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Exclusive UK Distributors for Jovy Systems
Anti-warp Jig Kit Developed using Jovy RE-7500
020 8361 1790 OR EMAIL firstname.lastname@example.org FOR INFORMATION.
This jig is for XBox360 Slim
Made from 4mm thick top quality aluminium
Height between jig and PCB at optimum for proper preheat
In our view the height of the PCB from the jig is of paramount importance
Preheating the PCB properly with large apertures to allow the bottom heater to heat the PCB from below is the proper way to reflow a BGA
Reflowing is the process of applying heat from below and above the PCB, so that the solder balls can be re-molten, ideally using a time-temperature profile. In particular the GPU is most affected as it generates lots of heat which appears to cause insufficient solder joint contact due to oxidation of the pads and/or attack from residual acids of the flux. The XBox360, PS3 and a lot of laptop GPU devices need to be reflowed.
During the reflowing process, the PCB should be placed on the rework machine's XY table, and both the upper and lower heaters should be warmed up. In the case of the Jovy RE-7500 this is just a couple of minutes as the dark IR emitters reach at least 80% of their maximum output.
Ideally the PCB is preheated using just the lower heater from room temperature to about 150°C. The Jovy's upper heater can be set high to assist the preheat process. At around 150°C, flux begins to activate. It is important to ramp up the temperature at less than 2°C per second to avoid thermal shock and uneven expansion of the PCB, which can cause even more warping of the PCB.
Recommended time for preheat stage is 90-120 seconds. With IR machines the time should be 120 seconds at least to allow the IR radiation to be absorbed by the PCB and in turn the BGA. Preheating properly is crucially important for the reflow process to be successful.
The soak stage is when the ramp rate is slowed down to allow the PCB and the BGA to soak up the temperature in readiness for the reflow. This is also the time when the flux is performing its duties such as cleaning and de-oxidising the pads on the BGA and the lands on the PCB. Furthermore the flux acts as a heat transfer medium and spreads the heat more uniformly across the BGA area on the PCB. The time between the end of the preheat stage and the end of the soak stage should be about 2 minutes and the temperature should reach about 185°C.
The reflow stage should be about 60 seconds in total and it is at this time that the maximum reflow temperatures will be reached. Once at the maximum reflow temperature, it is a good idea to keep at above the melting point of the solder alloy (217°C for lead-free solder) and it is recommended to exceed the melting point by 25°C which means 240°C or thereabouts, and keep at this temperature for up to 30 seconds.
This is known as TAL (Time above Liquidus) and this is when the intermetallic layer is formed by the dissolution of the base metals. ***
The cooling phase follows the reflow phase and it is best to allow the PCB to cool naturally to room temperature.
*** The K-type thermocouples used to measure the PCB temperature relies on good surface to surface contact between the probe tip area and the PCB. Placement of the thermocouple is very important and it is possible to under-read the actual temperatures of the PCB may be much higher than what is displayed. The reason for this is :
(i) The thermocouple is not placed under the BGA but on the edge of it for obvious reasons. Since it cannot be placed where it should, there is an inbuilt error due to this fact. Recent tests using a 0.25mm diameter K-type thermocouple placed under the BGA, showed temperatures between 20-40 deg C higher than shown by the thermocouple placed near the BGA.
Therefore it is highly recommended to limit the maximum reflow temperatures to around 195-200°C, as the actual temperature under the BGA may well be over its melting point. If need be the maximum temperatures can be incrementally increased in small amounts. Otherwise you may end up with delaminated PCB or BGA or worse popcorn the lot.
Our anti-warp reflow jigs have been carefully crafted to allow maximum exposure of the PCB from below which is so important for successfull reflowing. It is very important for the PCB to be heated as evenly as possible, and the largest apertures have been created on our jigs to allow the bottom heater to be effective. Using 4mm thick specially chosen aluminium, laser cut for best results, stay true to provide the consistently flat surface of the BGA area for the solder balls to attach to the BGA and the PCB perfectly. The solder balls on a warped BGA may not make contact with all the pads in the circuit board.
Our jigs are fitted with specially chosen stand-offs to provide consistent height, at the correct level above the jig surface, to allow the mounted PCB to be placed on the XY table without increasing the distance of the assembly from the bottom heater. Most jigs of similar design use much higher stand-offs which make the bottom heater less effective and are prone to expansion errors as these are often placed on top of the bottom heater and not the XY table. If the jig/PCB were to be placed directly on top of the bottom heater, then the temperature ramp rate would be too high and the possibility of expansion of the metal spacers could expand at different rates, causing stresses on the PCB surface. Using the Jovy in this fashion is not recommended as the radiation intensity can be dangerously high if the pcb assembly was to be laid on top of the bottom heater glass.
With our anti-warp jigs you will have the best chance of reflowing a BGA or resoldering it after a reball. We have extensively tested our design and the performance is outstanding.
In our view, the following are the most important issues during a reflow process.
1. Use a good profile to suit the solder alloy being used coupled with a wide temperature window no-clean gel flux such as Amtech flux products. Amtech flux products stay active up to 300 deg C.
2. Make sure the PCB is properly mounted on our anti-warp jig.
3. Make sure the thermocouple is well placed with at least 0.5cm of the shaft in good contact with the PCB and that the tip is looking towards the solder balls at a distance of 2mm. Put a little flux to help with the heat transfer to the probe from the PCB if there isn't any flux around the thermocouple probe.
4. Assume the temperatures below the BGA are higher than what is being displayed to avoid thermal damage to the PCB or BGA.
5. Allow the PCB to cool naturally down to room temperature.
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