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In late 2006 a friend of mine approached me with information regarding a recently released gaming console and a particular problem that seemed to be surfacing quite frequently. After receiving the details from this gentleman I proceeded to do a little research myself.
I?ve been in the PCB repair industry for approximately 7 years. I have access to all common PCB repair equipment. During the past 7 years I?ve successfully researched and developed solutions for many ?high profile? manufacturing defects. I have developed various techniques that reach beyond IPC standards. I have experience with various CSP and BGA packages of almost all construction. I?ve worked with various sizes of CSP and BGA devices ranging in size from 12x12mm to 40x40mm leaded and lead free configurations. I am self-taught in the following areas:
BGA & CSP Reworking
BGA & CSP Pad Repair
Pre-rework BGA & CSP fault identification
Pre and Post Rework X-Ray Inspection
Pre and Post Rework In Circuit Emulation Testing
My daily occupation involves the development and processing of solutions for factory defects within personal computers that contain highly populated and highly dense PCBs. These PCBs are usually so highly populated that heat sensitive BGA and CSP packages are usually found on both sides of the PCB and sometimes adjacent to each other.
Continued...
Discuss...
The information below does not specifically identify various troubleshooting techniques and identify specific equipment as this information is vital to the operations of my business and is not available to the public.
After purchasing a faulty console unit producing a three red light fault I proceeded to do a little investigating. Various initial board flexing produced mixed results. It seemed that any amount of board flex would either allow the system to start normally or make matters even worse. Applying my board troubleshooting techniques I was led directly to the two surface mount BGA packages of the CPU and GPU. Before I focused intensely on these two devices I did some standard probing on both top and bottom of the board looking for anything obvious. All probing is done with the help of a variable zoom microscope. Everything looked good from a ?top side? perspective. Next, I moved on to the surface mount Video RAM. For this I use a BGA solder sphere inspection camera. This camera is idea for inspecting the outer array of any given BGA / CSP package.
It?s common knowledge that if a bad solder contact lies beneath any BGA package (that is not a direct post assembly problem) MUST start at the outer array of solder spheres. In other words, solder balls under BGA and CSP packages don?t simply ?pop off? or lose contact anywhere under the chip. It WILL in almost all cases start at one given edge or corner.
The Video RAM sphere contact looked good on all 8 chips. I then shifted my attention to the S Bridge BGA package. All looked good there as well. I then progressed to the Video Processor. After careful inspection and applying various probing techniques I concluded that the GPU was not to blame for the 3 red light fault.
On to the Central Processing Unit or ?CPU?. This is where the fun begins. Immediately, I noticed that on the two corners facing the GPU each corner?s solder spheres appeared to be losing contact with their corresponding pads on the motherboard. (See Figure 1) This was verified by using the BGA camera in conjunction with a probing tool as a gentle ?lifting device?. After this discovery I proceeded to give it a test run. Using various techniques for probing PCB problems in the past I was able to stabilize the board to eliminate any board movement / flexing in order to effectively verify this was indeed the problem area. With the board stabilized and applied gentle pressure on these two suspect corners of the CPU and the board booted properly.
I am lead to believe that this is the problem area on the particular motherboard I had. Why did this problem occur? This problem occurred more than likely because of the following reasons:
HEAT ? The manufacturer installed two fans at the rear of the system to perform the cooling function. These two fans in my opinion do not serve the purpose of cooling this type of unit. To suggest that two outward blowing fans placed at the rear of the system will ?pull in? enough air around the unit to effectively cool the CPU and GPU is outrageous. Fresh - cool air needs to be introduced inside the unit and then passed through the CPU and GPU heat sinks. This now hot air needs to be evacuated from the unit?s casing. My version of this process can be seen in Figure 2.
HEAT SINK MOUNTING - The heat sink mounting clamp or what ever you want to call it is applying pressure onto the PCB. Any and all CPU and GPU heat sinks need to be anchored to the bottom metal chassis. Ideally you want the CPU and GPU heat sink to sit on top of each device. Then, screws need to be inserted from topside down through the heat sink, through the PCB and screw into the bottom chassis. This in essence will flatten the whole assembly as it?s tightened. This will eliminate board warping at normal operating temperatures.
PCB MATERIAL ? The PCB material used in this gaming system is poor and is extremely susceptible to board ?bending? or warping once heat is introduced. I cannot be more specific in this area as I?m not a professional in PCB manufacturing. Given my experience with various PCBs by various companies I can state that this PCB in this console warps extensively compared to others I?ve worked with in the past few years.
What can be done to repair this problem?
Answer ? Nothing currently. There have been many reports of people ?buying time? by ?reflowing? the GPU. First, chances are you?re reflowing the wrong chip.
You may ask.. ?If we?re reflowing the wrong chip then why are we seeing positive results? You?re seeing positive results because you?re action of applying heat to the PCB is warping the board. It?s changing the clearance under the problem area of the CPU allowing the solder sphere to temporarily make contact. If you attempt to ?reflow? the CPU you?ll find that you can?t get the CPU solder to reach ?liquidus? or ?melt? because of the pervious extremely high temperatures that it?s been subjected to up until the problem surfaced.
The extremely high operating temps of the CPU have dramatically raised the reflow threshold of the CPU?s solder spheres (it literally has broken the solder down). I have found that with 220 degrees C of bottom pre-heat temp and upper heater temps of 400+ degrees C is what it takes to allow the CPU spheres to reach liquidus. Once you reach this level of heat you almost certainly have damaged the CPU and bending or warping is apparent on the CPU itself. During this removal process only top rate BGA fluxes were used. Various profiles were also experimented with as well.
The only way that I can see possible to repair these units is to have access to replacement CPUs to use once the old CPU is removed. Also, another solution would be to buy a new replacement motherboard and properly cool it from the beginning of its service life.
Jan. 25 2007 - SMTRework
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