processes or solders for wave soldering systems are expected to have liquid temperatures some 30°C higher.

Component geometry

Partly to avoid the casing coming into contact with the solder paste, and partly to enable heat to efficiently reach the points being soldered, the geometry of THT components must be adapted.

In many cases the area of the soldering pin outside the plastic casing can be made large enough to avoid contact with the solder paste being applied. If this isn't possible, the component should be equipped with spacers to keep a recommended minimum distance of 0.3 mm.

Component volume

Component volume has to be considered when converting standard components to THT ones. The bigger the component is, the more heat it can absorb.

Also, the soldering points, which are mostly under the component, are covered up. Consequently, the soldering points aren't heated sufficiently and the soldering process isn't fully completed.

These types of components can generally only be converted or replaced when they have a small volume or number of pins.

Pin geometry

In addition to adapting the design of the casing, the connecting leads (solder pins) should be adapted.

Pins with a square cross-section are particularly well-suited. The pin mustn't be too long, or else the pin takes up too much solder paste, which won't then flow back to the soldering point.

This would lead to the component not being attached strongly enough or insufficient electrical load-carrying ability.

Minimum pin length should correspond to the PCB thickness. This sizing is also suggested when components are mounted on both sides of the PCB and pins sticking out hinder the placing of further components.

PCB layout

Through-plated holes are reliable and recommended. To allow for the pick and place machine's precision, the holes should be 0.1 to 0.25 mm larger than the component lead diameter or the pin diagonal with square pins as the case may be.

The maximum possible remaining width of the solder hole ring is limited by the nominal voltage of the component mounted. The width of the remaining ring should not fall below a minimum of 0.3 mm.

Solder paste application and amounts

Independently of the process, the solder paste application should completely fill the holes. It is recommended to have a slight hemispherical protrusion of paste on the underside of the PCB, so that as much solder paste as possible is available.

As a general principle, the amount of solder paste applied to the ring on the upper side of the PCB varies widely with the process.

Printing over the edges of the soldering ring should be avoided because of the risk of solder balls forming.

Currently, predominant screen printing processes in conjunction with popular stencil thicknesses of around 150 µm apply the same thickness of paste for all components. Consequently, there may be insufficient solder paste for components that require it in increased amounts, including THT components in some circumstances.

Possible remedies in these cases are multi-level stencils that give a thicker application only in the area of these components.

In general it is possible to apply sufficient amounts of solder paste for all components by setting the appropriate printing machine parameters. Where solder paste requirements vary greatly, a dispenser can be used as an alternative that allows the application of adjustable amounts of solder paste.

Flexible placing

Cost-intensive manual placing, which is an option for low component quantities, takes a subordinate role. In automatic placing processes, vacuum pipette handling takes predominance over picker arm systems. For vacuum pipette work, either a suitable flat suction surface already exists or one is created with accessories commonly called 'pick and place' pads.

These suction surfaces shouldn't materially affect the speed at which the machine places components. If surfaces are too small, the suction force may be too weak as a result, meaning that the component can't be moved at the required speed.

In addition, for placing machine processes the machine's free working height must be taken into account. This is where the need to limit components' height becomes clear.

Reflow soldering

To create the temperature profile, many kinds of forced convection and vapour phase reflow ovens are used. The use of infrared radiators is not recommended, since they often heat components too quickly, destroying them in the process.

Equally, unnecessarily long exposures - cycle times of more than five minutes - as well as exceedingly high momentary component temperatures over 230°C should be avoided.

Choose packaging individually

Packaging depends directly on the type of mounting. For small quantities and mounting by hand, low-cost ordinary bulk packaging is an option.

But if quantities rise and the full advantages of automated mounting are to be had, then there is a choice of tube feeder, tape feeder or tray feeder (see Figure 4). Any type of packaging must be custom-tailored to the component.

The best suited packaging for a component will depend on the placing process that is used in that instance.

Outlook

THT technology provides options for making the reflow process attractive for mounting PCB components.

Phoenix already provides THT male headers for pluggable connections with 3.81 and 5.08 mm pitches. Additional Combicon and PCB connectors can be checked on request and modified or converted if necessary.

In respect of suitable packing, there has to be close co-ordination with the customer in order to make production costs sustainable for all sides. With an eye to the future, we can be certain that these technologies will establish themselves as the new standard.

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