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Reworking Plastic and Other Heat-sensitive Components
December 31, 1969 |Estimated reading time: 6 minutes
By Bill Scheu
Notice how inexpensive hand-held hot-air soldering rework tools have all but disappeared from the assembly workbench? If the reason why is recognized, one might just avoid purchasing the tools that soon may be obsolete.
Companies that may be evaluating or weighing the suitability of new printed circuit board (PCB) rework equipment often ask tough questions when there is a new problem to be solved, or when available tools do not work well for the challenging application at hand. In the heyday of hand-held hot-air tools, which were selling in the $900 to $1,400 range, low-temperature/high-power hot-air rework systems with closed-loop temperature control were available, priced in the $3,500 range. Some might be surprised to learn that those systems, now more than 12 years old, are still in use and commanding 60 to 80 percent of their original value on the used equipment market.
Necessity of Thermal Control
Buying the right rework system for a particular application should start with considering equipment capable of solving really tough, long-standing problems. Even when considering the purchase of rework systems for advanced packages such as the ball grid array (BGA) or chip scale package (CSP), e.g., the new split-vision systems, the ability to rework plastic and other heat-sensitive parts is a must.
Virtually all hot-air systems have some means of digital control and display for setpoint temperature. The real test for the buyer: Do not trust the light-emitting diode (LED) that indicates, for example, 210°C. Rather, one must be certain at that temperature, a plastic part that will melt at 220°C will remain undamaged.
What, then, does it take apart from accurate temperature control to rework plastic parts, glob-top BGAs, edge connectors, surface mount connectors and optoelectronic parts? Knowing how to remove and replace such valued components successfully is usually sufficient to understand and replace other heat-sensitive parts. To that end, the focus here is primarily on plastic parts, since the methodology is the same.
It takes a specific level of energy to remove and replace any part. There are two ways to generate that which is required: temperature or wattage. The calories of energy required must also be delivered so as not to delaminate or burn the board. Since it usually takes even more energy to replace the part successfully, the ability to simply remove the part does not in itself solve the problem.
The first step in creating a thermal profile for reworking a thermally sensitive component is to determine the maximum temperature the part will tolerate. A good thermal tracking unit or temperature measuring device will enable the process engineer to determine this temperature. In fact, many plastic parts including glob-top BGAs will melt just above the flow point of the connecting solder. Hence, awareness of this will provide a proper starting point for testing.
The inside walls of edge connectors, for example, should be inspected carefully around the points of metal contact. Since metal will absorb the heat faster, the contact areas will show the first indication of pending damage or degradation.
After the maximum safe temperature has been determined for a part, the next step is to develop a plan for repair. The selected rework system should be checked for accuracy. For example, if the setpoint temperature indicator on the unit is set at 210°C, a temperature-measuring device should be placed in the hot-air stream to check that the unit is delivering the indicated heat level. An indication carrying even as little as a five-degree error can cause problems that include serious overheating of the part.
If the rework system directs its hot air using interchangeable nozzles, a nozzle larger than the component to be reworked is indicated. In operation, the part is placed under the nozzle and the heating cycle is initiated. If, after 4.5 min, the part cannot be removed (solder has not properly melted), the process is halted and the situation reevaluated. Heating any longer will increase the chance the flux and the solder joints will begin to degrade.
The solution is to add preheating, since the thermal mass of the board in the specific area of the component must be overcome. The ΔT — temperature difference between the top and bottom of the board — should be minimal. By focusing the heat source under the part, the energy can be directed to the task at hand.
The energy transfer from a preheating source can be regulated by varying the distance from the edge of the nozzle to the surface of the board. The distance depends upon the wattage of the preheater. The only concerns are the true temperature of the air and the exhaust of the spent hot air. For example, as the preheater moves closer to the board, the volume of ambient air mixing with hot air serves to increase the temperature of the air directed at the board. As a result, the calibration of the initial setup can become skewed. If the bottom preheat nozzle is positioned too close to the board, the initial calibration can also be distorted, resulting in a higher heating air temperature.
Rework Aids
Hot plates and infrared preheaters are not recommended for this type of rework. Reason: the thermal reaction times, energy transfer rates and efficiency are never consistent. However, they can be used for large metal- and ground-plane boards in limited applications, e.g., where the size of the board matches that of the preheater in area. These devices heat only under a PCB. They have little capacity to ramp and soak to perform properly engineered repair scenarios or to support the creation and running of complicated thermal profiles. They also are limited in their ability to preheat beyond the physical dimensions of the heating surface. Hot-air preheating can be ramped, soaked and — on some systems — synchronized with the reflow process, permitting duplication of the actual profile used in manufacturing the assemblies.
Efforts to attain zero-defect levels in repair and rework will eventually lead producers of some repair equipment to offer the advantages of hot-air preheating. For now, however, non-convective preheating options are a cheap way to offer the "apparent" features of proper preheating. BGAs and photoelectrical parts are sensitive to higher temperatures and any attempt to preheat with marginally controllable sources is risky.
Parts Replacement
Replacement of thermally sensitive plastic parts requires slightly more heating time than that needed for their removal. Normally an operator will remove a part at the earliest possible moment once it is freed of the board. Replacement, however, requires a full and complete reflow to be successful, since properly formed solder joints or connections must take time for the wetting process.
Simply applying the established removal time will not suffice; some joints will not be completely reflowed or formed properly. On small connectors and parts such as cell phone jacks and pager glob-top BGAs, the addition of 20 to 30 seconds is generally sufficient. On larger sockets and surface mounted edge connectors, the addition of 90 seconds in removal time is common. A visual inspection of the finished work is the best way to determine the proper heating interval.
Alignment of edge connectors can be done visually for installation. If the connector has exposed contact leads, tacking one or two before reflow is preferable. Some connectors and sockets have blind leads and require split-vision units for alignment. If only an occasional repair is required and the connections are of 0.030" pitch or greater, the operator can use a fine-line felt-tipped marker to outline the form of the part before its removal. Lastly, high-velocity hot air is sometimes used in low-power rework systems. With these units it is best to glue or tape the part into place before heating.
Conclusion
Plastic parts and temperature-sensitive components can be reworked with minimal effort and skill. The ease with which one may accomplish this with a given make of rework equipment is a good indicator of which rework system to select when time for replacement.
Bill Scheu, president, may be contacted at A.P.E. South Inc., 106240 Overseas Highway, Key Largo, FL 33037; (305) 451-4722; Fax (305) 451-3374; E-mail: bill@ape.com; Web site: www.ape.com.