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?

P.K.

Expert Panel Responses

There are three main sources of differences in reliability:solder joint behavior, component damage, and cleanliness. Hand soldering can involve tool tip temperatures of 400C and ramp rates of more than 100 C per second, and we need to consider the effect on the component.

Multi-layer chip capacitors, particularly larger ones, are a high risk for damage during hand soldering, for instance. If you have guidelines for rework that address the acceptable processes for specific components, you can rely on these guidelines for the hand-installation of the components.

The differences in solder joint reliability are hard to predict, and in most cases not your biggest concern, as long as you meet the applicable standards for soldering quality. Speaking of soldering quality, remember that your defect rate for hand-assembled product will almost always be higher than for automated assembly.

The cleanliness risk is real, but as long as you have well-developed processes for hand soldering that include ensuring cleanliness of finished assemblies, you should be in good shape.

In addition to the risk to the components being soldered, there are additional product risks, such as the risks of ESD or mechanical damage during the additional handling and processing necessary. It might not be easy,or even possible, to trace failures related to these additional risks back to the 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 are rarely worth it.

Fritz Byle
Process Engineer
Astronautics

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 reflow process but some care is required. The important thing to remember is to avoid excessive temperatures and contact times with the soldering tip.

The area of concern is components which cannot tolerate fast ramp rates and are prone to cracking or delamination issues.Hand-soldering heats parts up very rapidly and within a second or two the temperature is well above the melting temperature of the alloy. Reflow soldering slowly increases temperature and reduces the effects of CTE mismatches of materials as well as thermal shock issues.

It is important to be aware of the components ability to withstand higher temperatures and also ramp rate requirements as to avoid issues. The other point is to avoid heating the whole component but soldering the terminations only, this requires the use of the correct tip geometry.

Peter Biocca
Senior Market Development Engineer
Kester

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. Hand soldering is not a repeatable process. Oxides on the soldering iron tip or on the solder, pressure applied, heated contact area, flux application and operator skill can all have a significant impact on the finished solder joint.

One can have a very lovely solder joint on the outside but a bad joint on the inside. The interface between the bulk solder and the solder land or the soldered component lead can have a dramatic effect on the reliability of the finished solder joint.

Soldering results in an intermetallic compound (IMC) layer between the solder and the soldered object. The IMC layer in a SAC solder joint is composed of tin, silver and copper plus some of the solder wetted material. So if the contact surface is copper, excess copper will be part of the IMC along with tin and silver.

If the solderable interface is nickel, 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 solder joint. IMC thickness depends on two factors: temperature and time and the higher the temperature or the longer the time of heating, the thicker the IMC.

The ultimate goal of any soldering operation is to minimize the IMC in addition to making smooth fillets of the proper configuration as per workmanship standard IPC-610. Hand soldering operations vary from station-to-station(equipment dependent) and operator-to-operator, therefore hand soldering is the least favorable of all assembly methods as it is not a reproducible process. Hand placement of components into wet paste is discouraged as that operation often results in solder bridging/shorts or even displacement of adjacent parts.

If only loose parts are available, these parts can be re-taped onto a used reel. As an alternative, a matrix tray can be reused or created by simple machining and the loose parts nested in it for automated pick-up and placement. The ultimate goal is to minimize the number of soldering steps, only use reproducible processes (eliminate hand soldering) and minimize touches to the board to reduce board flexure and solder joint damage. Avoid hand soldering!

Gary Freedman
President
Colab Engineering

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 that hand soldering is a much less controlled process than the SMT process. If it handled correctly, hand soldering could introduce some quality issues. Here are some tips for hand soldering:

The solder iron needs to be kept clean. The black crud built up from metal oxide and charred flux should be removed frequently.
Choose the right solder wire. Incompatibility between the flux residue from solder paste and the flux used in the wire may cause issues such as white residue, low SIR or corrosion.
Avoid prolonged contact time. An excessive heat input not only may cause damage to the PCB and components, but also may cause copper erosion related issues.
Optimize tip temperature. A low tip temperature will cause cold solder joints, poor wetting, solder flags, icicles. If the tip temperature is too high, it may cause charring, grainy joints, de-wetting etc.
Use right iron tip geometry.
Use correct wire diameter, flux content and activity.

David Bao
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.

This process is, of course, one to avoid. When it happens though, try to keep it to a minimum. Having SMT components soldered later in the process involves several risks:

The temperature of the soldering iron tip is way higher than the reflow oven temperatures
Use of flux in excess can lead to later issues.
Once this operation is completed, a cleaning process will be required. A spot clean can be difficult and residues can be left on the assembly.

Note: it is not recommended to use through your processes a mix of flux types. A water soluble process in SMT followed by some rework/repair done with RMA or no clean flux and then a wave soldering with water soluble flux again can get you headaches - white residue is one of them.

So if you do this, remember to use a good, clean tip, flux pen (recommended instead of bottle) and an operator that is very well trained for this type of operations. Component terminations damage, lifted pads and burnt boards are just few of the unwanted results coming out of this practice. To summarize, you can get a good, reliable solder connection as long as you can accept the potential risks, increased costs and high probability of scrap.

Georgian Simion
Engineering and Operations Management
Independent Consultant

Georgian Simion is an independent consultant with 20+ years in electronics manufacturing engineering and operations.
Contact me at georgiansimion@yahoo.com.

Reader Comment

With shrinking geometries and the increase in functional densities the electronic components have migrated to Surface Mount Technology. To make use of the advantage of this technology in the real world one has to properly solder these components to the PCB and ensure a good interconnection is obtained, with proper wetting. Also, in this process one has to ensure that the components are not getting damaged because of excessive heating esp. moisture sensitive devices such as PQFN, LPCC, PSOF, LQFP, WSON etc. Towards this it is always advocated that these devices be Reflow soldered into the PCB with an evaluated profile.

For High reliability applications, it is preferred to make use of an 8 (hot) +2 (cold zones) so that thermal shocks and too quick cooling can be avoided protecting the components from CTE mismatch and mechanical fractures. Moreover, since the PCB can be treated as MSL level 3, a proper conditioning of the PCB through thermal baking shall be carried out prior to Reflow. By suitably adjusting the stage-3 of the thermal profile one can solder Pb-free and leaded components in the same reflow provided Bismuth is not present.

Dr. KUTTIYIL THOMAS OOMMEN THARAKAN, VSSC

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