Introduction
Not much to it, just order circuitboards from a vendor. How hard can it be, not much can go wrong? Or is it? Let’s see about basic requirements on receiving circuitboards.
Just a PCB in it’s shipped state
Looking great! Nicely vacuum packed in LDPE foil, fancy Humidity Indicator Card inside, product label on it. All set, good to go?
Different insights however?
IPC-1602
April 2020, IPC released the new 1602 standard. This is the forthcoming on the 1601 which already existed with guidelines for printed board handling and storage. Quite a lot of details on packaging, baking before assembly, moisture barrier bags, etc. More on this later on.
IPC-J-STD-609B
Marking, deemed a lightweight topic. But still there is a standard with all ins and outs for markings on a PCB and components.
Usually we only regard the brand and type of component. According to BOM, then we continue as usual. However, the encircled e3 does have it special intent. What many people do not know, is the plating nominator. This e3 in this instance tells us, the component lead is plated with tin (Sn). But various other indexes are called out in this standard. Doesn’t seems very important, but is critical knowledge to be able to achieve acceptable solder joints. Here shown as marking on a component but 1:1 translated to circuitboards.
Applying J-STD-609B to the PCB’s you will be ordering is mandatory when IPC-A-600 requirements need to be fulfilled!
There is also a mandatory requirement for marking of the finished assembly. After completion of the assembly (aka. soldering components onto the board) it is mandatory to note the used solder and conformal coating.
IPC-A-600
Sometimes underrated but very important. This document will enable the EMS to do a inbound or pre-assembly check on the received printed circuit boards. Did the manufacturer get everything right? Any blisters, scratches, solder mask voids that exceed acceptable limits? This standard is to be regarded as the fundament under the product. Regard the product as a house and the circuit board as a fundament as an experiment. Usually a concrete fundament is not noticeable on the average house or building, but a certain tower in Pisa (Italy) tells a different story….
Good foundation to expensive components?
Opinions might differ, but also the circuit board itself is a very important part of the total product. This is usually neglected with the expression “It’s the board vendor’s issue”. When it comes to product responsibility, the EMS has the final responsibility for end item quality.
Bare boards when received
What is noticed here?
- Bare boards, vacuum sealed
- Packaging label
- Humidity Indicator Card (HIC)
- Pack of dessicant
This may seem okay, but it is not! The sealed bag is made of LDPE. This type of film is partially permeable to water, this phenomenon is expressed by the Water Vapor Transmission Rate (WVTR). The WVTR is used to express a material’s property to allow water vapour to pass through it, nominated in grams per square meter in 24hours (g/m2/24hrs). The higher the WVTR value of the material, the more (rapidly) vapour can pass through it. The WVTR of LDPE film typically ranges between 16* and 23* g/m²/24hrs (*film thickness 30𝜇m). IPC-1602 and IPC-J-STD-033 advise a ‘Moisture Barrier Bag (MBB)’ with a WVTR not exceeding 0,031 g/m²/24hrs. The WVTR of Moisture Barrier Bags (MBB) should thus be about 510 to 740 times lower, than your typical LDPE bag.
Summarised: LDPE film will keep the boards dry to a certain level, okay for cement, salt or rice. However, not to the level a printed circuit boards needs to be processable without a baking cycle before subjecting it to the heat in the soldering process. LDPE is 510-740 times as worse as the minimum limit for WVTR!
Plastic film typically needs a metal layer to keep moisture out, exactly the reason why a bag of crisps is shiny silver on the inside. You wouldn’t like a stale snack, why permit your PCB’s to go bad?
There’s also a HIC inside the foil pack, this will discolor from blue to pink when subjected to moisture in the air. This will give the user an idea whether boards were exposed to humidity or not. The HIC also has its requirements. The minimum is three spots with a sensitivity of 5%, 10% and 60%. This picture shows a HIC which starts at 10%, being not accurate enough for this application.
Certain types of HIC contain cobalt bromide. This shall not be in direct contact with an ENIG finish.
Next item is a pack of desiccant, a big plus to have this inside the sealed package. Keep in mind the requirements from IPC-1602 – Appendix B which places requirements on the amount of desiccant required.
Last but not least is the label on the package. Mandatory requirement according to IPC-1602 and IPC-J-STD-609B is to have:
- Part number
- Date code
- Quantity of bare boards in the package
- b0 marker when Pb level exceeds 0,1% (609B). (leadfree finishes indicator not mandatory)
Now the doubt creeps in, can we use this stack of boards right away, straight from the seal?
NO, they cannot be used right now. We can observe the 10% indicator to have slight discoloration, a smoking gun for humidity in the seal. How to solve this issue and get these boards into production?
Two fold approach
- Laminate witness coupon
- Baking
A laminate witness coupon (strip or part of laminate with equal properties in the MBB). This sample is intended to evaluate relative moisture absorption. IPC-TM-650 provides methodes to asses the issues related to the amount of absorbed moisture. Also a manufacturer shall weigh the coupon to the nearest 0,0001 gram (0,1 milligram). If an agreement with the manufacturer is made to communicate this weight, then assessing the amount of absorbed moisture is easily determined.
A more reliable methode however is to integrate a baking step in the assembly process.
Specialised baking ovens enable the EMS to dry the bare boards prior to assembly. This is a very worthwhile tool to have. Not only for baking newly received boards, but also for drying after cleaning. repair, rework etc. Also, most EMS keep stock in bare boards, depending on the storage conditions (low oxygen/RH or ambient?) delivered boards accumulate moisture as well.
So a baking step in the assembly process is worthwhile to have.
Bear in mind; a baking profile must be created, there is no standard baking table like there is for devices (components) in the J-STD-033. IPC-1602 has an outline of the process in §3.4.4. A ball park figure is to bake at 105-125˚C during 4-6 hours, but this must be evaluated with actual products. Don’t overdo the baking by prolonging the baking time or upgrading temperature. This will introduce solderability issues or even delamination/blistering when baking is done too aggressive or too long. Thus, longer baking time is not necessarily better.
Moisture-Sensivity Level
Moisture-Sensitivity Level (MSL) is a nominator that indicates a device susceptibility to damage due to absorbed moisture when subjected to reflow soldering. As the definition says “device”, this is only applicable for components, not for circuitboards. Sometimes EMS companies take MSL 2a for their circuitboards, but this “rule of thumb” is to be avoided and not supported by IPC standards.
A better approach to determining moisture sensitivity for a circuitboard is to perform thermal stress testing according to IPC-TM-650.
Contact the author: ramon.koch@etech.training
