Ask the Experts
April 18, 2018
Reliability of Automated Soldering vs. Hand Soldering
On occasion, we have had to place a skip in our SMT placement machine due to unavailability of a surface mount component. The intention is to build the circuit card assembly and then hand place and solder the missing components when available. I accept that there are times when you have to rework/touch-up/replace components, but these should be kept to a minimum.
Can you comment on the potential differences in reliability for SMT machine placement followed by reflow soldering vs. hand placement followed by hand soldering of surface mount components?
Expert Panel Responses
There are three main sources of differences in reliability:solder joint behavior, component damage, and cleanliness. Hand soldering caninvolve tool tip temperatures of 400C and ramp rates of more than 100 C persecond, and we need to consider the effect on the component. Multi-layer chipcapacitors, particularly larger ones, are a high risk for damage during handsoldering, for instance. If you have guidelines for rework that address the acceptableprocesses for specific components, you can rely on these guidelines for thehand-installation of the components.
The differences in solder jointreliability are hard to predict, and in most cases not your biggest concern, aslong as you meet the applicable standards for soldering quality. Speaking ofsoldering quality, remember that your defect rate for hand-assembled productwill almost always be higher than for automated assembly. The cleanliness riskis real, but as long as you have well-developed processes for hand solderingthat include ensuring cleanliness of finished assemblies, you should be in goodshape.
In addition to the risk to the components being soldered, thereare additional product risks, such as the risks of ESD or mechanical damageduring the additional handling and processing necessary. It might not be easy,or even possible, to trace failures related to these additional risks back tothe manual assembly process.
In general, we try to avoid the "build short" practice as much as possible.The added cost of the manual labor and the reliability and quality risks arerarely worth it.
Fritz's career in electronics manufacturing has included diverse engineering roles including PWB fabrication, thick film print & fire, SMT and wave/selective solder process engineering, and electronics materials development and marketing. Fritz's educational background is in mechanical engineering with an emphasis on materials science. Design of Experiments (DoE) techniques have been an area of independent study. Fritz has published over a dozen papers at various industry conferences.
Hand-soldering of SMD's can be as reliable as an SMT reflowprocess but some care is required. The important thing to remember is to avoid excessive temperatures and contact times with thesoldering tip.
The area of concern is components which cannot tolerate fastramp rates and are prone to cracking or delamination issues.Hand-soldering heats parts up very rapidly and within a secondor two the temperature is well above the melting temperature of the alloy. Reflow soldering slowly increases temperature and reduces theeffects of CTE mismatches of materials as well as thermal shock issues.
It is important to be aware of the components ability towithstand higher temperatures and also ramp rate requirements as to avoidissues.The other point is to avoid heating the whole component but solderingthe terminations only, this requires the use of the correct tip geometry.
Senior Market Development Engineer
Mr. Biocca was a chemist with many years experience in soldering technologies. He presented around the world in matters relating to process optimization and assembly. He was the author of many technical papers delivered globally. Mr. Biocca was a respected mentor in the electronics industry. He passed away in November, 2014.
A process is only as good as its repeatability. Handsoldering is not a repeatable process. Oxides on the soldering iron tipor on the solder, pressure applied, heated contact area, flux application andoperator skill can all have a significant impact on the finished solderjoint. One can have a very lovely solder joint on the outside but a badjoint on the inside. The interface between the bulk solder and the solder landor the soldered component lead can have a dramatic effect on the reliability ofthe finished solder joint.
Soldering results in an intermetallic compound (IMC) layerbetween the solder and the soldered object. The IMC layer in a SAC solderjoint is composed of tin, silver and copper plus some of the solder wettedmaterial. So if the contact surface is copper, excess copper will be partof the IMC along with tin and silver. If the solderable interface isnickel, nickel will be part of the IMC along with tin, copper and silver. IMCs are notably brittle and the thicker the IMC, the more brittle the solderjoint. IMC thickness depends on two factors: temperature and time and thehigher the temperature or the longer the time of heating, the thicker theIMC.
The ultimate goal of any soldering operation is to minimize theIMC in addition to making smooth fillets of the proper configuration as perworkmanship standard IPC-610. Hand soldering operations vary from station-to-station(equipment dependent) and operator-to-operator, therefore hand soldering is theleast favorable of all assembly methods as it is not a reproducibleprocess. Hand placement of components into wet paste is discouraged asthat operation often results in solder bridging/shorts or even displacement ofadjacent parts.
If only loose parts are available, these parts can be re-tapedonto a used reel. As an alternative, a matrix tray can be reused orcreated by simple machining and the loose parts nested in it for automatedpick-up and placement. The ultimate goal is to minimize the number of solderingsteps, only use reproducible processes (eliminate hand soldering) and minimizetouches to the board to reduce board flexure and solder joint damage. Avoid hand soldering!
A thirty year veteran of electronics assembly with major OEMs including Digital Equipment Corp., Compaq and Hewlett-Packard. President of Colab Engineering, LLC; a consulting agency specializing in electronics manufacturing, root-cause analysis and manufacturing improvement. Holder of six U.S. process patents. Authored several sections and chapters on circuit assembly for industry handbooks. Wrote a treatise on laser soldering for Laser Institute of America's LIA Handbook of Laser Materials Processing. Diverse background includes significant stints and contributions in electrochemistry, photovoltaics, silicon crystal growth and laser processing prior to entering the world of PCAs. Member of SMTA. Member of the Technical Journal Committee of the Surface Mount Technology Association.
It is a well-known fact thathand soldering is a much less controlled process than the SMT process. If ithandled correctly, hand soldering could introduce some quality issues. Here aresome tips for hand soldering:
- The solder iron needs to be kept clean. The black crud built upfrom metal oxide and charred flux should be removed frequently.
- Choose the right solder wire. Incompatibility between the fluxresidue from solder paste and the flux used in the wire may cause issues suchas white residue, low SIR or corrosion.
- Avoid prolonged contact time. An excessive heat input not only maycause damage to the PCB and components, but also may cause copper erosionrelated issues.
- Optimize tip temperature. A low tip temperature will cause coldsolder joints, poor wetting, solder flags, icicles. If the tip temperature is toohigh, it may cause charring, grainy joints, de-wetting etc.
- Use right iron tip geometry.
- Use correct wire diameter, flux content and activity.
Director New Product Development
Metallic Resources, Inc
David Bao has more than fifteen years of experience in developing new solder paste, wave soldering fluxes and other SMT consumables. He currently serves as the Director of New Product Development at Metallic Resources Inc. He received a Ph.D. in Chemistry at Oklahoma State University.
Thisprocess is, of course, one to avoid. When it happens though, try to keep it toa minimum. Having SMT components soldered later in the process involves severalrisks:
Soif you do this, remember to use a good, clean tip, flux pen (recommendedinstead of bottle) and an operator that is very well trained for this type ofoperations. Component terminations damage, lifted pads and burnt boards arejust few of the unwanted results coming out of this practice.Tosummarize, you can get a good, reliable solder connection as long as you canaccept the potential risks, increased costs and high probability of scrap.
- The temperature of the soldering iron tip is way higher than the reflow oventemperatures
- Use of flux in excess can lead to later issues.
- Once this operation is completed, a cleaning process will be required. A spotclean can be difficult and residues can be left on the assembly.
Note: it isnot recommended to use through your processes a mix of flux types. A watersoluble process in SMT followed by some rework/repair done with RMA or no cleanflux and then a wave soldering with water soluble flux again can get youheadaches - white residue is one of them.
Engineering and Operations Management
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