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Important baking step before SMT process

SMT

What is PCB baking?

PCB baking is the pre-treatment to dry printed circuit boards and moisture-sensitive components before PCB assembly. It is to get rid of moisture left inside them or on their surface. With the PCB baking, the PCB assembly is away from soldering defects such as IC popcorning.

Why Long Stored PCBs Must Be Baked Before SMT?

The main purpose of PCB baking is to remove the moisture contained in the PCB because some materials used in the PCB easily to absorbed water from the external environment itself. In addition, when the PCB is stored for a long period there are opportunities to absorb moisture in the environment, and “moisture” is one of the main culprits that cause PCB popcorn or delamination. “Moisture” is also an accomplice to promoting oxidation.

Because when the PCB is placed in an environment where the temperature exceeds 100°C, such as a reflow oven, wave soldering oven, hot air rework or hand soldering…etc., “moisture” will become water vapor, and then rapidly expand its volume, when heated The faster the speed of the PCB, the faster the water vapor expands. When the temperature is higher, the volume of the water vapor increases. When the water vapor cannot escape from the inner layer of the PCB immediately after it is formed, it has a good chance of supporting it. Expanding the PCB, especially in the Z direction (vertical direction) of the PCB is the most vulnerable because the PCB is superimposed layer by layer, sometimes the vias between the layers of the PCB may be pulled off, and sometimes it may be Causes the separation between layers of the PCB. More seriously, even the appearance of the PCB can see blistering, bulging, cracking, and other phenomena. Sometimes even if the above phenomenon is not visible on the surface of the PCB, it has actually been internally injured. It will cause unstable functions of electrical products, or problems such as CAF (micro short circuit), which will eventually lead to product failure.

We follow the below procedure for baking process:

We put boards separate from each other onto the oven we are used racks for avoiding stack and vertical position

For Rigid-Flex PCB boards bake time depends upon the PCB thickness and Substance of PCBs

1.PCBs Thickness up to 1.0mm boards we kept in oven minimum 2hours at 120oc
2.PCBs Thickness up to 1.8mm boards we kept in oven minimum 4hours at 120oc
3.PCBs Thickness up to 4.0mm boards we kept in oven minimum 6hours at 120oc
4.PCBs made of polyimide/Thermount, all thickness PCBs are kept in oven minimum 6hours at 135oc
The process for Rigid PCB for different surface finishes

For HASL, Lead-free HASL and ENIG

1. All thickness PCBs are kept in an oven minimum of 2hours at 120oc

The hold time between baking and soldering dependent upon the storage conditions. At 50% relative humidity the suggested hold time is a maximum of 8hours. If boards are kept in the oven at 35oc then hold time can be used longer than the usual time.

Are all Components need to be baked?

The most common industry standard (J-STD-033) that governs the handling procedures and processes for moisture-sensitive devices (MSDs) can be difficult to understand. As a result, procedures are often implemented that tend to err on the conservative side, and manufacturers often assume better safe than sorry. One of the easiest ways to be “safe” rather than “sorry” is to bake all MSDs.
Traditionally, baking has been regarded as a cure-all for multiple soldering problems. However, in several situations, this is simply not the case. Many thermal processes are required (curing, solvent evaporation, etc.) in the component manufacturing process. Most components should not require any additional baking between the time they are removed from the manufacturer’s moisture barrier bag (MBB) and when they are soldered. There are only two reasons to require baking MSDs after removal from the MBB:

1. The MSD has a moisture sensitivity level (MSL) of 62. This is the highest (worst) MSL component specification to date, and J-STD-033 (in Table 5-1) states baking is mandatory and these components must then be soldered within the time limit specified on the manufacturer’s label. For these devices, baking is a must. However, applying this process to all MSDs just to be safe is overkill since it only applies to MSL 6 components

2. The MSD has exceeded its floor life.3 This can happen for many reasons but is often the result of avoidable situations:

I. The purchase of full reels of MSDs is required, and parts are leftover.
II. A slowdown in production or rush jobs causes MSDs to remain on the floor longer than planned.
III. Handling procedures for MSDs are not followed or are forgotten.
IV. The MBB is broken or damaged and/or the humidity indicator card (HIC) inside the MBB indicates overexposure to humidity.
V. The MSDs may have been received in an unknown condition from the vendor or customer.
With well-executed handling procedures and quality, a dry atmosphere cabinet maintaining relative humidity (RH) levels at 5% or lower and no more than 30˚C, most of these situations are avoidable.

If the reasons for baking MSDs are not one of the two reasons above, the decision to bake should be analyzed. Is it required or only being implemented to be safe rather than sorry? With the exception of the aforementioned two reasons or a specific customer requirement, baking should be avoided whenever possible. Unnecessary baking wastes time, money, and energy and only serve to reduce the reliability and quality of MSDs. Here are a few reasons baking should be avoided:

I. Baking MSDs can permanently damage the plastic tape and reels, carriers, carrier tape, tubes, and trays used to dispense the components. Damage can occur at temperatures as low as 45°C (113°F), making it impossible for placement machines to process these parts.

II. Baking exacerbates oxidization4,5,6 on the MSD’s solder terminations, one of the biggest causes of poor solder quality.

III. Baking takes a long time: up to 79 days for somewhat thick (2 to 4.5mm) components at the package safe low temperature of 40°C (104°F) to as little as 3 hr. at a package melting temperature of 125°C (257°F) for MSDs with MSL 2, with exposure time passed the expired floor life less than 72 hr. The higher the temperature, the shorter the bake time, unless components can be unpackaged, baked at a very high temperature, and then repackaged at a safer 40°C (104°F) temperature. At this safer temperature, bake times start at five days and can go up to 79 days.

IV. Baking consumes at least 20 times the energy as that of simply keeping the parts dry.7 And, since most MSDs do not require baking unless they are MSL 6 or the floor life was mistakenly exceeded, it would more reliable and economical to keep the parts dry rather than baking them.

Simply put, keeping MSDs dry is easier and more effective than baking. Dry atmosphere cabinets which maintain RH levels of less than or equal to 5% meet the requirements of the standard to stop the floor life clock when MSDs cannot be soldered before the floor life expires.

What’s more, maintaining MSDs in dry storage requires 1/20th or less of the energy (electric) required to bake, has zero bake time, and offers no risk to the component’s solderability or packaging.
Assemblers should consider using baking ovens for extremely sensitive MSL 6 MSDs or recover from the preventable situation of exceeding a component’s floor life. Otherwise, they could turn off the heated stock rooms, baking cabinets, and chambers and store all MSDs in dry – not hot – atmosphere cabinets without running afoul of the standard or diminishing the integrity of the component packages.
References
1. IPC/JDEC J-STD-033C, Handling, Packaging, Shipping, and Use of Moisture/Reflow Sensitive Surface Mount Devices, Section 1, Foreword, January 2007.
2. IPC/JDEC J-STD-033C, Table 5-1, January 2007.
3. IPC/JDEC J-STD-033C, Section 5.2, January 2007.
4. IPC/JDEC J-STD-033C, Section 4.2.7.1, January 2007.
5. Ray Prasad, “Baking and Bagging of PCBs for Lead-Free Assemblies,” rayprasad.com.
6. Intel, “Moisture Sensitivity/Desiccant Packaging/Handling of PSMCs,” 2000 Packaging Handbook, Section 8.5.2, 2000.
7. Typical electrical specification for desiccant dry atmosphere cabinet 15W average to dry, 600W to bake.
8. IPC/JDEC J-STD-033C, 5.2 and Table 5-1, January 2007.
9. IPC/JDEC J-STD-033C, Tables 4-1, 4-2, and 4-3, January 2007.
10. Typical storage or bake temperature specification for shipping trays and component tape and reel materials: 45°C.

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