Selective soldering--the future is now: optimize your through-hole soldering process down to the pin level.
Recessions and economic slowdowns rarely have an adverse effect on the evolution of technology. In fact, such times can serve as catalysts for process efficiency improvement, as is the case with soldering through-hole components on mixed-technology printed circuit board assemblies (PCBAs).
We all know that through-hole components will persist for a majority of applications. Usually, they will be connectors or switches that, due to the application and its environment and reliability expectations, require the extra mechanical robustness of a through-hole interconnection. Also, a number of power-related components do not yet exist in surface-mount packaging.
Wave soldering has been around for nearly 60 years and has not changed much. While some attempts to adapt wave soldering to solder both surface-mount and through-hole components offer improvement, in terms of process yields they all amount to essentially duct taping the process. Wave soldering never anticipated surface mount, and, therefore, we should not have high expectations. We have yet to see zero-defect wave soldering of surface-mount assemblies.
So, what to do? The first choice of many practitioners is to see if intrusive soldering (such as reflow of through hole, paste-in-hole or pin-in-paste) is applicable. With modern convection-dominant reflow ovens, intrusive soldering depends on whether the components can survive the incurred thermal excursions of the reflow profile; component survival may become a problem with the advent of lead-free solder and surface finish alloys that require higher reflow temperatures.
When you cannot run the mixed-technology board through and the volume or reliability expectations rule out hand soldering, we turn to selective soldering. As the name implies, we selectively solder only the through-hole components after the surface-mount components have been reflow-soldered.
The most common method of selective soldering uses masking pallets. The PCBAs are mounted in pallets with through-hole interconnections. The pallets are run through the wave-soldering machine, and the wave only contacts the through-hole interconnections-not the surface nmunt components that were previously soldered. You're thinking this option sounds pretty cool-I can use my existing wave solder machine with no new equipment! But some limitations exist regarding the proximity of the cutouts. Also, the pallets have to be beveled, which is expensive. Some high quality ones cost around $1,000 each--and, if you have any kind of volume, you are going to need quite a few of them. Don't forget the manpower necessary to load and unload the pallets.
Down to Details
Selective soldering falls into two categories: PCBA-level and micro soldering. We discussed micro-selective soldering in the December 2001 On The Forefront column, so now we will look at PCBA-level selective soldering. This methodology is slowly replacing wave soldering.
The most common types of PCBA-level selective soldering are point-to-point and multi-wave systems. Point-to-point systems use a fluxing spray head and a single point solder nozzle moved by a gantry or robot to select the exact solder locations on a PCBA. Multi-wave systems have multiple nozzles that solder several locations simultaneously. Both systems have advantages and disadvantages; the biggest tradeoff is between flexibility and application-dependent throughput or increased thermal transfer and speed.
Benefits Are Made of This
As with every technological advancement, selective soldering has obvious benefits: It allows users to optimize the solder process down to the pin level verses the compromise techniques in flux application and contact time that occur with wave soldering. Since flux deposition, nozzle height, solder dwell time and peel-off parameters are fully programmable and can be optimized for individual components, selective soldering is ideal for thermally-challenged through-hole components.
Selective soldering not only improves solder quality, but also minimizes thermal coefficient of expansion (TCE) issues with minimalistic surface mount components that do not survive the thermal shock of wave soldering. Selective soldering can even reduce or eliminate the bottom-side gluing process for surface-mount passives. Reducing hand soldering and eliminating secondary masking and gluing operations results in significant labor savings. And, since selective soldering is a data driven process, it improves your time to market by eliminating dedicated tooling and the lead time to order or make changes.
Selective soldering is also friendly to the environment since its flux and solder usagge and emission levels are significantly less than wave soldering. We know of cases where users operate their selective systems with self-contained flume extractors, eliminating roof venting and fire suppression systems entirely. When was the last time you had a plant visit from your local air quality agency?
Selective soldering is ideal for soldering PCBAs with high component density since it can maintain clearances between through-hole pads and adjacent surface-mount pads that are not achievable with masking pallets or bench-top solder fountains. Selective soldering also allows you to solder through-hole components on both sides of PCBAs without restrictions in component height. Masking pallets are limited in depth, so they do not adversely affect the dynamics of the wave soldering process; as a result, they cannot shield tall components on the solder side of the PCBA. Selective soldering can process tall bottom-side components, replacing hand soldering for those "ugly" through-hole components in the last assembly step.
The biggest advantage selective soldering offers is the ability to reduce solder defects, thus improving first pass yields. Soldering a PCBA at a faster rate in a masking pallet, only to have to perform secondary hand soldering, "touch-up" or other manual operations, does not result ill an efficient operation.
With the continued convergence of surface mount and through hole for the foreseeable future, we need to pay more attention then ever to through-hole solder joint quality. Selective soldering is rapidly becoming the preferred method to improve solder quality, reduce conversion costs and increase factory utilization. Remember, we're all in this together.
Acknowledgment: The authors would like to thank Vitronics Soltec.
Phil Zarrow (above) is president and surface-mount process consultant, and Bob Klenke (below) is a principal consultant, both with ITM Consulting, Durham, NH; (603) 868-1754;www.ITM Consulting.org.