Profiling BGA Webinar Supplemental (July 1, 2009):
Component Specific Specs
We discussed the need to define BGA specs separate from other components that have different reflow requirements. BGAs typically require more heat to reflow properly but typically there are many other “smaller” components that also populate a PCB that will overheat if you develop your process solely around the BGA. The following 2:40 min video reviews how you can bring both your BGAs and other temperature sensitive components into spec, striking a thermal balance that results in quality products.
Thermocouple Attachment
The following 1 min video shows one of the most reliable direct methods of TC attachment for BGA profiling.
…..but, who can always sacrifice a PCB in the process? We talked about some indirect/non-destructive methods for profiling BGAs that are suggestive, but inconclusive. In the fall I hope to have some results of a study that will help our industry come up with solutions that one can reasonably predict the temperature/profile of a BGA without destroying the PCB in the process or worse the BGA!
BGA Inspection
First there was SPI (solder paste inspection), then there was AOI, now there is RPI (Reflow Process Inspection)
You can see a prior blog posting discussing RPI at: https://profilingguru.com/reflow/what-is-reflow-process-inspection/
RPI works in the world of continuous reflow monitoring, where a profile is created for each and every production board.
In order to automate reflow profiling, a baseline/virtual profile is first established, where one runs a traditional profile with PCB, TC attachment and profiler while the on-board system of 30 thermocouples gathers the same profiling data and reconstructs and converts the traditional profile to a virtual representation. Once a virtual profile has been established, profiles can be collected for all production boards. SPC charting, cPk, traceability and process control are all possible.
So rather then the reflow process being a black box, where anything and everthing can go wrong…..
….alternatively, do you not only know what is going on continuously, but your BGAs using the techniques above are being monitored on a continuous basis.
Your Questions:
Q: Doesn’t the thermocouples utilized by the oven itself (assuming that they are calibrated and verified) provide the same basic information as the secondary set of TCs you are referring to?
ANSWER: No, the oven thermocouples and the secondary KIC TCs have completely different and separate functions. The oven TCs are typically located close to the heaters since their job is to turn the heaters on and off as the temperature drifts from the set points. The KIC 24/7 (or KIC Vision) TCs, located along the conveyor, help to automatically measure the profile that each PCB experiences as it is processed through the reflow oven or wave solder machine. This function is called Virtual Profiling.
Virtual Profiling (VP) provides process traceability as it logs the profile for each PCB, along with information on how this profile fits the established process window. The VP works in real time and offers instant alarm when the process (profile) drifts out of spec. Because it provides basic SPC charting, it acts as an early warning system for trouble ahead. Think of the KIC24/7 or KIC Vision as an automatic profiling system in real time.
Q: I encountered wetting issue with CSP and BGA, how do I solve them? / Q: How about wetting issue?
Answer; In some cases, but of course not all cases, wetting issues are a result of incomplete flux activation in the solder paste and an overall low temp soak, where the components did not reach sufficient energy levels before entering the reflow, TAL stage of the process. Many of these issues are related to Pb – free solder pastes, mixed RoHS components or a number of other variables.
I suggest that the best answer is to research the publications available on the Web for the most relevant solution. The following is a link that closely resembles the issue, but again, you will need to research the most relevant to your situation.
http://www.emsnow.com/cnt/files/White%20Papers/Henkel_Leadfree_Designing_Reliability.pdf
Q: How do you take measurements on each board without TCs?
Answer: KIC software algorithms compare what was observed at the time of the Baseline Profile to what is present within the oven during production. Using the 30 thermocouples in the oven, this data is communicated to the eTPU and the output is the PWI based on the specific process and the specification of that process.
Q: How well does the DPMO relate to the actual defect where there could be placement defects interacting with reflow?
Answer: DPMO is a parameter of only the thermal reflow process. If issues exist in placement or screen printing, it will not be reflected in the DPMO, since KIC is only monitoring the thermal process. Given that all other aspects of the SMT line is functioning properly, DPMO will give an assessment of the thermal defects assuming that the proper solder paste and placement is present at the time the product enters the oven.
Q: What about paste formulations?
Answer: KIC works with any solder paste manufactures to build the solder paste library that is present in the KIC software. This library is updated periodically and verified by the solder paste manufactures in most instances. The library however does not at any one time contain all information about all possible solder pastes. We try our best to be certain the information is present, but changes in formulation and engineering at the solder pate manufactures sometimes causes gaps that are beyond our control.
Q: How important is it to drill into the BGA ball and put the TC in it, vs. putting on the package, slip under the package, and on the bottom side of the board?
Answer: There are many variables in PCB design and component placement that directly and indirectly affect other components, in this case BGA. The best possible answer to this question is in the amount of data that is collected, how it is collected and how this information is applied to the specific PCB and BGA directly. Gathering as much information as possible, charting this info and drawing data driven values is the best possible formula for successful BGA reflow. Using all available data collection methods and positions aids in successfully reflowing this package.
As indicated during the webinar, we are currently commissioning a study to see if non destructive methods can be used in place of drill a hole.
Q: Does your software always choose an extended peak recipe?
Answer: No. Based on the type of recipe and profiles that are part of your normal production determines what path the KIC Navigator (auto-prediction) directs the profile. If your profiles are mainly RTP, the software looks at the values of the library data and suggests set points that will lead to a RTS profile. If your profiles are largely RSS, then the suggested set points will tend towards a RSS profile.
