The process for removal & replacement of surface mount Area Array devices (AAD's) has been in existence since the 1980's. During this time, the process for rework has been refined continuously to accommodate shrinking lead pitches, and increasing lead counts. Parts up to 2" x 2" containing over 2,700 connections can be reworked with yields greater than 99%. Conversely, flip chip & Quad Flat Leadless (QFN) parts down to 0.1" x 0.1" are routinely reworked with comparable yields. Process development has always centered around packages utilizing eutectic Tin/ Lead solder alloys. In the past two years some components are becoming available only in lead-free alloys. This change has already started to upset what had finally becoming a robust process for AAD rework. Following is a discussion of the impact lead-free solder will have on the various steps associated with AAD rework.

Site dress -
When removing an AAD from an assembly a portion of the solder from the part will remain on the assembly pad site. The method of removing excess solder from the pads is referred to as site-dress. The site dress process should leave either a flat pad, or slightly rounded pads with consistent volumes of solder. This process in not especially difficult, however any miscues could remove solder mask or pads and put the assembly in jeopardy of being scrapped. Three methods for removing excess solder are widely accepted.

1 - Vacuum removal:
Several manufacturers have vacuum solder removal tools on the market. The preferred equipment blows hot gas from a collar around a nozzle that vacuums the solder. This process requires that the solder pads be at an elevated temperature to work properly as the heating ability of the tool is not sufficient to reflow the solder. Typically this equipment is set-up with the removal tool so that the site dress process is initiated immediately after component removal when the assembly is still hot. For Tin/ Silver/ Copper (SAC) alloy solders this becomes more difficult as the assembly will loose 30°C to 50°C in a short amount of time. To implement this process for SAC alloys, the removal tool will likely need to go through another step to re-heat the solder pads and allow more time for vacuum removal. Using the vacuum removal tool when solder pads are not hot enough will result in pulled pads.

2 - Solder wick:
Use of braided solder wick by a skilled operator can be efficient at removing excess solder and leaving a flat solder pad. Only skilled operators should be used for this process as the danger of pulling pads and removing solder mask is high. Using solder wick for SAC alloys should not create much of a challenge to an experienced operator.

3 - Concave solder tip:
Solder tips are available for most soldering stations that utilize a concave depression to accept excess solder. This process is performed by lightly dragging the iron tip across the solder pads. Excess solder is collected in the tip, any solder pads that are void of solder will be redistributed with solder. In the hands of a skilled operator, this process is less prone to removing solder pads or mask as compared to the former two processes. The resulting pads will be left with a slight meniscus of solder. For replacement of a part this can present a problem with alignment as the component will slide off to one side of the pad. It is recommended that this process be used only when screen printing paste for reattachment of the AAD. Early investigations reveal little difficulty with incorporating this process for SAC alloy AAD's.

Rework of AAD's requires an appropriate thermal profile for both the substrate and AAD package. With eutectic Tin/Lead alloy this translates into a peak component temperature of 200°C to 210°C and substrate temperatures (as measured on the top-side) of around 120°C. New lead-free components typically utilize SAC alloys that reflow at 217°C to 221°C. These parts require a rework profile to achieve component temperatures of 230°C to 240°C, and substrate temperatures approaching 150°C. To accommodate this change in profile requirements is not as simple as cranking up the heaters. There are numerous changes that will need to be implemented to achieve an acceptable rework process for lead-free AAD's.

The first step for lead-free reflow profiling is to accurately monitor temperatures at various locations at the AAD and on the assembly. With higher temperatures, the delta across the component and substrate will tend to increase necessitating the need to monitor more locations than with eutectic Sn/Pb AAD's. Crucial areas to be monitored include solder spheres under the part and a location on the circuit board in an area away from the rework site. Ideally you would want to monitor center solder connections as well as corner connections. Type K thermocouples can be slid under the part to make contact with corner solder balls. To monitor the center you will need to either drill in from the bottom of the assembly and install a thermocouple, or use a hypodermic needle style thermocouple that slides in from an edge. Another method of monitoring temperature is the use of infrared (IR) sensors. This non-contact method will only monitor the top of the component or substrate. Particular care should be taken to validate the accuracy by correlating results against type K thermocouples. It should also be noted that color and surface sheen can change readings dramatically due to different emissive properties. IR sensors will likely be most useful as a monitoring tool when processing large quantities of the same assembly.

When running a thermal profile there is almost always a delta in the temperature from the center solder joints to the corner as the part is heated. In general, metal topped and ceramic parts are hotter in the center and cooler on the corners. Plastic epoxy overmolded parts are usually cooler in the center. The goal of a localized reflow profile is to minimize the delta across the part. A large delta will result in warpage of the component that could lead to solder opens and bridges. Forced convection reflow has been the standard for AAD rework for many years. Recent improvements in IR heating techniques have made this technology worth investigation. By monitoring temperatures at several locations in real-time, IR energy input can be varied to allow hot spots time to reach equilibrium with cooler areas. For applications where you will need to rework numerous different components and assemblies, IR is likely not the answer due to the difficulty in profiling safely. The danger with IR has to do with the rate of energy absorption of different materials and the ability of IR to heat very rapidly. If not monitored correctly you can burn an area of the circuit board or component in little time.

Damage to the circuit card or adjacent components during rework is not a new concern, but with an additional 30°C in the profile it is much more difficult to avoid when reworking SAC alloy components. Using forced convection to reach temperatures greater than 230°C on an AAD will result in temperatures in excess of 200°C around the rework site. Tantalum and electrolytic capacitors, as well as a host of through-hole parts will be damaged. For this reason the temptation of IR reflow begins to look attractive as the energy can be focused only on the part. Conductive heat transfer away from the part is minimal allowing adjacent areas of the board to stay well below 200°C.

Placing AAD components back on the assembly can be done using two different fluxing methods.

1 - Solder paste:
Screen printing either the pads on the assembly or the AAD is a process that offers the highest level of success with one major exception. Large components that have a tendency to warp will be more likely to bridge with the excess solder present when paste printing. The advantage for lead-free is that SAC alloys wet at a much slower rate, by having the paste in contact with both the component solder and substrate pads, the occurrence of open solder connections will be reduced. Finding insufficient reflow conditions with X-Ray will be easier because paste that has not fully reflowed will be more obvious.

2 - Gel flux:
Application of a thin layer of gel flux to the substrate pads, or dipping component solder spheres in gel flux is a reliable method for fluxing to attach AAD's. This process does not require a micro stencil to complete rework; this will aid prototype assemblers in both cost and lead-time. When working with eutectic Sn/Pb alloys the solder melts and as it wets the pads it centers the component on the pads. Sn/Pb wets almost instantaneously so once the part is centered properly you can be assured a metallurgical bond. With SAC alloys the aid of surface tension to align will be reduced. Lengthened wetting times will also make it more likely that metallurgical bonds will not be formed enough to provide robust mechanical connections.

Work in defining an optimized rework process for SAC alloy AAD's is still in its infancy. It is a safe assumption that many years of refinements will unfold before leadfree AAD rework is as safe and reliable as it currently is with eutectic Sn/Pb solder. It is the opinion of this author that the transition to lead-free will slow the evolution of our electronics industry as we focus engineering expertise on reinventing products & processes. Do not underestimate the reinvention that will be required to rework lead-free AAD's.

Stephen Schoppe
Process Sciences, Inc.
sms@process-sciences.com
512-259-7070

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