Category Archives: Rework
There are cases where the nameplate holder and or the contract manufacturer is limited by their capacity when it comes to the rework of a high volume of PCBs. In a majority of the cases this is caused by a poor design by the OEM and a part or parts need to be swapped out, an undetected manufacturing flaw or an installed component does perform as per the specifications of the part. For hundreds or even a few thousand of fully assembled PCBs there is generally enough capacity in terms of either personnel or equipment to get the rework project completed in the time frame required but when it gets in to the thousands or tens of thousands of units, capacity can be constrained.
One of the reasons for this capacity constraint is having the immediate availability of lots of hand soldering technicians. Typically there only a few soldering techs available as boards in higher volume are pushed to have as many components placed as possible by the automated assembly equipment. This means there only a few soldering technicians with the advanced skill set which are required in order to perform high quantities of the rework project. Hence these resources must be found within either the temporary labor pool (their number is steadily shrinking as the number of available positions in a high volume manufacturing environment has shrunk.
The other capacity constraint is the manual nature of a majority of the rework equipment on the market today. For example most BGA rework systems are of the “semi automated” approach where loading and unloading of the part and board is done via human intervention. In addition the printing of the solder paste as well as the site excavation and preparation is also done via a manual process. This slows down the process and again requires armies of highly skilled rework technicians in order to get through large volume projects.
One of the ways companies are solving this challenge is by outsourcing these very large projects. In this manner the responsible company can manage the logistics of the project (getting product from customers, the supply chain and the manufacturing location) and letting the outsourced rework service provider take care of the rework and repair of the printed circuit boards. One of the examples of such a high volume rework project and how a much higher throughput was achieved can be demonstrated by the following video.
When a select rework location consisting of an advanced area array device needs to be removed and replaced, there are many choices in choosing the rework stencil for the job. The type of rework stencil chosen depends on the following choice criteria:
The size of the component
The location of the component in relation to other components in proximity to the one be reworked
The skill level of the technician performing the rework operation
The availability of the equipment
Metal stencils for PCB rework are designed for very larg volume rework where the skill level and patience of the operator is high. In addition, because the process takes the longest the area where the cheapest PCB rework technician costs reside make the mini metal stencil the most pragmatic.
Stay in Place stencils are designed for cases in which the equipment set available to the rework technician is very primitive and the process window has to be the widest. In larger pitched devices the stencil can be hand placed and the device “fitted” tactily into the apertures. In many cases a reflow oven or even a toaster oven or heat gun will suffice for the re-attachment.
When there are cases where the devices are highly complex or they need to be fitted or “shoe horned” in to a very dense space on the board the StikNPeel(TM) style of plastic adhesive-backed rework stencils will do the trick. This also requires very little skill level relative to the min metal stencils.
Whatever your skill level match your rework stencil to the task at h
It is no secret that components on PCBs are getting smaller all of the time. Take a look here at the latest iPhone 6
While there is much being written about the processes for removal and replacement of the more complex devices such as QFNs and BGAs and PoPs, little attention has been given to the very small passive components.
There are some pretty big challenges in removing and replacing one or two 01005 sized package components.
They have some pretty hefty challenges including but not limited to:
There are numerous challenges associated with the manual rework of 01005 passive components. The greatest challenges include the following:
- Difficulty in seeing component with sufficient magnification and optical resolution
- Capability to allow in-situ process observation of the working area
- Securing the component handling process from pick up from tape to placement
- Controling placement of fragile components with very low mass (0.04g/1000 pcs.) so as not to physically damage them
- Force-controlled handling and placement throughout the whole rework process
- Removing components to be reworked without disturbing neighboring devices
- Hand placing new components manually with at least 10 µm accuracy
- Dispensing solder paste or solder paste transfer with consistent solder volumes
There are some very specific challenges in manually reworking this package including:
- There is an extremely high level of dexterity required of the rework technicians due to the size of the components. As a point of reference the pads are typically 3-7 mils (.076-.178mm) square with 2-4 mils (.051 to .102mm) spacing to neighboring pads.
- The temperature ramp needs to be low as the thermal mass of the components is small and the heat withstand properties are limited
- Designs employing these devices have tight spacings to control with even the most dexterous person-some parts are spaced 0.2mm or less
- Control of the volume of the solder needs to be done such that a very small volume of solder paste needs to be placed consistently and controllably on very small pads.
- Site cleaning is treacherous as even the smallest of forces can lift the pad
Like BGAs, leadless devices have primarily non visual inspection criteria. These non inspectable surfaces and criteria through visual inspection require other inspection tools in order to determine if a leadless device such as a QFN, LGA or PLCC has been properly soldered in to place.
Only through the use of X-Ray analysis of the bottom terminated components can there be objective evidence if the process is control or for 100% inspection criteria if the solder criteria have been met.
Also, each of the IO pads needs to be inspected via x-ray imaging to make sure that there has been consistent and uniform wetting all the way out to the pads. For double IO row devices X-ray imaging comes in handy for determining of there are any shorts.
By contrast the only visual inspection criteria of these leadless devices is to see if there is wetting evident on the I/O fillets and to make sure there is solder underneath the device as can be evidenced through the use of endoscopic inspection means. The endoscope can see the few mils of solder between the bottom of the package and the board.
Call BEST to help you diagnose and troubleshoot your leadless device process through x-ray imaging or have us perform AXI for your leadless devices on assembled boards.
Upon removal of the BGA the site needs to be prepared or “dressed” for replacement by the new or reballed BGA. This BGA site dressing needs to prepare the site so that the pads are as flat as possible and that they are inspected for any defects that may be present.
The site preparation can be done in one of many different ways. The most common manual approach to site dressing is the use of flux paste and solder wick to clean up most of the remnant solder on the BGA pads. By using flux paste and a blade tip of the appropriate size solder braid is used to excavate the remnant solder. One needs to take care in using this techniques as undue pressure can lift or remove the pads from the site location. In addition by “scrubbing” the surface solder mask can be damaged thereby exposing the underlying copper layers. This would cause there to be a defect condition on the board.
Another method for site preparation is the use of a non contact solder excavation tool. A heated nozzle blowing out hot air with the properly balanced vacuum source to remove the solder once it reaches reflow can be used to excavate the solder. It is important that the proper distance be maintained above the surface of the PCB in order to remove all of the solder. If it is not removed properly then there remain domes on each of the pads making it more difficult to attach solder balls on the bottomside of the component to these pads. In addition to performing this manually there are some machines which can be programmed to drive back-forth over the area of the BGA.
This is is an important part of the BGA rework process steps.
There is the easy and fast way to remove a BGA and the slower more analytical approach. In the quicker method a profile of a board similar in nature to the one where the BGA is being removed is recalled from the memory of the BGA rework system. It is simple “recyclcd” because by the experience of the operator it has a similar thermal mass to the board being worked on. While this a fast approach it may due for lower valued boards or when speed vs risk of board pr neighboring part damage is assessed.
In the more analytical approach the complete profiling of the PCB takes place (this presumes that a board can be sacrificed) . Usually a thermocouple is attached to the solder balls on the BGA (drilled in to the center of the BGA sphere), in to the die of the device and on one or more of its corners. In addition a thermocouple is also used in close proximity of the device being reworked so as to see what if any affect there may be on the reflow of neighboring or mirrored devices. Once the thermocouples are properly embedded the thermal profiles of both the bottomside heaters and the nozzle can be adjusted for maximum effectiveness.
Given these disparate approaches one can see that one way is less costly and consumes little time, yet is highly dependent on the knowledge, skill and experience of the rework technician while the second is dependent on having more time, having the availability of a board to sacrifice and having the finances to pay for these extra steps.
As the complexity of area array devices increases the need for BGA x-ray inspection and the capabilities of the x-ray system are increasing.
With the newest additions to the IPC-A-610 inspection criteria to include :
30% voided of the inspectable area
Sample of head in pillow defects
Sample of non-coalescing solder in the solder ball jpinsts and
Cracked solder joints it is becoming increasing necessary to have a very goos x-ray inspection criteria.
It is important that the x-ray tool you are using has:
3D like inspection capabilities
Software that measure the ball diameter, concentricity, roundness and voids performing the calculations for you. In addition the digital x-ray images are important when you have stacked package.
When there is volume production of the x-rays requiring the same areas of production quantities to be inspected it is very critical to have good PCB x-ray inspection capabilities.
Instead of using compressed air or dry brushes to removal remnant coatings, mask or other loose debris from a printed circuit board I have found a new tool to vacuum off the debris.
This handy tools will remove loose debris using a vacuum source generated by the input of clean shop air. The handheld cleaning wand can be configured with several different tips in order to create different vacuum pressures and cleaning nozzle diameters to remove debris. In addition the device has a an air velocity potentiometer which can also adjust the vacuum intensity.
The video below is a demonstration of this tool.
The tools can be cleaned out after vacuuming off a large amount of debris. It also comes with a micro filter in order to make sure that clean air is inputted.
Today’s modern electronics have numerous components that cannot be visually inspected after assembly due to hidden or buried interconnections which have allowed dramatic increases in interconnection density. These component types also allow interconnections to be “stacked” and components to be mounted closer and flatter to the PCB while allowing for signals on the PCB to go faster and with less interference. X-ray inspection is the only viable option in many cases.
One of the package types which requires PCB x-ray inspection is the component which has the greatest number of placed devices are leadless devices. These package types, whose specific designations includes QFNs, LGAs, PLCCs, QONs and others. These packages are commonly constructed such that there are both IO connections for the signal wiring as well as a larger ground connection for dissipation of heat through a ground plane. Many times these devices post reflow are only a few thousands of an inch off the surface of the board making visual inspection underneath the device impractical. In addition the IO connections are more commonly without a solderable surface on their edge making it difficult to determine if even wetting of these solder joints has occurred. Another challenge is investigating whether or not proper reflow of the solder underneath he package has occurred without voiding. These voids are formed when the expired flux species have not been allowed to escape because of the very low board standoff heights and the lack of a “channel” for the flux gases to escape. These voids can cause long term reliability problems of the solder joints.
The other common package type requiring x-ray inspection during PCB assembly is the BGA. In these packages an array of solder balls make the interconnection to the PCB underneath the package. Like the leadless device, most of the interconnection are therefore not inspectable. The other challenge, like the leadless device package is “seeing” the extend of the voiding in solder balls.
Other device packages which require PCB x-ray inspection services include but are not limited to “L” lead interconnections, stacked connectors and other high density interconnections. Without x-ray as an inspection tool the outgoing quality level cannot be assured.
BGAs or ball grid arrays are electronic packages with a grid of solder balls as the interconnect means and are located in rows and columns on the underside of the BGA in order to make the connection to the PCB. Due to these interconnections being on the underside of the device package, many of the solder joint interconnections cannot be made using optical means. Instead other tools such as endoscopes and x-rays are required to inspect this package type. Only in cases where the outside row is visible and available to an inspection microscope can these solder joints be inspected.
While endoscopic inspection can tell what the interfaces of the ball-package and the ball-PCB looks like, the main elements of the BGA need to be inspected via x-ray. In fact BGA x-ray inspection services are contracted by smaller PCB assemblers (due to the capital outlay of the equipment) to specialty companies to perform the BGA x-ray inspection services or perform the process validation services.
BGA X-Ray inspection services can be used for volume production, process validation or process troubleshooting. Volume production builds need to have these device locations inspected in order to confirm that they meet the inspection criteria is outlined contractually or by IPC’s A-610 inspection standards. Process validation can also one of the BGA x-ray inspection services performed by these specialty service providers whereby the process is run and inspected via a sampling plan to make sure that it is in control. Subsequent production lots or sample inspected per an inspection plan. Finally, process troubleshooting can be performed by having the BGA x-ray inspection service provider help document anomalies and defects and confirm those problem areas in the process.
BGA inspection service providers should have the trained and certified staff of electronics inspectors as well as the right quality x-ray system in order to provide these BGA inspection services.