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July, 2019
Handling Risk Mitigation in Hand Soldering: Questions and Concerns
By Michael Gouldsmith and Zen Lee, Thermaltronics
so simple. Some hand soldering systems are said to evaluate the quality of the solder joint by calculat- ing the intermetallic compound (IMC) formation and providing “closed-loop” feedback to the opera- tor.
T These systems suggest that rules of oper-
ation and complete management of the vari- ables are necessary to achieve the green or red light result. However, a more critical question is if a simulation process can provide a quantitative output result, rather than a qualitative output, on the formation of the intermetallic layer.
Simulation Challenges The argument put forward is qualitative
by nature, and therefore, is no better than the current method incorporated into hand sol- dering, which includes visual inspection, oper- ator skill, control of influencing factors, such as tip geometry, the manner in which the tip touches the joint, the duration of time on the joint, the angle at which the solder wire is fed to the joint, or the amount of solder being applied. It is important to remember that real-time
hand soldering is conducted in an open environ- ment, which is neither predictable nor easily sim- ulated. Basically, if the process cannot be set up like a reflow oven, in a completely sealed chamber, or if all the parameters are not controlled, then the use of a process, which is not “closed-loop” and incorporates red or green LEDs to determine cor- rect metallic formation is not much better than an educated guess.
here are products on the market that tout the simplistic go (green light), no-go (red light) way to solder. However soldering is not
The challenge is to design a multifunctional
simulation system and a process control device that not only incorporates all known variables, but also has the ability to make real-time adjustments as required. For example, simulation systems typically use an allocation of underlying random numbers
tion process. These include the tip and joint tem- peratures, the time to heat the joint, tip contact area with the joint, the solder type, additional flux, determining if the PCB is new or reworked, deter- mining if the soldering is through-hole or rework touchup, operator skill, operator habits, pad size, and the component pin size. While some data can be made available to the system, such as sol- der type and additional flux, the simulation process has no control over the variables themselves. As far as tip geometry is con- cerned, it is claimed that the simulation process judges thermal energy performance of the heater to make the correct determination regarding the formation of the IMC.
Closed-Loop? Given all the critical controlled and non-
Standard reflow oven profile.
(stochastic simulations). Random-number genera- tors are controllable (fixed sequence, streams, seeds) but are mixed with completely independent and separate random numbers. The simulated results are then measured against any variance. Unfortunately, some critical variables are not
controlled by the simulation process and are not included in the “input” data recorded by the sys- tems. Therefore, they are not fully capable of determining solder joint quality. There are a wide variety of factors in hand soldering that are not controllable by the simula-
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controlled variables, it is reasonable to assume that the hardware and software tech- nology in these hand solder systems is not technically capable of providing consistent validation results and lacks the closed-loop process necessary to confirm that a good inter- metallic layer has been formed. Information about one such system states
that at the preheat stage, when the tip touches the pad/load, two seconds will elapse before the solder becomes liquidus and the intermetallic formation takes place, at which point the system will detect whether the joint is completed correctly. It would also appear from the information provided that from the molten solder state to the reflow phase, there is a period of two to three seconds. On this basis, it could be concluded that these hand soldering systems operate using a predeter-
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