|Ask the Experts|
November 10, 2020
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 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.
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.
Senior Market Development Engineer
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!
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:
Director New Product Development
Metallic Resources, Inc
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:
Engineering and Operations Management
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.Dr. KUTTIYIL THOMAS OOMMEN THARAKAN, VSSC
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.
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