Feasibility of soldering fine pitch THT components Gerjan Diepstraten, Vitronics Soltec

Introduction: One of the major trends in circuit board assembly is the drive to smaller components and pitches. Where the focus is on SMD (Surface Mount Devices) and also THT (Through Hole Technology) the designers have the intention to go smaller and smaller. The result is less space on the boards but with increased functionality. The Roadmaps from IPC and iNemi mention a minimum pitch of 40 mils (1.00 mm) in the near future. The physical properties of the lead-free alloy, flux and solder mask give the engineer problems to set-up a consistent and robust soldering process for these smaller devices.

The method of soldering THT components depends on the number of components to alloy and available time on the machine. For low volume products a sequential method (point to point soldering) is most likely the most cost effective way. Available methods, apart from hand soldering, are robot, laser or selective soldering using a mini wave nozzle. For high volume assemblies a simultaneous soldering method is preferred because of the shorter cycle times. There are three main ways that THT components could be soldered. The first is in a reflow oven (using Pin in Paste technology). The second would be in a traditional wave solder machine using pallets that cover the small SMD’s. Final solution could be a selective soldering application which would involve dipping the complete assembly on to a dedicated nozzle plate.

Pin in Paste reflow: When a pin in paste reflow process is being considered for the fine pitch THT component soldering the complete SMD line has to be compatible. This starts before the oven with the solder paste, screen printing and component placement.

The requirements for the solder paste are:

1. Powder particle size should meet fine pitch SMD/THT criteria so would most likely be a Type 4 or smaller. 2. The flux should have good hot slump properties (to avoid bridging and solder balls). 3. Flux should be tacky to avoid solder paste dripping off in the oven during the heating process.

More critical is the clearance on the assembly that is required to have enough solder to fill the complete barrel. Overprinting solder paste is unavoidable. In many applications where fine pitch SMD components are used a step stencil might be needed to apply enough paste. This requires free space and increases the area for these THT components. Printing twice (forward and backward on stencil) would bring more paste in the barrel. A rough calculation shows that an aperture of 3.00 x 0.85 mm for a 4 mil (0.1 mm) stencil is required.

Summary: No additional process steps would be needed but space on the board would be required for overprinting of solder paste. Small pitch THT would be feasible with Type 4 or smaller paste. Wave soldering with pallets: Wave soldering is the most traditional way to solder THT components. Nowadays more and more SMD components are mounted on the solder side of the assembly. These small components need to be covered as the board passes through the liquid solder wave. Covering the SMD’s would avoid re- melting of the solder paste that connects them to the surface. This requires a small dam (= space) between component and pad of the lead to be soldered. Design rules for SMD pallets are generally available.

The soldering process as such is well understood but changes are that the smaller pitch devices have resulted in smaller apertures in the pallet. The flux as well as the solder has to penetrate into these smaller apetures. Possible shadow effects on the pallet may also result in defects. There should also be no gap between the bottom side PCB and pallet otherwise flux may flow between this gap. Should there be a gap between the PCB and the pallet then liquid solder could flow through this gap and could result in defects. Also flux that flows into any possible gap between the pallet and PCB is a potential risk for electro-migration should it not be activated.

The fine pitch THT should not have long leads. In today’s manufacturing processes engineers try to limit the length of the lead to a maximum 0.5 mm. Leads longer than 0.5 mm could result in bridges. Soldering in pallets needs turbulent waves to push the liquid solder into the apertures to achieve a good hole filling. If the force from the wave is too strong it could force solder between the PCB and pallet. Conversely a reduced force from the wave or too low of a pump speed could result in open joints. Wave solder processes with pallets can handle 1.27 mm pitch components. The IPC-A-610E requires that the lead should be discernable at the solder side of the joint (chapter 7.3.5). In cases of short leads this is critical and the industry may want to review the standard for these small pitch THT components.

Summary: Pallets require covering of SMD components. Space is needed in the design of the pallet to let solder reach the small leads. Wave soldering of fine pitch components in a selective pallet is feasible.

Selective soldering: In selective soldering the joints are soldered with a dedicated nozzle. SMD’s on the solder side would not be in contact with the liquid solder so as such pallets would not be needed. The free space that would be required would be 1 mm from the end of the pad to be soldered. All SMD’s that are reflowed before the soldering process only reach approximately 130-150 ºC during selective soldering so there is no risk for re-melting of the previously soldered joint.

The flux should be applied to the areas that are in contact with the solder. The flux should not spread (different from wave soldering) and stay where it’s applied. Important parameters here are the solder mask, flux properties and amount of flux. Too much flux in areas with SMD’s may cause electro-migration. The solder temperature is higher (up to 320 ºC) in comparison to the other processes which means a strong flux chemistry is required. Fine pitch point to point soldering can be done using either wettable or non-wettable nozzle technology. If there is the ability to tilt the board then the distance to larger components would be smaller (due to the solder angle). In today’s current production with small nozzles then 1.27mm components can be soldered. However, the protrusion length should be as small as possible to avoid bridging. Tools like SDC (solder drainage conditioner) will help to avoid bridging by adding additional nitrogen in the soldering area and disturbing the capillary force of the solder between leads.

Selective soldering by a multi wave dip soldering process with a dedicated nozzle plate is faster. This allows all solder joints to be made in one dip on a plate with nozzles mounted in the solder bath.

Design rules for the nozzle plate would need to be adhered to but a limited distance from pad to the next component is allowed. However, the component should have a free space of 1 mm to the nozzle. The risk for bridging increases when the board is dipped in a vertical direction. Bridging can be eliminated by the use of laser cut screens. Due to the small pitch screens can be used till 1.5 mm. Smaller pitch requires smaller screens. This technology would need to be improved in the foreseeable future. The alternative for screens are wettable strips that are mounted just under the solder surface to drain the solder away from the leads. The nozzle width is limited to 4 mm for dip nozzles. Smaller nozzles have a less consistent solder height.

Summary: Multi wave dip soldering can solder one board in 20 seconds. But results show that with fine pitch connectors a 4 mm width nozzle is the minimum width for a robust soldering process. Select wave point to point soldering with as small nozzle is the process that allows for the closest spacing, but cycle time could be an issue for higher volume products. Selective soldering with a tilted board and a wettable nozzle including the SDC option returned the highest yield for fine pitch in the DoE.

Contact: VITRONICS SOLTEC INC. 1629 Old South 5 Camdenton, MO 65020  573-317-3008 E-mail: [email protected] Web: http://www.vsww.com