PCB Fabrication

lead-free dfm: demands new tricks: lead-free dfm is a prime target for murphy\'s law. so make sure every detail is handled accurately.

by:A-TECH      2020-01-21
Lead design for manufacturing (DfM)
Proof involved, okay-
Established practices in PCB design layout, manufacturing, assembly and component management. However, lead-
The free DfM significantly changed the landscape of these four areas.
Oem outsourcing business
FreePCBs, but not fully aware of leads
Free DfM issue, could be a victim of expensive redesign
Work and lost timeto-
If they do not get in touch with the EMS provider, they will receive revenue.
In PCB design layout, key issues should be focused on component packaging, placement, wiring and post-cleaning
After the layout is completed.
In addition, as shown in Figure 1, the manufacturing and assembly drawings are generated as the output of the layout process.
In the process of DfM, it is essential to process and assemble drawings as they require special materials and pcb surface finishes to be able to withstand the higher temperatures required for leadsfree process.
Other calls are also required during assembly, such as heat distribution, special flux requirements, and some additional process changes. [
Figure 1 slightly]
For OEM, it is essential to understand the packaging and footprint of lead and lead
Free components may be different, and this difference may be the basis for costly manufacturing errors. Lead-
Free assemblies are encapsulated to withstand higher return temperatures, and they may or may not have a different footprint.
Therefore, it is important to carefully study the component footprint, package type and related component spec sheets during the PCBlayout process.
This special DfM step is easy to ignore, so the OEM will re-
Rotate the first PCB at a cost from $ fewhundred to more than $20,000.
A typical example is when OEMgives gives an EMS provider a lead
For the most important components, free PCB orders with wrong spec sheets, which are designed in a leading position
Free Assembly level.
Footprints will not match, resulting in re-
The rotation of the light plate, which is expensive.
Situations like this are rare and may occur once or twice in a few hundred times.
Nevertheless, they happen, usually due to OEM engineering assumptions and the lack of careful professional review.
Component placement may also introduce new expensive errors, especially if the PCB is a hybrid technology board with leadsandlead-Free components.
Keep in mind that these components require a different thermal distribution.
In the case of rework, this problem becomes more important.
For example, remove the lead-
Releasing free from the pcb requires a different nozzle, and there is more heat on the periphery to pull it, because a higher temperature is required for proper melting welding (FIGURE 2). [
Figure 2:
Lead and lead
Free components should be specified correctly and clearly on the assembly drawing, and do not leave any room for doubt, ambiguity, misconceptions or false assumptions.
For hybrid technology pcb, detailed instructions are particularly important, including how to handle engineering change orders and how clear they are listed in the assembly diagram.
If the assembly drawings are poorly recorded, it may cause problems such as the melting of the welding balls of the lead BGA, or even the complete melting of the ic.
An example of a bad OEM document will include failure to specify which components are potentially
Free of charge, especially the problem with the amixed technology board.
In this case, the leader
After the circuit board is returned, the free element will fall off the board. The reason?
The Assembly has not gone through the correct high temperature thermal profile;
Therefore, the components are not welded properly on the board.
Even with a good quality control process, this particular type of error is hard to capture because in some cases the part number and lead-
The free components are the same.
Manufacturing PCB material types play an important role in Leadsfree DfM.
Considering the lead-
Free items require higher temperature distribution, it is important to use base materials compatible with lead
Rapid manufacturing and assembly of PCB. FR-
4 has always been the traditional material of leadbased PCBs.
Laminated materials like FR408 with higher heat distribution are more suitable for lead-free assembly.
Table 1 shows the thermal distribution compared to traditional FR-
4 and higher Tgmaterial, fr408.
During the material identification process specification sheet, the manufacturer\'s reference data and other documents need to be thoroughly reviewed to fully understand the best PCB Laminate based on the properties required for processing performance.
If clarification is required, it is better to contact the manufacturer directly and ask specific questions.
EMS providers must also continue calibration and fines
Adjust its assembly and manufacturing equipment.
Must be at the top.
Notch shape enables the device to handle tighter leads efficiently and accurately
The free assembly process for windows compared to the lead assembly.
Because these fast thermal cycling windows are so closely linked
Free assembly, the margin of error is very small.
Thermal cycling with lead, with 20 [degrees]to 30[degrees]
Latitude and components can stay for another 30 to 60 seconds without adverse effects.
Lead is impossible.
Because the operation window is much shorter, it is free.
In many cases,
Free PCB surface treatment is different from traditional lead PCB. Hot-
Leveling of air welding (HASL)
Lead-based soldering tin is a common lead plate. A lead-
Container Material not compatible with leadfree assembly. For lead-
Free Assembly with or without nickel/gold immersion (ENIG)
, Silver immersion, high temperature organic welding preservatives (OSP)
And a special clue. free HASL.
Surface treatment is important for leaders
Free PCB manufacturing.
They need to withstand higher temperatures without affecting their wetting properties and are compatible with lead
Free assembly of materials.
As shown in Table 1, there are several trade-offs for these surface treatments.
Shelf life, cost, return cycle and solder joint flatness are all unique.
Precious metals are used in the process of chemical nickel/gold immersion and silver immersion.
Depending on the number of exposed surfaces, additional charges may be required for these finishes.
Therefore, from the acost/performance point of view, it is better to choose a surface finish that can prove reasonable in a specific final product application.
Assembly: There have been many defects since the lead time-
Flee components require higher temperatures when assembling, and the procedures followed are critical, including careful study of component spec sheets, thermal distribution, flux and solder paste and their different temperature properties
The first article in DfM is important for any type of component, but more important for leads
It is free because different heat distribution is used. Also,for lead-
Fleeing the PCB assembly to find possible defects becomes more intense than the lead assembly.
Quality control measures embedded in the assembly process must receive more detailed attention than a given assembly.
Possibility of lead presence
Free holes, graves and other Free related defects
Stone Effect and tin must be formed.
The importance of using advanced equipment and trained technology to target these defects cannot be underestimated. High-
AOI and high-resolution x-
The primary task of tracking the problem is the ray system.
These systems and their support software can not only detect defects, if any, but also verify whether they are real defects (FIGURE 3). [
Figure 3 slightly]
Lead labeling equipment
Free use is another aspect of tivtivedfm.
This is important to avoid the possibility of a regular exit process mixing with leads
Free and incur additional costs and time delays.
Similarly, appropriate training should be given to technicians and other support personnel to manage and operate these leadersfree systems.
A trained eye-free eye can detect if a component or assembly is faulty.
Component Management buyer is key to effective leads
Free Accessories Purchase
They must be well trained, well educated and qualified to do the job effectively.
This position in the EMS provider is critical. Alas, some component suppliers failed to change component names to clearly identify potential customersFree components.
Therefore, the burden and responsibility of procurement falls on cross-sourcing
Reference to lead parts replaced by lead
Free equipment, and buyers must be able to suggest the right delivery
In the replacement equivalent.
Staff of procurement management and trained personnel should also be familiar with the composition inspection to accurately separate and detect lead and leadFree components.
The team should include strong inspection criteria to closely review and verify component value, footprint, packaging type and leadsfree status.
Lead content detector is an important tool for inspection management.
It not only checks for lead content, but also searches for all other RohS substances to ensure that the ingredients meet the requirements.
Incoming inspection prevents the use of wrong disparts and is used to verify suspicious parts from brokers.
The best practice is to buy from reputable manufacturers or dealers.
While brokers also sell parts, in some cases they are extracted from discarded PCBs or recycling systems.
It is not a good practice to use pull components because their reliability can be questionable.
For component changes or replacements during DfM, they must be made through an engineering change order with proper records, which in turn should be published directly from the document control department.
This process is critical and should not deviate, as it ensures that the relevant project can be accurately supported by the relevant documentation.
This is especially valuable for leaders.
Because these pcb needs more attention to detail.
Finally, \"Stock Returns\" is an equally important aspect of the leadfree DfM.
Here, cross-checking is essential to ensure the lead
When free and lead parts are returned to stock/stock, they are separated.
These different components are relatively easy to keep separate when entering.
However, it can be difficult and wrong to return the inventory process --
When components are in one container with thousands of other components, it is easy to appear, and these components may or may not be potentialfree.
Zulki khan is president and founder of Nexlogic Technologies, San Jose, California.
You can contact him at zk @ nexlogic. com.
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