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Reclaiming Costly Components Without Losing Quality
Martin Mini-Oven 04 handles a diverse range of BGAs, QFNs and CSPs.
By Scott Rushia, Martin, Manchester, NH
Today's demanding rework applications require reliable solutions that directly address the specific challenges involved with each type of rework process. Having to scrap boards and components can greatly impact the profitability of any job and is avoided whenever possible. Therefore, many companies require a quality rework or reball solution with reliability and repeatability. These are commonly needed for applications in the military, medical, aviation, aerospace and networking sectors.
Various environments — from start-ups to original equipment manufacturers, contract manufacturers, and repair services — frequently seek an economical way to reball Ball Grid Arrays (BGAs) and Prebump Quad Flat No-Lead packages (QFNs). Many need the capability to safely heat very large server PCBs for assisting with the removal or replacement process of Surface Mount Devices (SMDs) without sacrificing quality.
Reballing holder with BGA after reflowing.
The industry has seen a significant increase in the number of requests to reball BGA packages, thus allowing components to become usable again. There are many reasons why one would want to reball a component. Perhaps a replacement is difficult to find, the part is obsolete, or just plain unavailable. Often the component is a costly one and not cheaply replaced.
Quick Turn Needed
In some instances, a quick turn is needed, or as is often the case in military, only lead-free components are available and specifications call for leaded. When that is the case, the lead-free balls need to be removed with heat and vacuum so the pads aren't damaged, and replaced with leaded balls. Another scenario would involve leaded ball BGAs that were shelved when the switch to RoHS standards was implemented. Instead of scrapping the BGAs, they can be easily reballed with lead-free balls. Both of these practices are common and are becoming widely accepted.
Reballed BGA is good as new and ready for reuse.
There are various commercially available reballing kits. Some come in a preform (with the proper size ball, pitch, and ball pattern) that gets manually aligned to the BGA. This can be quite expensive and is not very flexible. Other kits match up a frame and mask to the BGA and use solder balls bought in bulk containers (more economical, more flexible). But it is how they are then reflowed that varies. Typically, there have been two widely used solutions for reflowing solder balls — ovens and rework systems. If you use an oven, you would need to shut down the SMT line to run the BGAs through. Not an ideal situation if you are trying to save money by reballing. In fact, money would be lost because of interrupted production. If a rework system is used for reballing, you inflict the same damage to your bottom line. Instead of using the rework system for the tasks it was purchased for — removal or placement of components — you are tying it up for reballing.
An alternative would be a small, intuitive, stand-alone bench top unit that is simple to use, programmable to comply with today's standards, while providing a cost-effective solution.
Small Benchtop Unit
So how does a small benchtop unit handle the task? The unit is powered by IR lamps and reflectors to produce a heat source that is focused on the part. An optimized component heating process is the result of the combination of IR radiation and convection technology, providing efficient temperature distribution with a powerful 500W IR bottom heater. Process gas can be used to minimize unwanted oxidation.
QFN, pre-bumping mask and holder.
A frame, mask and holder are used in the reballing process. The frame is inserted into the holder and the component added, flux is applied and the mask aligned. Masks can be modified to accommodate different sizes by using Kapton tape to define the array. Or, for more difficult arrays, custom masks can be made to match the ball pattern. The solder balls are distributed on the mask, filling the openings. Any vacancies can be filled by brushing. Excess balls are discarded, or collected depending on operator choice. The unit is placed in the mini-oven, a program selected and the unit started. Through a porthole, the progress can be observed with or without a microscope as needed. At the end of the cooling process, the component can then be removed and inspected.
With the ability to store and edit up to 99 profiles, operators can save trial and error efforts. Adjustable hold time, temperature offset, and temperature damping provide process stability. Simple, safe rapid technology soldering profiles rigorously apply settings within the permitted thermal limits, heat at the highest advisable thermal ramp rates and maintain within a safe temperature zone. The result is a reballing process in as little as three minutes.
QFNs are becoming increasingly popular as PCBs become smaller and smaller and are an ideal choice for applications where size, weight, and thermal and electrical performance are important. Higher density boards with QFNs can be difficult to rework. Due to the proximity of neighboring components, most locations cannot be screen printed to aid in the replacement of a QFN. These situations require a method commonly referred to as QFN pre-bumping.
Because of the very small amounts of solder on the contacts of QFNs, the use of residual solder in rework is not recommended. It is imperative that the pads of the board are cleaned; normally there is no possibility of applying new solder to them. One way of providing solder to the joints is to add it to the pads of the components. QFN pre-bumping is the process of adding solder to the device in preparation for the placement and reflow steps.
Martin Expert 10.6-HXXV: unique hybrid (IR + Convection) underheating.
With the Martin approach, the stencil that is used to paste the QFN stays with the device during reflow. This allows the stencil designer to choose any size aperture, ground pad pattern and stencil thickness — whatever is needed to achieve the desired paste bump size to meet the paste coverage requirements. Normal stencil aspect ratios do not apply and are not needed. All of the solder paste that is applied gets reflowed because the stencil is not removed prior to the reflow process. Using nitrogen will help assist the reflow process by reducing oxidation on the pads.
The QFN is then cooled and ready to be handled with either an SMT placement machine or a rework system because the leads now have new solder applied. Then it's a matter of simply adding tacky flux to the land pattern on the board, placing, and reflowing.
The industry has seen increased demand for large board, high value rework. Reworking large PCBs comes with a risk due to the high cost of producing these over-sized PCBs. A single unsuccessful rework task can destroy the entire board. One must be careful to avoid excessive heating and disturbance of neighboring components.
The added challenge of higher temperature, lead-free chemistries has brought the need for better PCB under-heating to the forefront. International Electronics Manufacturing Initiative (iNEMI) rework projects have recently focused on temperature uniformity and repeatability which is important in establishing manufacturing guidelines to prevent component temperatures from exceeding temperature and time limits.
In response to these needs, there is now an under-heating system available which combines IR radiation with convection. It is a package that combines a powerful 10,000W hybrid under-heater in a compact control unit and is especially suitable for the repair of large PCBs, such as high-value server boards up to 500mm x 700mm.
This type of underheating combines the best of both worlds. It provides not only a more powerful and larger under-heater, but in addition to infra red technology, it employs the added benefit of convection heat flow. It is the perfect compromise between the speed and power of IR pre-heat, and the stabilizing effect of convection. This hybrid method promises a more equal temperature distribution across large boards. Users can rely on precise and effective heat flow for reworking any assembly. This reduces thermal stresses and keeps PCB distortion to a minimum.
A well thought-through equipment concept should enable process certainty. Martin rework systems are designed to provide the user with a production capable platform that is repeatable for demanding, multiple shift rework environments. The consistency of performance has shown board temperature variations under 10 degrees across panels up to 500 x 700mm. A hot air reflow pen is capable of providing consistent temperature of 150 to 450°C with ±1 percent repeatability.
Considering the very high cost of some large PCBs, an appropriately selected rework system can quickly pay for itself. A technology-refined solution with hybrid heating can provide an increase in productivity and quality, as well as many advantages that come from modular production.
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