Reworking lead-free array packages
07 January 2007
Craig Brown focuses on heating, the biggest issue introduced by lead-free rework, and how best to reflow array package components on large double-sided PCBs
Over the years, circuitry and components have become smaller and more delicate while they have grown in complexity and functionality. At the same time, fierce competition has put severe pressure on every aspect of the manufacturing cycle, and throughputs and profitability are key. Against this backdrop, rework, far from being an optional or occasional task, has become unavoidable. It is also one of the riskiest, most demanding parts of the electronics cycle, especially where array packages are involved. Indeed, quality, efficient rework is the single most important factor influencing successful, profitable BGA implementation. Now, with the introduction of lead-free processes, the quality of rework becomes even more crucial, as lead-free materials call for higher processing temperatures that may stress or cause irreversible damage to heat-sensitive board materials and components.
Whether using lead-bearing or lead-free processes, and whether reworking conventional or array package components, the principal steps in rework, (desoldering, removal, site cleaning and reattachment) remain largely the same. These are so well documented and understood by rework professionals that it is unnecessary to describe them here. Rather, this article proposes to focus on heating, the biggest issue introduced by lead-free rework, and how best to reflow array package components on large double-sided PCBs.
The issues:
Higher Temperatures
Whereas leaded solder melts at 183°C, and reflows at 210-220°C, SAC, the most popular lead-free solder alloy, melts at 217°C and reflows at between 235°C and 250°C. This is very close to the temperature at which components will be damaged, the IPC limit for component die, at 260°C, for example, is already too high for some component manufacturers. It is clear from this that processing windows are tighter than they have ever been. It is also clear that temperature profiles, ie: target temperature levels, ramp speeds, soak times, delta temperatures, and heater size and positioning must be precisely established and controlled if faults such as warpage, melt, open circuits, shorts, bridges, delamination, blistering and even component loss are to be avoided. That said, higher temperatures are necessary and in order to create a reliable joint, lead-free solder must be heated to at least 230°C. As process variations are in the order of +/- 5°C, it follows that the lowest temperature that is possible in order to achieve a reliable solder joint is 235°C, or, according to NEMI standards, 245°C.
BGA Lid Damage
When working at these temperatures, one of the biggest challenges is maintaining a low temperature on the reworked BGA lid, which is often made of heat-sensitive plastic. In large BGAs, high temperatures will cause the lids to warp down in the corners, a major cause of bridges, especially where the component is difficult to solder down correctly. This can be due to the use of poor BGA substrate materials.
Delta T
Another factor to consider when reworking is the temperature difference, or delta T, between different parts of the assembly. A good rework process, with tight process control, will ensure that the difference in temperature between the solder balls being reworked is close to 5°C and under 10°C.
[Ramp Speed
Time is money and the faster the throughput, the better the profit margin. This is as true for rework as it is for any other part of the manufacturing cycle, so process control is key.
Heaters and Air Temperature
When heating array package components, convection heaters, which use hot air forced through a nozzle, are more practical than conduction heaters, as they have an immediate effect, yet will allow temperatures to ramp gradually throughout the assembly and the package.
A factor of major importance is the size and position of the heaters relative to the PCB. If heat is concentrated only on the area to be reworked, the substrate will reach temperatures that are far in excess of what can be tolerated while the rest of the PCB remains cool. This will cause the PCB to warp, stressing its interconnections, while the very high local temperatures can blister the solder resist.
To avoid these problems, the standard approach to heating lead-process PCBs involves heating not just the target BGA, but the entire board, to reflow temperatures by setting air temperature to a maximum of 220øC. In this way, component surface temperatures may reach 190° to 200°C, while solder heats to 220°C.
Unfortunately, for a number of reasons, some companies have discovered to their cost that the same approach cannot be used for lead-free PCBs because the higher air temperatures would be disastrous, especially for the board's underside. This is because, like the topside, it is populated with delicate components and connectors. Their plastic parts can end up warped and deformed, necessitating additional rework, or their leads can partially or fully reflow. Thus, components and connectors may elongate, sag, stretch the joints or even drop off the underside of the board.
Some rework professionals will try to avoid these risks by pre-heating their lead-free boards from underneath to well below the 217°C melting point, and then heating the BGA directly from above until reflow is achieved. Although this is better than no pre-heat at all, the BGA will nevertheless be exposed to temperatures over its tolerance limit in order to get its underlying solder balls to reflow, incurring the risk of heat damage.
The Ideal Profile For Double-Sided Boards With BGAs
Taking all of these issues into consideration, it becomes clear that array package rework on large double-sided PCBs requires thermal profiles that are carefully programmed and continuously controlled. These should be used on reflow equipment that is large enough to house and support the PCB to protect it from warpage, powerful enough to pre-heat the entire board quickly, and accurate enough to only reflow the part that is to be reworked, within precise time and temperature limits.
Naturally, the ideal thermal profile for any given board will depend on its size, mass, component types and component density. For a large double-sided PCB with array package components for example, this will include a pre-heat stage. This uses a large array heater positioned below the board to heat the entire assembly to a temperature below the solder melt point, ideally 190°C. At this point, the large underside heater is switched off and is replaced with topside and underside heat nozzles that are concentrated locally on the BGA rework site to take it to reflow temperature for a brief period. Thanks to the pre-heat stage, and to their working in tandem, the local heat nozzles can use lower temperature air flow to achieve target temperatures. This significantly reduces the risk of heat damage to the BGA and its lid, as well as the surrounding soldermask.
Furthermore; by keeping the rest of the PCB at a cool 190-200°C, the rework site temperature can be ramped back down to safe temperatures quickly and easily, eliminating the risk of local warpage on the board. Additionally, because the rest of the board remains below melt temperatures, there is less risk of damage to the underside devices, and solder reflows only where necessary. It is also possible, using this approach, to keep delta T across the board to within very tight tolerances.
Conclusion
Experience, patience and a great deal of trial and error have gone into the development of an outline rework heat process for double-sided boards that serves as a valuable basis for developing optimal product-specific thermal profiles using dedicated software.
Precision and speed are key to successful, profitable rework, and it is not always enough simply to 'turn up the temperature' on existing hardware. It is essential that rework equipment is powerful enough to work comfortably and accurately at the higher temperatures needed for lead-free processes, while closed-loop feedback and dedicated control software are becoming key to quality, fast, low cost array package rework.
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