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Printed circuit board (PCB) rework is not only basic repair, such as replacing a capacitor or resistor. Modern high-density PCBs, featuring BGA and QFN packages, fine traces, and multiple layers, present challenges that extend far beyond basic repair skills. If mishandled, defects such as cold solder joints, pad lift, or corrosion can compromise the entire system.
This article will dive into the details of advanced PCB rework with more complex activities, including:
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Rework of Ball Grid Array (BGA) and Quad-Flat No-Leads (QFN) packages
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How to repair cold solder joints
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Methods of controlling corrosion and contamination
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Solutions to lifted pad issues
What Tools Are Required for Advanced PCB Rework?
Advanced circuit board rework requires specific tools to ensure accuracy, prevent damage to components or pads, and maintain overall board reliability. The most important tools are:
Rework Station
- Hot Air Rework Station or Infrared Rework Station provides controlled heat for reflowing and part removal such as QFNs, BGAs, and fine-pitch ICs.
- Preheater (PCB Preheating Plate) reduces thermal stress by evenly heating the board before localized heat.
Soldering Equipment
- High-Precision Soldering Iron with temperature control, for touch-up soldering and repair of pads.
- Soldering Tips of Various Shapes (fine tips, hoof tips) to accommodate different shapes of pads.
Inspection and Measurement Tools
- Microscope or Video Inspection System: For the detection of solder bridges, tombstoning, or micro-cracks.
- X-Ray Inspection Machine: Essential for hidden joints such as BGA or QFN center pads.
- Multimeter and Oscilloscope: For continuity testing and functional testing after rework.
Soldering Materials
- Solder Wire / Solder Paste: Lead-free or leaded, as needed.
- Flux (Liquid or Gel): Improves wetting of the solder and prevents oxidation.
- Solder Wick and Desoldering Braid: Excess solder removal and repair of solder bridges.
- Component Handling & Cleaning Tools
- Vacuum Pick-up Tool or Precision Tweezers
- Isopropyl Alcohol (IPA) and Brush Cleaning
- ESD Protection Equipment
BGA Rework: Problems & Professional Fixes
Ball Grid Array(BGA) devices are comprised of solder ball arrays on the back (input) side of the component, directly attached to PCB pads. It gives high density and good electrical results, but inspection and repair are legendary nightmares.
Common Failures in BGA Package
The failures of BGA usually have a number of pronounced manifestations, which have very different causes.
When too much solder is used, there will be a solder bridging issue. Since the closely neighboring balls connect together during the reflow process, they form an electronic path where there should not have been one.
Head-in-Pillow (HiP) is another compound failure whereby the BGA ball and the solder paste on the PCB pad both liquefy but never unite to form a single inter-reliable connection, a failure which can occur due to component warpage and/or PCB warpage, or highly unequal heating.
Considerations for BGA Rework
The probability of successful BGA repair depends on many important factors, the first of which is the necessity to use special equipment. The BGA repair tools include a dedicated rework station with a focused infrared or hot air head and a professional pre-heater is provided as a matter of course, since the pre-heater introduces a gradual increase in the overall temperature of the board, thereby preventing thermal shock, and the rework station introduces focused heating to a specific component.
It also requires very careful temperature profiling, with the heating occurring in a certain curve, such as the ramp, soak, reflow, and cooling steps, balanced to the size and number of layers of a PCB, as well as to the melting temperature of the solder itself; otherwise, an improper profile can ruin both the BGA and the PCB itself.
How to repair a BGA?
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Diagnoses: BGA fault should be provable. Check the solder balls with an X-ray machine to find out about any cracks, empty spaces, or bridging. A thermal scan with a load will also show the existence of a cold spot due to a failed connection.
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Targeted heating: A good reballing/re-replacement will need a sophisticated rework station including both a preheater (to warm the whole board and avoid warping) and a small spot heat source (hot air or IR). There can be no compromise on a temperature-controlled process.
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Repairing Process: The normal process is to place flux, remove the old component with care, pad the hole circle area clean, line up new solder balls (reballing) or a new component, and reflow with a certain thermal profile.
Look at this video as a reference for repairing a BGA.
QFN Rework: Challenges & Solutions
Like BGAs, Quad Flat No-Leads(QFNs) are no-lead. Rather, they possess perimeter pads and commonly a large thermal pad on the bottom. It makes them small and heat-dissipating, although it poses severe challenges to repair.
heatGeneral QFN Failures
Most of the failures of QFNs are associated with the soldering challenges at the underside that is not visible. An inadequate amount of solder on the perimeter pads can ruin the connection, but an excessive amount of solder is readily capable of making bridged connections between fine pins.
The giant heat pad is more susceptible to trapped air holes, which incapacitate the heat dissipation and lead to overheating.
Also, non-uniform expansion during the reflow process can cause a condition known as tombstoning, where one end of the component is lifted entirely off the pad.
Reworking Process of QFN
Inspection and preparation: carefully examine the type, location, and severity of the defect using a microscope. If the defect is difficult to detect visually, it has to be tested using an X-ray machine to verify the presence of defects lurking behind the component.
Prepare the PCB rework tools: Preheat station, hot air/infrared rework station, flux and solder paste.
Component removal: Preheat the entire board to avoid thermal shock, and heat the QFN using hot air/infrared according to the temperature profile and remove it once the solder has melted.
Table: Optimal Temperature Range for QFN Repair
| Stage | Temperature Range | Time/Rate | Description |
| Ramp (Preheating Stage) | Up to 150 ℃ | 1–3 ℃/s | Gradual heating to reduce thermal shock |
| Soak (Thermal Stabilization Stage) | 150–180 ℃ | 60–120 s | Stabilizes board temperature, activates flux |
| Reflow (Peak Stage) | 230–250 ℃ | 10–30 s | Solder fully reflows, ensures wetting |
| Cooling (Solidification Stage) | Room temperature | ≤4 ℃/s | Controlled cooling to prevent stress/cracks |
Pad cleaning: Remove any residual solder with desoldering braid and clean the pads. Inspect the pads for lifting or damage and repair as necessary. Reapply solder paste to the PCB pads, controlling the quantity to prevent bridging/voids. Pre-tin component leads if necessary.
QFN alignment reflow: Precisely align components with pins over matching pads. Reflow according to the temperature profile to ensure uniform heating and prevent tombstoning.
Inspection and verification: X-rays inspect the solder joint quality and confirm that there are no bridges through a microscope. Perform electrical and functional testing to validate the success of the rework.
Repairing BGAs and QFNs requires specialised inspection and repair equipment. For most individuals and manufacturers, seeking professional PCB rework service support is a better choice.
Locating and Repairing Cold Solder Joints
Cold solder joints happen when solder is not melted thoroughly, resulting in a weak, unreliable physical and electrical connection. They may appear dull, grainy, and/or cracked.
Reasons and Fixes:
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Reason: Lack of sufficient heat in the initial soldering procedure, displacement prior to solder having set, or contamination at the pad or part lead.
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Fix: Put flux on the connection, then reflow the connection with a soldering iron or hot air, and the right degree of heat and wetting to produce a smooth, shiny appearance.
Corrosion and Contamination Control
One of the main killers of electronics is liquid damage. It is capable of instantaneously producing short circuits and corrosion that will eventually make a hole through copper traces and component leads over time.
Repair Steps:
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Cleaning: A complete cleaning with >90% isopropyl alcohol and a soft brush is always the first step. Specialized cleaning may be necessary, especially when dealing with serious corrosion.
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Inspection: High magnification inspection should be performed to examine green/blue corrosion on prints, under components, and connectors.
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Scraping and repair: Light corrosion may be scraped off, and the trace re-tinned using solder. Extensive corrosion that has corroded out a trace will need the trace repair procedure listed above.
Lifted Pads & Damaged Vias
- Lifted pads are caused by excessive mechanical force that separates the copper pad from the PCB substrate, frequent rework, or overheating.
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- Repairing methods: After cleaning the area, epoxy adhesive is used to fix the pad. A tiny jumper wire is used to restore continuity if the electrical connection is interrupted. To prevent more damage, careful heat control is required.
- Vias join various PCB layers and can break because of improper handling during rework, heat stress, or drilling flaws.
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- Repair methods: Copper rivets or tiny wires can be used to repair damaged vias. Conductive epoxy is sometimes used to restore interlayer connectivity.
Conclusion
Professional PCB repair is a careful art that combines diagnostic expertise with a surgeon-like skill. Effectively solving BGA, QFN, trace damage, and corrosion issues necessitates the correct tools, such as microscopes, rework stations, and, in many cases, X-ray equipment, a thorough understanding of the thermal and physical tolerances of the board, and an interest in applying it.
When the project is a board that has a suspected BGA failure, QFN concerns, or there is a lot of physical damage to the board, the best course of action is to work with a specialist.
To get the professional diagnosis and repair services that meet the highest standards of quality, contact ELEPCB to have a professional consultation and a quote.
FAQ
A1: Advanced PCB rework requires specialized equipment such as hot air or infrared rework stations with preheaters, precision soldering irons, and quality flux. Inspection tools like microscopes or X-ray machines help verify hidden joints, while solder wick, tweezers, and vacuum pick-up tools ensure precise component handling. Cleaning with isopropyl alcohol and proper ESD protection is also essential for reliable results.
A2: Diagnosis is indirect and drawn inferentially without an X-ray. Typical methods include placing moderate pressure against the component with the board powered up to detect whether the problem is intermittent (usually indicative of a cracked joint) or the use of a thermal camera to detect an overheating component that has shorted out. Nonetheless, X-ray inspection is usually necessary to provide conclusive evidence.
A3: Yes. A typical symptom of BGA failing solder joints is intermittent failures. Cracked joints swell and shrink as the device warms and cools, making and breaking contact in the meantime. These are problems frequently observed in game consoles, GPUs, and mobile phones.
A4: Yes, this is one of the usual symptoms of a bad thermo-connection (on a QFN package). If there are holes or dislodged solder below the large centrally located pad, heat transfers to the PCB, causing the chip to overheat, and thermal protection activates. Repair requires breaking the component, re-soldering it, and remounting it so that the thermal path is solid.
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