|Ask the Experts|
November 28, 2018
Is Pre-Bake Standard for Rework?
We subcontract all board assembly, however on occasion we need to perform rework. This can include adding a resistor, replacing a BGA or even PTH connectors. The PCB's are mainly FR4, however polyimide materials are occasionally reworked. Is there a rule or standard that can be applied for pre-bake requirements prior to rework?
|Expert Panel Responses|
The pre-bake consideration is also dependent on the components residing on the board. You can check the various IPC standards for this. Remember that increased reflow temperatures for lead-free solders have shortened the typical floor life for components and boards. The MSL levels can vary, but in general 80 to 125 deg. C is a common range for up to 24 hours. There are also "bake and bag" dry storage solutions out there, depending on your rework backlog.
The rule of thumb used by us in our rework area for FR4 and Kapton based materials is 105 degrees C for 8 to 12 hours to insure that all moisture absorbed into the pcb materials are driven out and we also bake out the components that will be used as well during the rework. Same profile unless the parts are not rated for that temperature bake out. Then one must increase the length of bake out at a lower temperature but most parts can be baked out at a higher temperature without compromise. BGA's are the most susceptible but other overmolded qfn style packages are also susceptible. Good question and good luck. Rework requires patience.
VP Engineering Services
STI Electronics Inc.
Pre-bake prior to rework is highly recommended. This is especially the case for BGA and PTH, less so for SMT resitistors. The requirements should be based on the materials, not the fact that you are going to perform rework since the thermal excursions should be no more agressive than your original reflow profile. Do not forget to pre-bake your BGAs too. We have seen many cases where PBGA components delaminate due to moisture.
VJ Technologies, Inc.
I would recommend IPC/JEDEC J-STD-033B which is a specification for handling, packing, shipping and use of moisture/reflow sensitive surface mount devices. It is available on the JEDEC website after logging in. We follow their recommendations for our manufacturing process.
Director of Engineering
Interconnect Systems Inc.
In my experiences and experiments it never hurts to pre bake epoxy with ED copper being heated at high temperatures. Prevents heat shock and helps eliminate pad lifting. 250 degree F for 2 hours IPC also has these instruction that you should follow so you comply for your reliability.
Applications Project Manager
Quick Turn Flex Circuits LLC
I cannot answer your specific question, but I would like to point out that if you are subcontracting, you are doing it to save money. If you are doing rework, you are not saving money. You are spending more. I would carefully review your vendor and possibly either select an alternate or bring it all in house. You should be able to eliminate this rework altogether. Obviously I do not have all the details, but those are some thoughts to consider.
President & CEO - Retired
Bliss Industries, Inc.
No, it is not a common practice to pre-bake an assembled PCB prior rework, unless the PCB storage conditions (eg high humidity over long period of time) are bad.
Managing Director, Asia Pacific
Sorry to those who said no, but that is incorrect. The correct answer is: it depends- on a lot of factors.Here is what you should consider. First, what TYPE of PWB material and component requiresrework, and what KIND of rework? And what is the cost of the assembly? Is itreally cheaper to throw it away? If it is a small chip capacitor that needs to be added,and this can be done with a soldering iron and the PWB material is a standardrobust FR-4 or polyimide material, then a bake is not usually necessary. Soldertouchup, solder bridge removal, tombstoned part rework, stuff like that where asolder iron is used and the PWB will not be heated significantly (significantlymeans not above its glass transition temperature (Tg) as listed in the IPC4101B slash sheet)usually can be performed without any danger to the PWB or thepart. Polyimide PWBs are more easily damaged than most FR-4 PWBs if the solderiron rework is not done properly, however, and may fracture or delaminate ifstresses are applied from the solder tip. If the CCA needing rework is a high-cost assembly,and/or the component being reworked is going to be removed and re-used, and/orthe type of rework will be solder fountain, hot air or hot gas where the PWB Tgwill be approached, then the CCA should be baked per IPC 1601 guidelines andreworked within an hour or two after the bake. This is a very common practicefor most companies, indeed, it is a no-brainer for some whose assemblies andcomponents may cost several hundred or even several thousand dollars. Remember,the cost of scrap is not the cost of the parts and direct labor alone, it isthe cost of re-ordering replacement parts to build another CCA to satisfy thecontract, re-receiving those parts, re-stocking those parts, re-assembling,re-testing, re-shipping, etc. etc..... It might just be a good idea to bakethat CCA needing rework to ensure it is done without damage.
How many times did you see: It depends! Well, this isanother one. The PCB material type is very important - the polyimide is highlyhygroscopic so the baking before any type of reflow process is recommended. BGAand TH connectors replacement involves reflowing wide areas of the boards anddelamination due to moisture ingression in the PCB structure is pretty common. For minor rework like replacing a chip resistor, capacitor, etc. the baking isnot a must, however a pre-heating of the surrounding area is not a bad ideaespecially when you process flex-rigid or flexible circuits.
Engineering and Operations Management
Pre-baking is a requirement for all field failure returns that require rework. If the rework is for an assembly that is still in process, baking should not be an issue because it is still in a controlled temperature and humidity environment. My rule of thumb, if you don't know where it has been, bake it before rework.Ray Clark, TT Electronics - IMS
I recommend that all assemblies be preheated and to use a hot air pencil to remove the components that need to be removed. When you do not preheat the assembly you are thermally shocking the component and surrounding components and the board assembly in general. If you do not preheat you are cutting corners. I have seen ceramic components crack because of thermal shock. I have seen glass seals crack from thermal shock.Chuck Erbe
If the rework is concentrated on removing and replacing MSD such as BGAS and CSPs, baking the populated board is necessary in order to eliminate the moisture presence on the assembled board. This is clearly defined and reflected on topic#6 of IPC/Jedec J-STD-033A (Handling, Packing, Shipping and Use of Moisture Reflow Sensitive Surface Mount Devices). However, it is important that the appropriate bake duration for the PCBA and the component to be reworked is considered as per table 4-1 since some PCBs and MSD parts are sensitive to baking.Manolito Mapaye, Venture Manufacturing Services
IPC-7711/21 Rework and Reapair of Electronic Assemblies, and IEC EN 61192-5 RWL of Electronic Assemblies, tell to perform adequate pre-bake boards (PCBA) before rework.Gabriele SALA, GS Consultant, Italy
A few thoughts from a pcb manufacturers view with experience in flex-rigid:Andreas Schilpp, Wurth Elektronik
Drying conditions are also dependent on the copper layout, big copper areas make drying difficult and should need a much longer drying time.
A very smooth drying could be performed in a drying cabinet but this needs a long time, i.e. a week.
And last but not least Polyimide is not always the same critical material considering moisture intake. There are new Polyimide-glass constructions that have even less moisture intake compared to FR4.
So many divergent views! Do we need to bake or not is still not clear. Can we decide that it is relative to component density on PWA or the type of component being reworked on PWA? What is the impact on other soldered components when we bake the PCB at 105 deg.C?Dayanand.K, Tata Power SED
As a provider of contract PCB level damage repair and rework services, it us our standard practice is to pre-bake all boards requiring any type of hot gas rework process for a minimum of 12 hours at 100-125 degrees C prior to commencing rework. We also pre-bake any components that will be used in a hot gas process unless they are received in a vacuum sealed static bag with a moisture indicator card and desiccant.
Circuit Technology Center
With no idea of storage of the assembly, the PCBs need to be baked. Also, moisture sensitive devices need to be baked to avoid any thermal shocks and pop corning. Also, inspect the terminations of the devices before populating into the PCBs.Dr. KUTTIYIL THOMAS OOMMEN THARAKAN
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