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Selective soldering--the future is now: optimize your through-hole soldering process down to the pin level.



"Any tool can be the right tool."--Red Green

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 soldering

Process that uses metal alloys with low melting points to join metallic surfaces without melting them. Tin-lead solders, once widely used in the electrical and plumbing industries, are now replaced by lead-free alloys.
 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 Applying liquid solder to the underside of printed circuit boards in order to bond the chips and discrete components that are placed on top of the board and whose metal leads (pins) extend through the board.  has been around for nearly 60 years and has not changed much. While some attempts to adapt wave soldering to solder solder (sŏd`ər), metal alloy used in the molten state as a metallic binder. The type of solder to be used is determined by the metals to be united. Soft solders are commonly composed of lead and tin and have low melting points. Hard solders (i.  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.

Decisions, Decisions?

So, what to do? The first choice of many practitioners is to see if intrusive soldering (such as reflow (1) The process of heating and melting the solder that has been screen printed onto a printed circuit board in order to bond chips and other components to the board. Surface mount chips (SMT) use the reflow method. Contrast with wave soldering. See also reflowable text.  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 Selective soldering is the process of soldering only through-hole electronic components onto a printed circuit board that has surface mount components on the under-side. This is usually done because the surface mounted component is not glued into place, instead solder paste is used . 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 Pronounced "sod-erd." Permanently attached by a hard metal bond. In order to replace a chip soldered to a circuit board, it requires heating the soldering joints until they melt. Contrast with socketed. . 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 bev·el  
n.
1. The angle or inclination of a line or surface that meets another at any angle but 90°.

2. Two rules joined together as adjustable arms used to measure or draw angles of any size or to fix a surface at an angle.
, 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 Gantry
A name for the couch or table used in a CT scan. The patient lies on the gantry while it slides into the x-ray scanner portion.

Mentioned in: Computed Tomography Scans
 or robot to select the exact solder locations on a PCBA PCBA Printed Circuit Board Assembly
PCBA Physically Challenged Bowhunters of America
PCBA Polk County Builders Association (Florida)
PCBA Punjab College of Business Administration (Pakistan) 
. 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 See thermal wax transfer printer and direct thermal printer.  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 The time cargo remains in a terminal's in-transit storage area while awaiting shipment by clearance transportation. See also storage.  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 Noun 1. coefficient of expansion - the fractional change in length or area or volume per unit change in temperature at a given constant pressure
expansivity

coefficient - a constant number that serves as a measure of some property or characteristic
 (TCE TCE

trichloroethylene.

TCE Environment A volatile chlorinated hydrocarbon that boils at 88ºC and is highly soluble–1000 ppm in water, with various industrial uses Toxicity Peripheral neuropathy, carcinogenic.
) issues with minimalistic surface mount components that do not survive the thermal shock Thermal shock in mechanical models

Thermal shock is the name given to cracking as a result of rapid temperature change. Glass and ceramic objects are particularly vulnerable to this form of failure, due to their low toughness, low thermal conductivity, and high
 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 Fire suppression systems are used in conjunction with smoke detectors and fire alarm systems to improve and increase public safety Types
  • Fire sprinkler systems (wet, dry, pre-action, and deluge)
  • Gaseous agents
  • Wet and dry chemical agents
 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.

Conclusion

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 We're All In This Together can refer to:
  • "We're All in this Together", an OST from the High School Musical Soundtrack.
  • We're All In This Together (sketch), a Malaysian sketch about school life.
.

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 ITM

See: In-the-money
 Consulting, Durham, NH; (603) 868-1754;www.ITM Consulting.org.
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Title Annotation:On the Forefront
Author:Klenke, Bob
Publication:Circuits Assembly
Date:Apr 1, 2003
Words:984
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