-
- News
- Books
Featured Books
- pcb007 Magazine
Latest Issues
Current IssueThe Growing Industry
In this issue of PCB007 Magazine, we talk with leading economic experts, advocacy specialists in Washington, D.C., and PCB company leadership to get a well-rounded picture of what’s happening in the industry today. Don’t miss it.
The Sustainability Issue
Sustainability is one of the most widely used terms in business today, especially for electronics and manufacturing but what does it mean to you? We explore the environmental, business, and economic impacts.
The Fabricator’s Guide to IPC APEX EXPO
This issue previews many of the important events taking place at this year's show and highlights some changes and opportunities. So, buckle up. We are counting down to IPC APEX EXPO 2024.
- Articles
- Columns
Search Console
- Links
- Events
||| MENU - pcb007 Magazine
Practical Processing of Lead-Free Assemblies (June 2002)
June 18, 2002 |Estimated reading time: 9 minutes
lead free; lead-free; lead,free; lead, free; lead_free; lead - free; lead=free; lead'free Practical Processing of Lead-Free Assemblies By Alan Rae, Vice President Technology, Cookson Electronics, Inc.
The question is not whether, but rather when, your customers will ask you for lead-free assemblies.
Passage of the European Community’s Reduction of Hazardous Substances directive, the so-called RoHS, has taken place, and the date of implementation will likely occur in 2006. This move is a concern to many manufacturers, but while lead-free processing is hardly a trivial issue, it is not a difficult one for the majority of assemblies. For simple boards, reliable lead-free assembly is relatively easy. Even for complex boards, by identifying the challenges and technical options, as well as issues related to laminates, packaging materials, solders, surface finishes, and wave and reflow processing equipment, the move to lead-free can be uncomplicated.
In fact, many consumer OEMs are already shipping large quantities of lead-free products in Asia and Europe, and the automotive industry has been shipping literally millions of lead-free assemblies since 1997. And, while results from the European Community’s IDEALS program, two NCMS studies, and a very thorough NEMI lead-free study will not be published until later this year, findings from these studies are already motivating worldwide industry standardization on tin-silver-copper for reflow processes and tin-copper for wave processes.
Cookson Electronics had questions about the prevailing issues that manufacturers migrating to lead-free processing must address, and how these issues will affect a manufacturer’s operations. In order to better serve our customers and our markets, we wanted to understand how a change to lead-free would change assembly and manufacturing processes.
To that end, we recently completed a comprehensive study of surface mount reflow (SMT) and through-hole (TH), including both wave solder and pin-in-paste techniques. We looked at solder alloy, surface finish, flux, etc., from the point of processibility, and then tested every joint electrically and visually, following up with air-to-air and liquid-to-liquid thermal cycling.
We undertook our own analysis because, in spite of the many excellent studies recently completed or soon to be published, we wanted to test the processes and materials reflective of our diverse customer base. Whether making backplanes or pagers, cell phones or avionics, our customers use a wide variety of alloys and surface finishes, so we wanted to be able to answer questions such as, “What happens if my customer asks me to change surface finish – will I need to change my wave soldering flux?”
For this independent project, our test vehicle needed to be large enough to simulate a real-life assembly situation, possess a range of SMT and TH components of various pitches and geometries, have a component layout such that soldering defects would be expected in certain areas to give a basis for cross-comparison of results, contain enough solder joint opportunities to produce a statistically meaningful dataset, and utilize current off-the-shelf Pb-free materials for fabrication.
To satisfy these requirements, our test vehicle was a 12″ x 10″, two-sided, four layered, 0.062″-thick board comprised of six separate circuits. Each circuit had equivalent SMT component composition such that the board could be sectioned for distribution into the different environmental test chambers. The board laminate material was a high performance FR-4 with a Tg between 175 and 180° C. The bottom side of the board was used for the wave soldering experiments (arrangements of components on a single circuit is shown in Figure 1 , with the feed direction into the wave indicated by the arrow). The SMT components included SO16s, SOT23s, 1206 and 0805 chip resistors, and fine-pitch QFP80s. Both vertical and horizontal orientations were present for all components, deliberately giving rise to so-called “design violations” for wave soldering. It is in these areas, more than others, that soldering defects are expected to occur.
Based on six replicates of the component arrangement in Figure 1 for each test vehicle, there were 4,416 SMT wave-soldering opportunities per board. The test vehicle also had different types of TH components, including (2) 96-pin connectors, (2) 50-pin connectors, (2) DIP16 chip carriers, (1) PGA 256 socket, and (4) arrays of ten ¼ W axial-leaded resistors. The PGA 256 socket and two of the ¼ W resistor arrays were confined to a separate center panel on the board.
Figure 1 - Arrangement of SMT components for one circuit on the bottom of the test vehicle for wave soldering. Arrow shows feed direction.
The top side of the test vehicle was used for the paste/reflow experiments and showed a denser arrangement and higher variety of components compared to the wave soldering side. It also contained six circuits; the component arrangement for a single circuit is shown in Figure 2 .
Each circuit was comprised of an array of normal, fine, and ultra-fine pitch SMT components. This included chip resistor arrays (1206, 0805, 0603, and 0402) in both vertical and horizontal orientations, (3) QFPs of various pitch, (2) PBGAs and (3) mBGAs, (1) PLCC28, and (1) DPAK. The same array of TH components was available for the reflow experiments to evaluate pin-in-paste performance.
Figure 2 - Arrangement of SMT and TH components for one circuit on the top of the test vehicle for reflow soldering.
Based on six replicates of the component arrangement in Figure 2 for each test vehicle, there were 11,544 SMT reflow-soldering opportunities per board. For both wave and reflow, there were 660 through-hole soldering opportunities per board.
The vast majority of the surface mount and TH components on the board were daisy-chained with circuit terminations at the edges of the board (two card-edges connectors per circuit). These edge connections were used for making electrical resistance measurements before and during the reliability tests. The test vehicles were treated with five different surface finishes, including: Hot Air Solder Leveling (HASL) (Sn/Pb HASL for Sn63Pb37 baseline alloy and Sn-Cu HASL for the Pb-free alloys), Electroless Ni/Immersion Au (ENIG), Immersion Tin (I-Sn), Immersion Silver (I-Ag), and Organic Solderability Preservative (OSP). The I-Sn finish was only tested in the reflow study, not in the wave study. The alloys evaluated were tin-lead, tin-silver, tin-copper, tin-silver-copper with 3% and 4% silver, tin-silver-copper-bismuth, tin-silver-copper-antimony and tin-zinc-bismuth.
The results of our tests provided answers to many questions that are likely to be asked as assemblies are asked or required to migrate to lead-free processes.
How does lead-free compare to a eutectic solder?
We tested reliability between 550C to 1250C for both air-to-air and liquid-to-liquid. In common with other studies, we found no major difference between the alloys and surface finishes. What we did find was that large resistor components (1206) failed frequently due to joint cracking under thermal stress.
Do I need new equipment?
For really simple boards, your existing reflow oven is probably fine. For complex boards, you will need to use a multi-zone oven – 7-zone and above – with integral flux management and nitrogen capability. Wave solder pots of corrosion-resistant cast iron are fine, but stainless steel components are attacked by lead-free alloys over a period of months. Retrofits of specially coated stainless steel for wear-prone components are available.
Do I have to print differently?
Lead-free paste printing must take into account the fact that the molten solder will not spread to cover pad areas left open in the print process. All alloys and surface finishes show this effect.
So I need nitrogen?
Nitrogen buys you 100C, allowing you to solder at lower temperatures with a better finish joint but incurring nitrogen cost. The different rate of flux evaporation in nitrogen, as opposed to air, has been blamed for “tombstoning” in some assemblies when 0201 components are reflowed in nitrogen. Lightweight components can also float on flux exudates. When specifying new reflow or wave equipment, nitrogen capability is a wise investment.
What special board material will I need?
A third of the US industry has switched to higher Tg materials – 1700C vs. 1400C – for a greater margin of safety in rework. For complex, professional electronics boards, these materials work fine in a lead-free environment, but take care that the time to delamination of the grade you choose (typically the T260 rating) is also improved over regular FR4. With single-sided boards for TV or other applications, FR-2 boards can be soldered lead-free with care.
What surface finish should I use?
Lead-free HASL is available and works well. OSP, despite the rumors, works well – even in air – as long as the number of reflow cycles is limited. Tin and silver finishes perform well, and nickel-gold is fine, but as OSP, tin, and silver usage increases, gold’s use is reduced.
At what temperature do I need to run my solder pot?
In most cases, the same temperature as with eutectic tin-lead: 2500-2600C.
Will I be buried in dross?
No. Our study showed no statistically significant increase in dross – even in wave soldering well above 2600C. What’s more, because of the higher value of lead-free dross, integrated dross management and solder reclaim systems may experience more rapid payback.
Will I see abnormal stencil wear?
Stencil wear is unaffected by lead-free processes.
Will I have to reflow at 2600C?
Not necessarily. On smaller boards with low thermal mass, 2300C is possible. More complex boards require higher temperature and attention to detail on heat distribution.
What considerations are there in soldering through-hole components?
With pin-in-paste assemblies, tin-silver-copper alloys showed far fewer defects than other alloys. With wave soldering, the most dominant factor is flux composition.
What about “popcorning”?
Although epoxy molding compound makers have produced compounds that will comfortably take 2600C for JEDEC level 2A, most components work well with standard molding compound. Reliability of large ceramic components, such as 1206 resistors, may be an issue because of thermal mismatch with the circuit board material..
How do the joints look?
Tin-rich lead-free solders often look a little less shiny than eutectic joints due to the formation of relatively large crystals during cooling. This is seen in all alloys and surface finishes. Voids are also an issue, particularly in fine-pitch BGA joints, because the high surface tension of tin-rich alloys prevents the escape of bubbles from flux solvents or microvia in pad voids, but voids can be minimized through careful process and materials control. Tin-silver-copper alloys and tin-lead appear in our study to give fewer voids than other alloys.
How is cleanup affected by lead-free?
Air-reflowed boards are tougher to clean than nitrogen-reflowed boards because of flux residue oxidation, but equipment and surfactant vendors have systems that perform well on lead-free boards.
What precautions do I need to take?
Because lead contamination can have a negative effect on solders, segregation of lead-free and lead-tin solder in processing and rework is required.
What alloy should I use? There are several tin-silver-coppers.
Much of the work in Europe and the USA has been on 3.8-4.0% silver with 0.5-0.7% copper. Japanese work focused on 3% silver to reduce costs. (In our study, the 3% and 4% silver alloys performed about the same.) All these alloys melt around 2170C. Make sure that your solder manufacturer has cross-licensed the alloys you need to cover the global market.
What’s next?
We’re extending the study to finer pitch components to address some of the issues occurring with 0201s and CSPs as well as evaluating the second-generation pastes now becoming available.
It is important to keep in mind that the industry is moving irrevocably toward lead-free, and for those assemblies that have successfully adapted their lines to lead-free, the reward can be a competitive edge. By understanding how lead-free processes will affect results, changes can be made in advance, ensuring a better end-result the first time, rather than enduring a costly trial-and-error period.