While mass soldering processes dominate soldering applications, hand soldering remains critical in electronics manufacturing. Existing processes (e.g. convection oven reflow, vapor phase, wave flow, selective wave flow, etc.) are very efficient and offer high volume capacity but may not always be suitable. One of these special situations is when the thermal mass of a few components to be soldered is relatively larger than the other components on the same substrate. In that case, we must consider a separate process for these components (i.e. separate reflow step or hand solder). While hand soldering may appear to be simple, the same critical process elements exist as in mass soldering and must be controlled to achieve reliable solder joints. These include:
1. Heat source
a. Reservoir soldering irons
b. Constant tip temperature soldering irons
2. Heat transfer media
a. Soldering iron tip. Size and configuration
3. Cleaning material (flux)
a. Type. No Clean, RMA, OA, etc.
b. Amount
4. Time to achieve soldering
a. Operator dependent
b. Point of contact for solder and iron tip
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Should it become necessary to use hand soldering for large thermal mass issues, the following determinations and techniques need to be considered.
1. Soldering iron selection. Large thermal mass components require a lot more energy transfer to achieve soldering temperatures. Heat reservoir type irons may not be capable of delivering this required amount or may have inadequate recovery time during repeated uses. These types of iron store the energy (heat) in a large thermal mass area and transfer it when the tip comes in contact with the material to be soldered. Irons with constant tip temperature monitoring use a different technology to maintain the tip at a selected constant temperature regardless of the size of the component to be soldered.
2. Tip selection. Tip selection is critical to effective heat transfer. Think of it as a gate which if inadequate will prevent efficient heat transfer or cause excessive thermal loss and tip deterioration. Tip configuration is determined by the size of the materials to be soldered. Maximize the contact area of the tip to the material; the greater the contact area the more efficient the heat transfer. Minimize tip non-contact areas as this promotes heat loss away from the joint. In contrast, too small a tip restricts heat transfer and increases soldering time which may result in unreliable solder connections.
3. Cleaning Material Selection (Flux): Flux type is a trade-off between not having a post solder joint cleaning step or a more aggressive pre soldered components cleaning. While cleaning (oxide removal) is the primary function of the flux, facilitating heat transfer from the iron tip to the components is also critical. This involves having the proper amount of flux delivered to the connection. Core solder is common in hand solder applications, which has the flux encapsulated in the center of the core. The amount of flux is determined by the size of the flux channel in the core solder and is a matter of choice. Too little flux will prevent proper pad cleaning and thereby prevent proper heat transfer .
4. Time to Achieve Soldering. Allowing sufficient time for the components to come to proper temperature is perhaps the single most critical parameter in soldering. Mass soldering processes control this parameter by machine settings like the speed of the conveyor. Hand soldering however depends on the operator and is thus subject to a great degree of variability. For large mass components, more heat transfer is necessary to reach temperature (and thus more time). The best indicator is visual, such as when the solder melts – although this can be deceiving. The wrong way is when the soldering iron tip melts the solder first and then the melted solder deposits on the components. It causes improper intermetallic (non-wetting) and the bond between the solder and the components/PCB pads is compromised. It is important to understand that as the material to be soldered increases in mass, time to reach soldering temperature increases significantly and there is no means available for the operator to know when that occurs. Technique of soldering now becomes critical. Placing the solder iron tip directly on the components to be soldered and contacting the core solder to the same components surfaces (not the soldering iron tip) will permit the solder to be melted by the heat from the components rather than the soldering iron, eliminating “Transfer Soldering”. Now, when the operator sees the solder melt, they know proper temperature of the solder surfaces has been reached.