Welcome to Zephyrtronics! The World's First Low Temp Approach to the Electronic Benchtop.

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	OVERVIEW:
	The introduction of the hot air pencil as a device for reflowing solder sprang from a certitude that processing PCB's at the bench --- whether in prototype, design or rework and repair ---  should always mimic the initial production reflow processes and equipment as much as possible. The hot air pencil made its entry into the PCB production world shortly after the advent of surface mounted technology and during the industry transition away from through-hole technology.  Even the briefest study of any state-of-the-art, high volume PCB production process --- along with the most basic semiconductor data books --- reveal  precise handling methodologies and exacting solder reflow prerequisites. And since it only logically follows that all PCB tasks performed at a benchtop should be held to the same equal standard of high-volume production, some fundamental conclusions can be made as just how to achieve premium solder joints at the benh:
				A Hot AirPencil is Shown Above Used in Concert With a Bottom-Side Preheating AirBath.

• The PCB Assembly should always be carefully thermally ramped at 2ºC to 4ºC with a brief, but required preheating / “soak” stage just before every task (prototype/design/low volume production and/or rework/repair.
• A non-contact and non-contaminating method of reflow (convective) is best suited for placement soldering of ceramic chip resistors and capacitors.
• A non-contact and non-contaminating method of reflow (convective) is best suited for placement of fine pitch and ultra-fine pitch semi-conductor (QFP's).
• When hot air is used to either remove or replace SMD's, the air stream should be pin-pointed precisely to the targeted lead and land only either with hot air pencil “airtips” as part of a Hot Air Pencil or with low velocity hot air nozzles as part of a larger SMD/BGA Hot Air Benchtop system..
• Components adjacent to the SMD that is being removed or replaced must never be exposed to the elevated reflow temperatures.
• PC boards and their components should never be subjected to temperatures greater than originally seen during initial production reflow.
• An accelerated cool down of the solder joint is desirable and recommended to insure robust joints and to prevent accidental skewing, misalignment, and/or tombstoning.
• Static-sensitive electronic components should be protected during PCBA rework from accidental static discharges. The use of an ESD wrist strap by the rework engineer or technician is recommended.

CONVECTIVE vs CONDUCTIVE REFLOW:
A Brief History. Within the global electronics industry today, reflow soldering can generally be divided into two different processes: convection and conduction. Both of these soldering processes have been successfully used for many decades now. However, with the advent of surface mount technology (SMT) and the miniaturization of electronic devices, it appears that the physical contact required during conductive soldering is rapidly losing favor with process, manufacturing and quality engineers. The utilization of high volume conveyor ovens and hot air equipment (convective) is rapidly taking precedence over the traditional contact methods (conductive) such as soldering irons, hot bars, and thermodes.

The successful reflow of solder and the creation of an electrically sound and physically strong solder joint ---between the lead of an electronic component and the tinned copper pad/trace on a printed wiring substrate --- requires serious attention to all of the process parameters involved. One may be surprised to learn the variables present in even the simplest of printed circuit board assemblies (PCBA's).

The Hot AirPencil is Shown Above Soldering a Quad Flat Pack (QFP). Notice the 100% Non-Contact of the AirPencil to the Delicate Leads.

Indeed, within the most nominal printed circuit board assembly, a host of dissimilar materials can be found: copper, fiberboard, ceramic, plastic, tin, lead, silicone, laminates, and more. Each of these distinct materials have varying thermal expansion rates which, if ignored, can cause many compounding problems when subjected to solder reflow temperatures.

Ceramic Chip Resistors & Capacitors and the Arrival of the Hot Air Pencil. As the transition from Thru-Hole to SMT accelerated, an immediate drawback with the use of soldering irons and hot tweezers was discovered: miniature ceramic chip capacitors and chip resistors were cracking or fissuring when being replaced to the PCBA. Because of this cracking problem due to the unequal expansion rate between the ceramic chip and its leads when heating with a contact type soldering device, an application of pin-pointed hot air over the chip became the instrument of choice and known as a Hot Air Pencil.

Today, most every manufacturer of ceramic capacitors and glass diodes have published papers and warnings that they are not responsible for damage done to their ceramic caps induced by hand soldering irons because of the above stated problems and state that soldering irons are prohibited as soldering tools with their devices.

While one could remove a chip capacitor or resistor quickly with a heated contact tool (soldering iron or hot tweezers), it was recommended that the removed chip never be re-used after removal. More importantly, the replacement of these tiny ceramic chip capacitors and chip resistors with contact type tools was rapidly becoming prohibited within many electronic firms to prevent the cracking/fissuring problem described above. Instead of the soldering iron and hot tweezers, the hot air pencil quickly gained favor as a preferred tool for replacing the ceramic chip SMD's. Moreover, the hot air pencil proved to be very effective in the removal of chip capacitors, chip resistors, and SOIC's as well when utilized with standard (non-heated) tweezers.

Also, hot air pencils rather than soldering irons became a preferential tool for preparing or "touching up" pads. Soldering irons can leave tags or ice cycles on the pad causing co-planarity problems between the lead/pad interface. A hot air pencil leaves a smooth pad. This process is now known as hot leveling and underscores the many intrinsic advantages of the hot air pencil's "convective & non-contact" reflow over that of "conductive & contact" reflow type tools.

CONTACT vs NON-CONTACT REFLOW:
A Brief History. The beneficial applications of the hot air pencil quickly transcended any perceived limited scope of utility with only ceramic chip capacitors, resistors, and SOIC's. Around the globe, the hot air pencil was also rapidly becoming the tool of preference for the replacement of a newer type of SMT...the Fine and Ultra-Fine Pitch Devices (FPT & UFPT).

Between 1988 and 1995, the demand by engineers and designers for higher and higher I/0 counts on integrated circuits pushed electronic packages to new extremes. Whereas earlier SMD's had lead counts at 100 or less with pitch spacings typically between 0.040" and 0.050", these new FPT and UFPT components were rapidly gaining market share with I/O counts as high as 250 plus. To complicate the manufacturing and rework processes further, these FPT and UFPT devices now had pitch spacings between the leads as little as 0.015".

The problem that the FPT and UFPT presented to hot contact tools was not so much in the reduction of pitch spacing, but the related miniaturization of the leads themselves. This miniaturization of the component's leads, more than anything else, presented the greatest disadvantage for any contact/hot bar method when compared to the non-contact (hot air) method. The leads became so small in overall footprint (width, thickness, and even extension of the toe) that they proportionately became extremely susceptible to bending, twisting out of their axis and deforming whenever touched by a contact type tool.

The Hot AirPencil is Shown Above Soldering a PLCC Package. Notice the 100% Non-Contact of the AirPencil to the Delicate Leads.

Advantages of Hot Air Pencil Over Hot Contact Tools. Herein lies the tremendous advantage of a non-contact / hot air approach to reflow over that of the former contact type. The AirPencil's hot air simply never "touches" the component or its tiny, fragile, and weak leads. The opposite is true of the hot contact reflow tools where the tool can "stick" to SMD leads; or bend SMD leads upwards/downwards causing co-planarity problems; or tweak SMD outwards/ inwards causing electrical shorts or opens; the hot air pencil is not impacted by this host of problems.

Other Advantages of Non-Contact Reflow. A convective hot air tool allows no opportunity for introducing contamination into the PCBA as opposed to the hot contact tools. Dirt, grime, residual flux and adhesives can and often do stick to contact tools cross-contaminating the PCBA creating even more quality problems.

Also, a hot air pencil can be used to gently pre-heat the surrounding area of the targeted component for removal/replacement allowing for lower temperature settings and shorter reflow dwell times.

Of course, the preferred and most quality-effective, auxillary compliment to the Hot Air Pencil is a temperature-controlled, bottom-side, forced-convection preheating system, today also known as an AirBath™ pre-heater.  Such a mild pre-heating of the PCBA right at the bench can really be of great assistance when processing PCBA's with heavy ground planes, multi-layers, or large heat sinks.

-- David Jacks, 1996 Los Angeles, California

ABOUT THE AUTHOR: David Jacks was Director of Engineering with the two largest soldering equipment manufacturers in the world for nearly 13 years before founding the Zephyrtronics company in 1994 with his business partner, fellow engineer and great friend, Randy Walston.

David's professional design career stretches from the early 1970's. His designed products have been spotlighted in feature articles in both Popular Science® and Popular Mechanics® magazines. He has designed products, tools and appliances marketed by Sears®, Black & Decker®, RadioShack®, Motorola®, Snap-On Tools®, Rubbermaid®, CooperTools®, Farmer Brothers® and Brewmatic®. David holds multiple patents (utility and design) for his many inventions; has authored technical articles for national journals, and routinely speaks to electronic professional societies.