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Step 6: Component Placement
December 31, 1969 |Estimated reading time: 8 minutes
Cost savings is the primary reason for the growing interest in through-hole reflow (THR) technology. Its cost advantages become obvious when one considers that today's wired components not designed for SMT usually comprise only 5 to 10 percent of the components on a printed circuit board (PCB), but the cost for putting them on the board is disproportionately higher. This step describes the integration of THR into SMT processes.
By Magnus Henzler
The primary objective is to process SMD and THR components using the same machines and procedures in a single step. Using THR makes it possible to eliminate the otherwise necessary wave- and selective soldering (or press-fitting), saving time in terms of handling and logistics while reducing manufacturing machinery requirements and floor space. SMT/THR production flow typically consists of:
- Pressing solder paste onto the underside of the assemblies
- SMD placement
- Reflow soldering
- Applying paste to the top side
- SMD placement
- THR placement
- Reflow soldering.
The basic method for using THR connectors is the "pin-in-paste" procedure, involving insertion of the terminal pins into drilled through-holes filled with solder paste, and then soldering using the reflow method. The result of the solder paste melting in the through-hole and on the contact tip is a solder joint similar to classic wave soldering.
Before THR can be integrated into the automated SMD manufacturing process, several issues regarding component selection and soldering must be considered. These include compatibility of components with higher temperature profiles, suitability of components for vision systems, pin grid, height and weight, positioning and seating force, form of the contacts, PCB layout, solder paste application, and reflow.
The primary task when changing over to the THR method, apart from selecting suitable connectors, is validating the manufacturing process. A few rules must be observed with regard to the PCB layout and design of the template for screen printing and the doctor blade parameters. Automated fitting of odd-form components may require greater expenditure and expertise. Any "reflow-capable" connector can be placed automatically. Only new-generation automated placement machines can process higher-level components and create greater seating force for components such as snap clips. However, minor factors such as the lack of feeders and automated packaging also may create problems.
THR connectors and connectors for wave soldering have been found to be suitable for reflow soldering. Temperature profiles with temperatures of around 190°C for a few minutes, as well as short periods of exposure in the range of 220° to 240°C, are not a problem with a suitable plastic. The pending use of lead-free solder in electronics production make temperature increases of 30° to 40°C necessary. These connectors are compatible with these temperature profiles as well (peak maximum 260°C).
In THR, the housing must not come into contact with the solder paste, yet optimal heat flow to the solder joint must be guaranteed. The component should have a minimum heat capacity and not draw heat away from the solder pins or mask them.
The components also should be designed to fit into standard process packaging. Pin length must be appropriate to PCB thickness and type of application. Pin projects also should be about 1 to 1.5 mm on the underside of the PCB so a final assessment of the soldering joints may be made in accordance with IPC-A-610C.
Based on placement precision of automation equipment and component tolerances, the diameter of drilled through-holes should, depending on pin sizes, be 20 to 50 percent bigger than pin diameter. For connectors with few contacts, the drilled-hole diameter can be smaller. Fastening should not require a lot of snap-in force, since SMT equipment supports relatively little press fitting force (10 to 20 N).
The surrounding ring width of the solder land should be optimized in reference to the required clearance and creepage distances and the space available below the component. The more defined the solder globule to be formed is and the greater the mechanical load capacity must be, the wider the ring of the land for soldering should be, a minimum of roughly 0.25 mm.
The amount of solder paste necessary for THR soldering joints is generated mainly on the basis of the volume in the drill hole and a defined push-through amount. Thus, the amount in the template area is negligible and, in contrast to pure SMD components, the template thickness is of little relevance. Today's template thicknesses of 150 µm, trending to 100 µm, require no new procedures. The template cut out should be 0.1 mm smaller in diameter than the solder pad to avoid solder pearl formation. When estimating the solder paste amount, remember that only about 50 percent of pure solder remains for form-ing the solder joint. The other elements are auxiliary materials such as flux (Figure 1).
Figure 1. THR soldering joints should be at least 75 percent filled.
A template for screen printing exists with which SMD fine-pitch and THR structures can be supplied with their respective amounts of solder paste. For screen printing (open system), the parameters are best set so that the solder paste is pressed into the THR holes 1 to 1.5 mm for a 1.6-mm thick PCB.
The suitable choice of solder paste based on the system can lead to better results. Other possibilities for increasing the solder paste volume (fill ratio) include pressure doubling and using a closed-pressure system, with which a 100 percent fill can be achieved.
Tape-on-reel packaging is especially well suited for processing with conventional pick-and-place loading systems, as standard feeders can be used. Removal from the reel is done with standard vacuum pipettes applied to the suction surfaces on the insulating sleeves, or applied on the shrouds of the connectors.
Tape-on-reel feeders more than 75 mm wide are not commonly used. The fitting speed must be adjusted to the component, which is why the performance range of the fitting device must be taken into consideration with regard to component size when THR product selection is made.
Like SMT components, THR components must be easily recognizable. For this reason, THR components often have black insulating housings to enable a high-contrast picture when using a camera (Figure 2).
Figure 2. Black insulating housings facilitate recognition during automated fitting.
The pins of the component subsequently are inserted into the drill holes, and a portion of the solder paste is pushed out of the drill hole (pushed through). The pushed-through paste loses contact with the remaining mass in the drill hole, to the head of the pin.
Forced convection or vapor-phase reflow ovens provide heat management capable of reacting to the requirements of different assemblies. THR connectors are suited for a wide range of temperature profiles. No special adjustment of the soldering process is necessary concerning THR solder points.
After the heating phase, the PCB reaches a temperature range for melting the solder, and the actual THR soldering process begins. Even before reaching melting point temperatures, the solder flux begins to flow. Densification of the solder globules progresses, and the concentration on the head of the solder tag is apparent. The tag head reaches liquidus temperature; the solder is already melted in this area. The solder mass moves up on the flanks of the terminal toward the drill hole. Capillary action pulls the melted solder into the gap between the component pin and the PCB hole.
There are two essential factors when inspecting the solder joint: the degree to which the hole is filled and the peripheral wetness in the soldering globule area. The fill ratio can be checked only by random sampling by grinding cross-sections. Optimal THR solder joints have a fill ratio of nearly 100 percent, with a minimum of 75 percent (IPC-A-610 Revision C). Gas pockets resulting from the process do not have a negative effect. Assessment of solder joints usually is done using a microscope. IPC-A-610 Re-vision C also defines the minimum conditions for solder peripheral wetting between pin and sleeve on both sides and the percentage of solder land on the primary and secondary sides. Thus, the solder globule formation and peripheral wetting also must be checked.
In general, even with minimum rings, through terminals create acceptable solder menisci. With formation of the menisci on both sides of the circuit board and concurrent peripheral wetting of nearly 360° of both globules (acceptable minimum is 270°), THR solder joints meet the IPC requirements.
Template geometry and doctor blade parameters have the most impact on the fill ratio in through plating when using open doctor blade systems. ZVE studies show that conventional template parameters and a minimum amount of solder paste meet minimum requirements of IPC-A610-C.
The range of products for THR technology continues to expand. DIN components in compliance with IEC 60603-2 are just one example. Insulating housings made of glass fiber-reinforced PA-46 plastic can withstand temperatures up to 260°C, making this material suitable for lead-free solder processes. To simplify changeover, some THR variants are layout-compatible to existing press-fit connectors. Some PCB connectors also are black to enable better recognition for automatic placement.
Figure 3. Connector with SMT pins for signal and THR connections for the shield contacts.
Some differential connectors also have SMT/THR connections (Figure 3). The terminals of the signal pins are of SMT design with tested coplanarity of less than 0.1 mm, and the shield contacts with THR connections. This combination offers the HF advantages of SMT terminals. This supports data rates of 10 Gbit/s and more. In addition, the reflow-soldered ground terminals assure robust placement with strain relief. The permissible auxiliary forces of a THR connection are between four and eight times greater than comparable components in pressfit technology.
Conclusion
Product optimization, observance of layout rules, template design, and adaptation to existing production steps all mean that THR connectors can be integrated into SMD processes. In principle, both of these connection technologies can be processed concurrently using the same procedures and equipment.
Magnus Henzler, product marketing manager, may be contacted at ERNI Electronics Inc., 3005 E. Boundary Terrace, Midlothian, VA 23112, (804) 228-4100; E-mail: Magnus.Henzler@erni.de.