SMD Technologies used in DC/DC Converters

SMD pinned parts are increasingly in demand by our customers. This is mainly due to the following reasons:

1. Pick-and-place machines allow fully automated production, reducing costs
2. Only one assembly process (reflow oven) is required, reducing manufacturing time
3. SMD assembly allows automatic optical inspection (AOI) of the solder joints, increasing reliability.

In double soldering process manufacturing, where a partly completed SMD board goes through a further wave soldering process to assemble the through-hole technology (THT) components is commonly used for the following reasons:

1. The components are large and need a lot of heat transfer to make a good solder joint (e.g. a power inductor with heavy duty copper contacts). If the reflow oven temperature is raised or kept above 260°C for a long time to allow the larger components to reach soldering temperature, the smaller components could become over-heated and fail.
2. Connectors or other mechanically-stressed parts often need THT mounting as an SMD connector would break off in use (the weak point is the glue bonding the copper track to the PCB, not the solder joint to the track or pad).
3. The components need to be mounted to a heatsink, so they need to be lifted up off the PCB on legs (e.g. power transistors).

DC/DC converters with THT pins are still sold in large numbers because if the PCB has to go through double soldering process manufacturing, it does not matter if the converter is SMD or THT. However, customers are always looking for ways to avoid dual process manufacturing if they can. Traditionally THT components such as connectors, fuses, electrolytic capacitors, transistors and transformers are being increasingly offered in SMD pinouts. Sometimes, the DC/DC converter is one of the only THT parts on the board and has to be hand soldered as an extra assembly process. This is because the DC/DC converter cannot cope with the reflow oven temperature stress.

Long term, RECOM needs to increase the number of SMD parts in its portfolio to stay competitive and meet customer demand.

As indicated in the introduction, it is not sufficient to take a standard THT design and simply change the pins to SMD versions. The main issues are the following:

1. The thermal stress in an SMD reflow oven is much higher than in the THT wave solder process where the PCB itself shields the converters from the heat of the molten solder bath. In a reflow oven, especially for taller SMD components, the temperature of the parts closest to the heating elements can be much higher than the temperature down by the pins. For example, to get a temperature profile of 265°C peak measured at the pins, the top of the converter might be close to 300°C. This temperature difference is worse for high volume manufacturing processes where the oven is often fixed at a higher temperature and the speed of the conveyor belt increased until the peak pin temperature just peaks at 265°C. This high temperature gradient can cause internal components with high coefficients of expansion (especially ferrites) to crack or break the solder joints. Encapsulation (potting) can help to even out the internal temperature gradients, but the material itself can also expand under the high ambient temperature and crack the case.
2. A DC/DC converter also contains SMD components. Especially with vapour phase reflow ovens, the temperature of the entire converter can exceed the melting point of the solder, causing the internal solder joints liquefy and expand. If the part is fully potted, the liquid solder inside the converter has nowhere to go except to force its way between the potting material and the PCB, which is the path of least resistance. The result is a higher probability of short circuits on the converter PCB after reflow soldering. Therefore, potting materials should be avoided in SMD parts.
3. THT pins do not need to be very precisely aligned as the PCB holes need to be over-sized to allow the solder to wick up and make a good joint. The advantage is that the pins can be out-of-true by as much as 0.25mm without overly affecting the assembly process. On the other hand, SMD pins need to be very precisely aligned and flat (coplanar) to within a tenth of a millimetre. If even one pin is bent out of line, it will either not make contact with the pad or lift up its adjacent pins so that they do not make contact with their pads. Maintaining a precise coplanitarity is one of the most difficult manufacturing challenges for SMD pinned devices.

There are at least seven different ways we know of where SMD pinning can be realized. Each method has its advantages and disadvantages.

SMD pins are embedded into an injection moulded plastic carrier. The PCB is then soldered on to the projecting pins. This is one on the most commonly used SMD pinning methods.
This process is used with the following RECOM products (and many others):

Advantages:

1. The pins are accurately positioned by the mould tooling with precise coplanarity.
2. The projecting pins on the other side of the carrier can be made any length so the same concept can be used for different thickness PCBs and both DC/DC and AC/DC products (where the board is often inverted and mounted on long pins)
3. The thickness of the carrier can be reduced for a low-profile design or increased to leave space for SMD components mounted underneath the PCB. A double-sided PCB layout significantly reduces the footprint size.
4. This method is well suited for mass-production. V. The solder joints are inspectable.

Disadvantages:

1. A specialist supplier is needed who is able to embed metal parts in a plastic mould. Only small sized pins can be successfully embedded as larger pins would absorb too much heat from the injected thermoplastic which would result in moulding errors.
2. The initial tooling costs are high.
3. For large flat carrier sizes (e.g. DIP24) the plastic frame can warp during the soldering processes due to the residual stresses in the plastic carrier. The carrier may need reinforcing ribs if it starts to twist, warp or bend during customer reflow soldering. Changing an existing tooling to add the extra rib material is expensive.
4. The thin pins are easily bent or misaligned in transit which could cause coplanarity problems, so the packaging is complicated and more expensive.

Bent square THT pins fixed in a header are glued into position on the PCB and soldered into place using pin-in-paste reflow soldering or hand soldering.
This process is used with the following RECOM products: R-78T series

Advantages:

1. The square pins are accurately positioned in a plastic header and bent with precise coplanarity by a mechanical bending jig.
2. Square pins are stronger than other SMD pins and can be used to support larger and heavier products or higher-current designs.
3. Square pins are cost effective as header pins are commonly used connector pins and made in very high volume.
4. The pins are usually gold plated so solder well without the need for solder flux.
5. This method is well suited for mass-production if a pin-insertion machine is available.
6. The solder joints are inspectable.

Disadvantages:

1. A accurate tooling jig is required to precisely fix the pins
2. The pins need to be glued into place to stop them moving during assembly or during the customer reflow process. This is an extra manufacturing process.
3. The pins need a large amount of space on the PCB, so this method is suitable for larger PCBs only.
4. Longer pins are easily bent or misaligned in transit which could cause coplanarity problems, so the packaging is complicated and more expensive.

Half-vias are added to the edge of a flat PCB and gold plated to connect between the top and bottom layers.
This process is used with the following RECOM products:

Advantages:

1. The solder pads connected by half-vias are inherently coplanar.
2. The half-vias are part of the PCB production process, so no additional pin assembly process is required.
3. This is a relatively cost effective solution.
4. This method is useful for multiple pins as the production cost does not increase significantly with the number of connections.
5. The pins are gold plated so solder well without the need for solder flux.
6. This method is well suited for mass-production.
7. The solder joints are inspectable.
8. There are no pins to get bent or mis-aligned in transit, so packaging is simplified and the risk of transport-damage is reduced.

Disadvantages:

1. Plated half vias add cost to the PCB manufacturing
2. The underside of the PCB must be completely flat, so only top-mounted components can be used. This increases the size of the PCB significantly.
3. The vias need a relatively large amount of space on the PCB, so this method is not suitable for very small sized products.
4. Care has to be taken that the product does not misalign during customer reflow soldering as the entire PCB floats on molten solder. For example, the ROF design shown on the previous page has half vias at each corner so that the connections are symmetric and the pull of the surface tension on the joints is evened out.

A lead-frame is stamped out of metal sheet. The converter PCB is ...