Beyond the obvious benefits of surface mount technology (SMT) over traditional through-hole technology (THT or Thru-Hole), there is one immediately recognizable advantage with thru-hole: the strength of the solder joint.

In thru-hole assemblies, the leads of a component first penetrate through the substrate (some are even clinched on the other side) and afterwards, the lead is soldered.   And with plated thru-hole PCB's, the solder wets down inside of the plated hole adding strength to the pad/lead interface.

High Reliability Soldering. It is easy to appreciate why such solder joints provide high expectations for reliability (Figure 1).

However, with SMT, only the tiny solder joint alone a top of the substrate's surface must provide all of the mechanical connection in addition to the electrical connection. Therefore, the quality of a solder joint on SMT assemblies is far more critical than traditional thru-hole solder joints requiring greater attention process parameters when surface mount soldering

For integrity and strength at each solder joint, it is imperative that an SMD's leads always have fillets at the toes, heels and sides. This is true whether the soldering is made during initial production or later in rework, but. unfortunately, while such fillet criteria has been standard in production processes, it is still universally mostly ignored at the bench where PCB prototyping and rework is performed.

Most inferior SMT solder joints made at the bench result from two hangover processes used with THT: a.) using solder wire rather than solder paste; and b.) using contact soldering irons rather than non-contact hot air.

Where's the solder wire? There is not a single high-volume SMT soldering that utilizes solder wire. Ponder that. Either the popular use of solder paste in reflow ovens or molten solder with wave soldering are the proven, established norms.

So what about solder wire? Historically, solder wire (typically cored with flux) was a staple with THT for decades at the bench. For over half a century! Where wave soldering was not feasible or affordable, the long assembly line of personnel outfitted with soldering iron stations was the status quo for all those years in electronic production.

An old, disappearing paradigm. And sitting alongside each soldering iron there was always the ubiquitous spool of solder wire. And so it followed that the individual rework bench was a mere snapshot of the assembly line: soldering iron, spool of solder wire, wick, bottle of flux, etc.

This world changed beginning in the late 1970's and early 1980's with the introduction of surface mounted chips when components became smaller and could be soldered on both sides of a PC board and the old traditional methods of soldering at the benchtop were becoming obsolete.

The advent of SMT created a proportional decline in the traditional THT assembly and with it traditional contact soldering iron and solder wire. The trend is irreversible. Try reflowing a BGA with an iron and solder wire.

Curiously, as solder wire and the traditional conductive soldering iron were together transcending into history at the production level, they were still struggling for survival at the rework bench, and with lots of problems. (Figure 2)

Soldering iron woes: Some of the more notable problems related contact soldering irons and solder wire with SMT were: lack of solder fillets at the toe, heel and sides on both J-leaded and gull wing devices; inducing pad damage and co-planarity problems due to hand pressure from the iron while contacting the chips; and cracking ceramic devices due to sudden application of direct conductive heat from soldering irons to these delicate devices. Necessity became the mother of invention: the hot air pencil.

First, soldering paste has proven to be a near perfect compliment to high volume SMT production reflow. The process is as follows: A.) Paste is dispensed or screened to the printed circuit substrate; B.) components are seated into the paste at the pick and place sequence; C.) preheating at first tunnel of oven activates flux within the paste (and warms the board); D.) solder reflow in last tunnel as solder paste wicks up SMD leads yielding premium solder joints replete with fillets; and E.) quick cool down of the PCB assembly for strong solder joints.

Paste is Trumping Wire. While it is somewhat new to the rework bench (there were some early converts years ago), solder paste is now rapidly gaining favor over the use of solder wire when prototyping new designs or reworking SMT assemblies.

Thermal Profiles & Solder Paste: With a convective preheating device, (Airbath™) under the board assembly, solder paste can be reflowed with a hot air pencil from above yielding solder joints that rival those from the conveyor oven.

More Forgiving. Solder paste melts and reflows at the lead yielding fillets at the toe, heel, and sides of the device right at the bench. (Figure 3) In fact, the use of paste can even forgive some co-planarity problems as the solder wicks up the lead from the pad.

Preheating during benchtop rework is as integral to the reflow process as it is in initial production. Also, activation of the flux during pre-heating cleans and enhances the wetting process resulting in smooth and shiny joints that will feather out to the a thin edge.

"These basic processes have the advantage that they are simple, require minimal training and virtually never cause damage to the assembly," Jerry Green, a senior manufacturing and quality engineer in San Diego points out, "you can actually see the solder reflow and become shiny" as the AirPencil passes over the leads. Green emphasizes it is critical that the SMD sit flat on the pads, that the paste be non-rigid, that the SMD not be moved once preheating has begun, and proper inspection be made after reflow.

Solder paste, not wire? Yes, as in high-volume production processes, instead of solder wire? Right at the bench? You bet.

Forced convection from a hot air pencil like in high-volume production processes, instead of contact, soldering irons? Right at the bench? You bet.

Ramping at 2°C to 4°C as in production? Preheating the PCB allowing flux activation before final reflow as in production? All at the bench? You bet.

Video: How To Use Solder Paste?

Some Final Thoughts: Use of solder paste coupled with a hot air pencil and a warm air bath at the bench is rapidly gaining favor due to the many evident advantages over solder wire and contact soldering irons. A few last points:

1.) It is not advisable to use solder paste with a soldering iron as it gets very messy;

 2.) Hot air pencils are less effective without pre-heating. Green stresses that "preheating cannot be rushed";

 3.) Air pencils require low velocity hot air so not to blow solder paste/balls across your PCB;

4.) A hot air pencil is not to be confused with the larger hand-held hot air jets with larger nozzles that enclose the entire component. The air pencil provides only pin-pointed hot air to one lead at a time.

Finally, for a quick, step-by-step, color pictorial presentation of how to solder SMD's with solder paste and a hot air pencil, please visit our helpful primer at this direct link: "SMD Soldering Made Easy at the Bench". Or watch a live video demonstration of how to use solder paste with surface mount chips.

David Jacks was Director of Engineering at three Fortune 500 corporations along with the two largest soldering equipment manufacturers on earth for 13 years before launching Zephyrtronics in 1994 with fellow engineer, Randy Walston.

David's professional design career stretches from the early 1970's. His original products have been spotlighted in feature articles in  both Popular Science® and Popular Mechanics® magazines and have ranged from commercial coffee brewers and radio frequency controlled residential garage door openers to hobby glue guns, professional heat gun paint strippers and sophisticated industrial soldering equipment.

He has designed products, tools and appliances marketed by Sears®, Black & Decker®, RadioShack®, Motorola®, Stanley Tools, Snap-On Tools®, Rubbermaid®, CooperTools®, Weller®, Hakko®, Ungar®, Farmer Brothers® and Brewmatic®.

Any electronics catalog of soldering equipment, tools and products today reflects David's long and enduring influence on the printed circuit board industry world-wide.

David holds many patents (both utility and design) in North America, the European Union, Japan and around the world.

His patented inventions have been cited as prior art by firms from IBM to Mitsubishi. He has authored technical articles for international journals, and routinely speaks to electronic professional societies.

Now in his old age, David's keen interests is in encouraging inventors and designers to "stick with it" and never to surrender their dreams, and to "make the impossible possible" through science and technology.