Through-hole rework of thermal dissipating assemblies: step-by-step instructions are given for a new method of reworking expensive through-hole heat sink assemblies.

Because of its familiarity, through-hole technology is not as glamorous or exciting as the evolving surface-mount technology. Besides decreasing assembly size, the implementation of surface-mount technology has shifted resources to new components and processes. However, when through-hole rework is required, the method must maintain the assembly's initial integrity.

In particular, large thermal heat-dissipating assemblies with through-hole components remain a vital rework issue. The fragile barrels, mask and pads of these expensive assemblies can be easily damaged using existing manual rework methods.

Accordingly, a new rework method was developed that incorporates computer control of temperature, time and motion for solder pot modules and preheaters. A hot gas system is used to remove the solder from the barrels before resoldering (Figure 1). In the new method, four major steps are required for successful through-hole repair: preheat, component removal, component insertion and resoldering.

[FIGURE 1 OMITTED]

Preheat

The key element in this step is a high-wattage system that provides overall assembly heating within a reasonable time frame.

The board is loaded into a motorized carrier system and automatically positioned in the preheater to set temperature. It is then immediately and automatically conveyed from the preheater to the reflow module. Separate preheat ovens or systems may be used for a manual or batch-type method, but safety must be ensured when transferring the heated assemblies. In addition, heat loss due to handling must be considered.

The assemblies must be preheated for the following reasons:

* To minimize heat absorption of the overall assembly during the reflow process. Excessive heat may damage the mask or cause board delamination or discoloring. Preheat also reduces the power and capacity of the system required.

* To reduce the thermal gradient, thereby minimizing localized PCB warpage.

* To reduce the time that the process or localized heat is applied, thereby minimizing component overheating. Leads may separate from the low-melt plastic housing. Leads will remain in the board barrels, resulting in difficult removal or overheating of the assembly while manually removing them.

* To establish the same starting point for the reflow process.

* To provide uniformity of the solder joint temperatures, thereby minimizing open solder joints.

* To relieve board warpage caused by humidity of in-field or off-line storage. The relationship of the solder wave to the board must be level.

Component Removal

The key elements in this step include:

* high mass heat medium such as a 2,500 W, 35 lb. capacity solder pot

* process controlled heat with time and temperature fixed

* selective heat/solder contact to component only

* board positioned level and accurately to solder wave; a robust carrier platform and computer-controlled board positioning.

The board is accurately positioned over a restricted solder wave whose shape matches the lead pattern of the component immediately after being preheated (Figure 2). The motorized z-axis controls the speed and height of assembly (leads) to the solder wave. Low temperature solder flows for a few seconds against the bottom of the board, transmitting heat simultaneously to all component leads.

[FIGURE 2 OMITTED]

Heat (temperature and time) to the assembly must be limited to prevent mask, component, barrel and board composite damage. When all joints are molten (timed duration), the component is lifted from the board. However, care must be taken not to remove the component before the biggest heat sink joint is molten. If not, damage may occur to leads and barrels.

The system's large thermal mass provides a uniform low temperature throughout the process. In contrast, high-temperature vacuum desoldering tools or manual wicks can damage pads and masks by overheating from excessive tip temperatures. Increasing the temperature necessary to overcome assembly heat dissipation can easily damage the board. Gauging the exact time to stay on the pads, tip positioning, and scraping of the pads and masks due to excessive pressure prevent the use of these manual methods.

Using solder as the heat medium provides several advantages. No prying of bent leads with a heated tip is needed. No reheating of partially desoldered joints occurs, thereby eliminating pad delamination. Damage primarily occurs at this time, because the operator cannot determine which joints have been successfully desoldered. The operator must then reheat the joints without the benefit of solder to act as the heat transfer medium. In the case of multi-lead connectors, hundreds of joints may have to be reheated, significantly increasing the probability of damage.

With the new method, the average removal reflow time is 10 seconds. The assembly has less heat being applied--at a lower temperature and significantly less time--and much less operator requirement.

Component Insertion

The key elements in this step include:

* high capacity preheat, as described in the previous section

* high capacity non-contact heating to minimize assembly heat absorption and tip abrasion

* high capacity vacuum flow and force for fast and complete residual solder removal

* minimal maintenance to provide continual and long-term usage.

After component removal, the barrels must be completely clean for component insertion (Figure 3). This part of the process is critical because the solder must be heated and quickly removed while attached within a massive thermally conductive assembly. For the same reasons stated during removal, wick or vacuum desoldering tools cannot be used. While pad damage may be easy to see, mask damage may not be readily apparent.

[FIGURE 3 OMITTED]

For component insertion, two methods or a combination may be applied: hot gas/vacuum or low pressure air.

Method 1

A hot gas module provides overall bottom side heat to the board (providing the preheat benefits) while establishing a flat assembly platform. Hot gas at a controlled temperature and flow is directed onto the residual solder of the PCB pads. The heat is channeled to multiple solder barrels, thereby ensuring gradual temperature ramp-up and minimal thermal shock to achieve molten solder.

The high vacuum flow system simultaneously removes the excess molten solder into the collection chamber. The holes are completely cleaned, allowing full insertion of the replacement component, particularly when position tabs must be fully seated.

The fixed position and clear stereomicroscope viewing of the process eliminate the fine operator dexterity required of conventional methods. The composite vacuum tip ensures no abrasive or grounding metal contact with the assembly (Figure 4).

[FIGURE 4 OMITTED]

Method 2

After component removal, an air-cleaning hood is lowered against the board surface. Low-pressure air is applied to the lead pattern, forcing molten solder from the holes. Method 1 may be used for barrels (ground pin) that are not cleaned. However, some customers do not allow pressured air to contact their assemblies, so they cannot use this method.

Resoldering

The key elements in this step include:

* high mass heat medium such as a 2,500 W, 35 lb. capacity solder pot

* process controlled heat with time and temperature fixed

* selective heat/solder contact to component only

* board positioned level and accurately to solder wave; a robust carrier platform and computer-controlled board positioning

* controlled z-axis velocity to eliminate bridging.

The component is fluxed manually or automatically by machine. The board is loaded into the motorized carrier, using the same system as for removal. Fixturing may be required to reduce board sagging. The board is positioned to the preheater for preheat and then positioned to the solder module.

The board is lowered into the solder wave (Figure 5). Lead-to-solder-wave entry speed, duration and withdrawal speed are computer controlled. A slow, uniform withdrawal speed is critical for correct solder peel to supply repetitive successful soldering. The board is returned to the load/unload position.

[FIGURE 5 OMITTED]

Conclusion

Massive heat sink assemblies are thermally robust and perform as designed to effectively dissipate heat. They are great for their designed function and for soldering, but they are difficult to rework. They are assembled in a highly controlled process with nonrestrictive heating. Yet when rework is needed, serious heat limitations exist due to the fragile and delicate nature of the barrel, pad and mask.

The new rework method provides a controlled process using low operator requirements. The method may also be used to repair ball grid arrays, chip-scale packages and flip chips and for selective through-hole soldering and removal. While an investment in equipment is required, other methods may not provide a safe and reliable rework on these expensive assemblies.

Richard Garnick is lead process engineer, Benchmark Electronics, Huntsville, AL; e-mail: [e-mail]. Ronald Wachter is marketing manager, Air-Vac Engineering, Seymour, CT; e-mail: [e-mail].