Printed Circuit Board Assembly & PCB Design SMT Electronics Assembly Manufacturing Forum

Printed Circuit Board Assembly & PCB Design Forum

SMT electronics assembly manufacturing forum.


Rich Croteau

#14712

BGA Repair Problems | 12 August, 1998

Have an SRT w/preheat, 18 layer board all copper, 456 BGA needs to be removed and replaced. The manufacturer is Chip Express and during reflow it warps up or down. Causing either opens or shorts on the corners. I was using gel flux with multiple profiles. Now going to try dispensing paste with Camelot using a profile on the lower end of the window. It should reduce the peak temperature and time in reflow, hopefully will not warp. We have baked the parts prior to trying to reflow them. Could use help with this problem part, "Chip Express" Thanks in advance, Rich

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Mike C

#14714

Re: BGA Repair Problems | 12 August, 1998

| Have an SRT w/preheat, 18 layer board all copper, 456 BGA needs to be removed and replaced. The manufacturer is Chip Express and during reflow it warps up or down. Causing either opens or shorts on the corners. I was using gel flux with multiple profiles. Now going to try dispensing paste with Camelot using a profile on the lower end of the window. It should reduce the peak temperature and time in reflow, hopefully will not warp. We have baked the parts prior to trying to reflow them. | Could use help with this problem part, "Chip Express" | Thanks in advance, | Rich Here are a couple of things to try 1) Make sure the board is not restricted from expanding. Do not hold the PCB on the edge or with tooling pins. The board needs to expand. 2) Consider a longer dwell (and higher temp) in the preheat stage with the bottom heaters, this will help to minimize localized warping. I had the same problem with a board (unfortunately I had a POS rework station without bottom side whole board preheat), and I had to resort to tooling that held the board flat around the BGA, but allowing the rest of the board to flex as it needed to. But I would only use that approach as a last resort. Hope this helps Mike

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Bob Willis

#14713

Re: BGA Repair Problems | 17 August, 1998

| Have an SRT w/preheat, 18 layer board all copper, 456 BGA needs to be removed and replaced. The manufacturer is Chip Express and during reflow it warps up or down. Causing either opens or shorts on the corners. I was using gel flux with multiple profiles. Now going to try dispensing paste with Camelot using a profile on the lower end of the window. It should reduce the peak temperature and time in reflow, hopefully will not warp. We have baked the parts prior to trying to reflow them. | Could use help with this problem part, "Chip Express" | Thanks in advance, | Rich The problem you have is related to the heat input to the device. You need to get more heat pre heat into the board prior to applying heat to the topside. The speciafication for rework may be of value to you. REMOVAL OF BALL GRID ARRAY

BGA (Ball Grid Array) Component Removal The following procedure may be used to remove and replace Plastic Ball Grid Array (PGA) components. It may also be used for Chip Scale Packages (CSP). Initially if the components are to be re-used or retained for failure analysis the board assembly should be baked to remove any moisture in the device. This will also prevent any damage to other BGA parts in close proximity on the circuit board. To reduce the amount of solder spiking on circuit pads and BGA during component removal a liquid flux is flooded under the component using a fine needle dispenser. In many cases flux may be eliminated as all the solder on the surface needs to be removed. An uneven surface will make placement of parts difficult. A specially selected nozzle is applied over the BGA and hot air (215-225oC) is applied. Hot air is forced into the nozzle and will, in time, cause the solder joints to reflow. Nitrogen may be used as an alternative to air for both component removal and replacement. The rework process is best monitored with a thermocouple lead noting pre heat, peak temperature and the time when reflow takes pace. The lead may be placed inside the nozzle or, better still, under the centre of the BGA using thin cable. When complete reflow has occurred a vacuum pickup is applied to the top of the device and the part raised from the board surface. If the centre of the device base has been at 200oC for over 10 seconds all the ball terminations will have probably reflowed. Care should be taken in confirming reflow. The same technique is used with infra red rework equipment, In this case the IR light beam is focused on to the device and surrounding board until reflow is detected. This may be determined by lightly touching the side of the device before lifting the component with a vacuum pickup tool. Touching the side lightly easily detects reflow of all joints like traditional surface mount component removal. The pad surfaces should now be checked for any damage. Prior to cleaning the surface of the pads the surface should be allowed to cool for a couple of minutes. Re Balling BGA Devices To replace solder balls on to a BGA first all the uneven solder or solder balls should be removed using a conventional de soldering tool. When the surface of all the pads is flat either solder paste may be applied to the part, a liquid flux dispensed or flux paste stencil printed on to the BGA. A clean stencil is then placed on to the BGA and solder balls placed in the stencil. The over sized apertures allow the balls to roll in to the holes. Make sure not may are placed into the stencil, with care any open apertures can be manually filled. Slowly lift the stencil free from the BGA surface. It may be necessary to either remove extra ball which have slipped through the apertures or position balls that are not present. Either way this technique is faster than placing all the balls manually. The BGA may now be reflowed in a normal reflow oven making sure the manufactures soldering recommendations are followed. It is recommended that the devices are placed on a flat pallet for passing through the reflow oven. After reflow check that all the balls have reflowed and no paste is present on the base of the device. If required the device may be recleaned before re use. It is recommended to check around the outer plastic to laminate interface. If any cracking has occurred this is the most common place to see "pop Corning" The component may now be reused on the assembly. The repositioning of the device and soldering is conducted using the same process conditions used to remove the device. The process conditions should be specified and include the maximum temperature for pre heat and reflow and the duration of each cycle. In the case of large QFP and BGA devices under board pre heating is recommended. Pin-In-Hole Reflow The use of custom stencils may also be employed for prototyping or pin hole reflow. Often it is difficult to achieve sufficient paste quantity during normal stencil printing. A manual paste printing operation into a selected component through holes may be conducted prior to normal printing. This will allow a near perfect through hole solder joint during reflow. Cleaning the Repair Stencil Remove the majority of the paste with the stencil blade and place it in paste container for re use. Remember to seal the lid to prevent drying out of the paste. Using the same cleaning solution used in production for cleaning stencils wash out the stencil and stand it upside down and pour the cleaning solution through the apertures. An automatic cleaning system may be used provide the stencils can be held and that the pressure of the washing action will not bend the side walls of the stencil. After cleaning check all the apertures are clean, dried solder paste in apertures prevents satisfactory printing just as in production operations. Rework stencils may be produced for any surface mount pattern. A custom set may be produced from the wide range of stencil apertures held by Chemtech. For further information on rework stencils contact Chemtech UK Limited. REPLACEMENT PROCEDURES

BGA Component Replacement As it is not possible to see the pad and terminations during replacement all the pads should be flat prior to replacement. Solder wick may be used with a soldering iron or a vacuum de-soldering tool with a flat ended vacuum tip. A flat blade soldering iron tip is also a good method for pad flattening. If there is no evidence of spiking on pads due to the correct use of flux, solder removal may not be necessary. Removal of the solder to flatten the surface does make life easy. Pads may be re-coated with flux prior to placing the new part. A tacky flux gel can be used as an alternative to flux. When applied the flux gel holds the part in position even with board movement. The board should be cool when the flux gel is applied. Flux or gel may be used on boards for rework as there is already a coating of tin/lead left from the removal process. They should not be used on copper or gold board for prototype assembly. Solder paste may be used as an alternative. In this case a mini stencil is used to print the paste on to the pad array. Small rework stencils have become very popular today and are available from a number of stencil manufacturers like Chemtech Limited. The mini stencil consists of a 0.006" stencil with foldable side walls. The side walls are only partly etched to allow the four sides to be folded. This provides four side walls which prevent the paste moving off the stencil during printing and a surface to hold and position the stencil. If a mini stencil is used without side walls Kapton tape may be placed around the edge of the stencil. This holds the stencil onto the surface of the board and it prevents paste spilling out and contaminating the board surface. Using paste may seem a little time consuming but it is the preferred process for replacement. It overcomes coplanarity issues, provides the fluxing agent and replicates the original process for assembly. A new BGA part is placed on to the surface of the fluxed pads or paste deposit using the corner alignment pads on the board or using guide marks. The marks are invaluable for component positioning. Special optical alignment systems are often used for part positioning. These image the pads and the base of the device and align them together. This is an expensive option and can be avoided if care is taken during design to add corner marks. If alignment pads are not available on the board design they can be temporarily applied in two corners of the device with a pen prior to initial removal of the BGA. If special optical alignment is not available on the rework equipment this is a necessity. When the rework cycle is complete they may be cleaned off and no one will know how the alignment was achieved. The part is then reflowed using similar settings to those used during component removal, allowing an additional 10-15 seconds for complete wetting of the pad surface. Lightly touching the side of the device can demonstrate complete reflow and the surface tension will pull the part back into line. This is not a necessary part of rework but can be useful during operator training. A full demonstration of the rework and repair of surface mount components and BGA parts is illustrated on the EPS training video tapes. A separate video is available for x-ray inspection of solder joints either before or after rework. Solder paste printing using micro stencils for rework and repair Make sure that the paste being used is the same as in production for printing. Often a dispense grade with larger ball size is used in repair areas and may not be suitable for stencil printing fine pitch parts. Before printing paste onto a board try printing paste images onto a flat surface like copper laminate, glass or smooth hardboard. This will allow an opportunity to perfect the repair printing process. When the correct technique has been mastered then check the cleanliness of the stencil base to prevent contaminating the board surface with paste. The stencil is unique as it provides ease of handling and prevents solder paste from spreading on to the board surface like other repair stencils. Both these feature are achieved by use of a folded support wall. Due to the often limited room available around components only a small area is available around the stencil apertures. Repair printing relies on the skill of the operator. The stencil designs for BGA devices are slightly oversized from the design rules commonly used to day. This is a pad size equivalent to the size of the ball on the device. Using a slightly over sized aperture also allows the BGA stencils to be used for ball replacement. Solder ball replacement is often required if the BGA device is going to be reused on the product. This is often a necessity if expensive ceramic devices are used. It is also quite feasible on plastic parts if care has been taken during component removal. Oversizing of the apertures also makes manual printing easier to achieve in what is a confined position on a fully populated assembly. When a device is to be removed the same care should be taken as was used during its installation. Check the edge of the device between the plastic and fibreglass substrate for any cracking. If any cracking is visible then the part should not be reused. Engineering should be informed about the potential problem. Plastic BGA devices just like plastic QFP (Quad Flat Packs) are prone to cracking during reflow. This is due to moisture being present in the devices and expanding during reflow. The devices when dispatched from the supplier should always be received in sealed desiccated foil bags. Inside the bags should be a humidity indicator indicating near zero humidity. If parts are provided to production or repair areas not protected engineering advice should be sort before use. Re Balling BGA Devices To replace solder balls on to a BGA first all the uneven solder or solder balls should be removed using a conventional de soldering tool. When the surface of all the pads is flat either solder paste may be applied to the part, a liquid flux dispensed or flux paste stencil printed on to the BGA. A clean stencil is then placed on to the BGA and solder balls placed in the stencil. The over sized apertures allow the balls to roll in to the holes. Make sure not may are placed into the stencil, with care any open apertures can be manually filled. Slowly lift the stencil free from the BGA surface. It may be necessary to either remove extra ball which have slipped through the apertures or position balls that are not present. Either way this technique is faster than placing all the balls manually. The BGA may now be reflowed in a normal reflow oven making sure the manufactures soldering recommendations are followed. It is recommended that the devices are placed on a flat pallet for passing through the reflow oven. After reflow check that all the balls have reflowed and no paste is present on the base of the device. If required the device may be recleaned before re use. It is recommended to check around the outer plastic to laminate interface. If any cracking has occurred this is the most common place to see "pop Corning" The component may now be reused on the assembly. The repositioning of the device and soldering is conducted using the same process conditions used to remove the device. The process conditions should be specified and include the maximum temperature for pre heat and reflow and the duration of each cycle. In the case of large QFP and BGA devices under board pre heating is recommended.

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