Rework Stencils

Designed for manually applying solder paste to circuit boards in certain areas, the rework stencils that BEST produces are laser cut stencils designed to replicate the original manufacturing process of a section of a board. These stencils can come without flaps, or with flaps like many of our other stencils, in order to prevent the splattering/spilling of solder paste onto unwanted areas. These mini stencils are custom made following the Gerber or CAD files presented by the customer for their rework process in order to fix any board necessary. StikNPeel one-use stencils are also another method of rework simplifying the process, making it easy and quick.

BGA Rework Preheating the Board

It is critical to get the PCB up to temperature prior to removal when working with lead free solders. The optimal board temperature is  around 125 degrees C. By preheating the board there will be less thermal stress on the area being reworked as the temperature difference between the site location and the rest of the board will be minimized. This reduces the thermal stresses on the laminate as well as making sure the rework cycle is not to long.

Some modern preheater use multiple zones whereby the board immediately underneath the rework can be preheated to one temperature and the other areas of the board can be preheated to another temperature. This further reduces the stresses seen on the laminate right underneath the are being reworked.

Make sure you consider the preheating cycle when you are developing the profile for rework the BGA.

BGA Profiling

BGA Profiling Methods

There are several BGA profiling methods available to correctly identify and construct a BGA rework  profile. The method used is a function of the availability of a sample or test profile PCB and component to be reworked.  One method assumes that a profile PCB and part is available, while another assumes that no dedicated profile PCB exists. Whatever the method used it is critical to read the joint temperature.

Profiling Using a “Test” PCB

This is the preferred BGA  profiling method as it provides the most accurate joint temperature readings, serves to reduce the chances for damage to nearby components on the boards being reworked, while reducing the chances for board damage or warpage. In this method the thermocouple is attached to the solder ball either on the side of the BGA ball using conductive epoxy or into the BGA joint by drilling through the backside of the BGA and filling it with thermally conductive epoxy. The thermocouple is then properly routed between the balls or on the underside of the PCB.

Example BGA Profile Ouput from Thermocouple

BGA Profiling

QFN Rework

QFN rework refers to the repair and restoration of quad flatpack no lead electronic chip components. These chips do not come with an external connection pad as other devices like BGAs, QFPs or SMT devices.  In a QFN leadless package, the interconnections are all on the bottomside of the package.

The removal process is straightforward for this device family. Simply add some paste flux near the site area and reflow the site.  Bottom side heat is recommended in order to make sure the board and the part is not unduly stressed.

The surroundings of the QFN are generally packed tightly and this poses a problem in that the locally acceptable way of solder application using a stencil cannot be practiced. The alternative is to dispense a large number of contacts in a slow process in which solder paste is dispensed.

In terms of placing the device back onto the PCB there are numerous methods available. The QFN rework method that is simplest and requires the least amount of equipment and has the highest first pass rework yield is part bumping process. Here the part is bumped by pasting solder paste onto the bottom of the device and reflowing it. After cleaning the uniformly “bumped” part can be placed, after applying flux paste, onto the lands of the board.  Reflow per the developed profile in order to make sure the device is soldered properly to the board. Follow with x-ray inspection inspecting per the customer or applicable IPC standards.

BGA XRAY Inspection-Advanced Capabilities

BGA, leadless device and other complex rework requires the use of X-Ray inpection for sufficient quality control. This tool  provides precise image capture and  reporting. It also  offers BGA and BGA/device void measurements, along with enhanced PCB xray inspection capability.

Only qualified and experienced in order to perform sorting and other related operations. Whether it is meeting your criteria, or offering viable tips or suggestions, Engineers help develop, monitor, and implement a range of strategies to achieve desired outgoing quality levels. They can also meet any form of customer specific inspection or sorting criteria set forth by clients.

Inspection criteria may relate to solder ball roundness, void percentage or ball size. Once this has been designated, the software tools can automatically measure each ball on the BGA. This helps ensure precise and repeatable sorting, along with other pertinent measurements and factors. The software can even create 3-D image renderings for detailed reviews.

With an advanced XRAY capability , individuals have better visual representation of BGA ball size measurements. They can also determine pass/fail criteria, along with the root cause of potential system failures. With advanced capabilities, the software can even download all BGA analytical data for quality control and problem tracking purposes.

With advanced capabilities, the best x-ray system offers a wealth of benefits for users. This includes precise interpretation of results, along with identifying root causes of failure. The system enables product screening for void detection, especially for solder joints and solder ball diameters. Contamination or foreign intrusion can be detected as well.

Product screening covering broken wire bonds amongst other screenings can be done by the system as well. Perhaps the most innovative aspect is the high-resolution x-ray imaging, which can be secured from a myriad of angles, views, and perceptions.

BGA Reballing with Preforms

If you work in a lab or in a repair facility and find yourself having to do BGA reballing, the process may seem difficult at first. On one hand you may want to have the job completed by an outsourced service provider specializing in component rework, but on the other hand you may also want to keep the costs low. In such cases, you could opt to do it yourself.

There are many benefits of going down this road. For starters, you have a lot more control over the project when you do it yourself. This is because you would know exactly what to do in order to make sure that you carry out the process to your specifications. To add to that, it is also not very expensive doing it on your own. The only cost required is the reballing kit and little bit of your time.

By purchasing the BGA reballing kit and reballing the part on your own you can control the timing of the rework. The reballing kit should be manufactured such that it is simple to process in order to keep the first pass yield rates high. By using a reballing preform which uses double-sided tape and is sized to the outside dimensions of the package, simpler processing can be assured.

BGA Reballing Process

BGA reballing involves removing the remnant solder balls from a BGA package and adding new solder balls into same. It involves a simple procedure that can be carried out by any individual when need be. It can involve either lead and lead-free solder balls.

The tools and devices used for the above BGA reballing process are not that technical at all. They can be handled by any individual. In fact repair depots repairing laptops and other computer accessories can perform this process with a simple tool set and reballing preforms. These preforms are preloaded fixtures matching the pattern, allow and ball size of the package allowing the user to easily replace the grid pattern with solder balls on the bottom of the package.

The the tool set for BGA reballing includes at a minimum the following:

1. Soldering iron with blade tip
2. Solder wick
3. Flux
4. Isopropyl alcohol (for cleaning)
5. Heat source for reflow (it could be as simple as a toaster oven)
6. Reballing preform

Reballing preforms are custom-made and are made to the correct pattern via the part data sheet or by taking exacting measurements of the device. When done properly this 90% plus yielding process can be completed in as little as 10 minutes per device.

BGA Underfill Rework

There are times when BGA and CSP packages are underfilled for product durability and reliability reasons. Increasingly handheld devices, such as cell phones and PDAs, automotive electronics, and military applications, have a need for greater miniaturization and higher performance. In these applications, mechanical stresses can induce early failures. Solder joint performance under drop and bend tests, two test which are extremely demanding but required for handheld devices, is increase with the use of underfills.

One of the downsides of underfill is the lack of reworkability from a yield and cost standpoint. Most if the dunerfills are some type of epoxy with a softening point but just below that of the liquidus temperature of the solder. This coupled with the lack of volume control of the underfill as well as the completel underside coverage of the udnerfill makes it challenging from a rework perspective.

Two steps are usually required in the rework of these package types. First: the device is properly removed from the PCB (Baked our first so that the part can be assured of being properly salvaged). Second: the residual underfill is removed from both the package and the PCB. Typically the underfill forms a seal around the component as well as being drawn between component and PCB by capillary action. In addition if placed under a shield it may be ttahced to the shield. If the material can be heated to a plastic state to become vicious, a tool, combined with a shear force, can be used to pry the package from the PCB. Once removed, some form of mechanical or etching process will be needed to remove the residual underfill material from the PCB. This is where pads and laminate or components can be damaged thereby impacting the yield.

The yield detraction is usually over 30% but with some care and time this process can be completed and the device itself salvaged.

Underfilled BGA rework can be performed when care is taken.

IPC-a-610e

ipc-a-610E

The IPC-A-610E, Acceptability of Electronic Assemblies is the most widely used specification published by the IPC for the electronics manufacturing industry. This publication is a visual document that provides comprehensive accept/reject criteria for handling of electronic assemblies, mechanical assemblies, component installation, location, orientation, soldering, cleanliness of assemblies, marking of assemblies, coatings, laminate conditions, discrete wiring assembly and surface mount assemblies.

Worker Proficiency students will be evaluated on their mastery of the IPC-A-610E material through end-of-course tests that have both open-book and closed-book components. Students must receive an average of at least 70% on both tests. Upon satisfactory completion of the course, participants will receive an IPC certification that is valid for 24 months . Everyone who successfully completes the worker proficiency certification will receive a copy of the IPC-A-610E Acceptability of Electronic Assemblies.

This class does NOT include any hand soldering instruction. Please see the Assembly classes for hand solder training.

Topics Include:

• Terms and Definitions
• Handling Electronic Assemblies
• Mechanical Assemblies
• Component Installation Location/Orientation
• Soldering
• Cleanliness
• Marking and Coatings
• Laminate Conditions
• Discrete Wiring Assembly
• Surface Mount Assemblies

This is a 3-day, lecture only course that teaches students the accept/reject criteria in the IPC-A-610, for all three classes of PCB assembly. Everyone who successfully completes the Worker Proficiency certification and examination will receive an IPC-A-610, IPC-A-610 Worker Proficiency Handbook and an IPC Certificate of Training.

Please visit BEST Inc. at

http://www.solder.net/training/courses/IPC-A-610-CIS_description.asp

for more information.